EXECUTION OF A C++ PROGRAM


EXECUTION OF A C++ PROGRAM 

How a c++ program is proceed
                                                                         C++ PROGRAM
                                                            
step 1.                                                          
                                                                            EDITOR


step 2.                                                          PREPROCESSOR


step 3.                                                                        COMPILER


step 4.                                                               LINKER


step 5.                                                                       LOADER


step 6.                                                  EXECUTION




According to structure.  Processing a c++ program

As a programmer you need to be concerned only with step 1 that is you should learn , understand , and master the rules of programming language to creat programs.

PROGRAMMING WITH PROBLEM ANALYSIS CODING EXECUTION CYCLE


PROGRAMMING WITH PROBLEM ANALYSIS CODING EXECUTION CYCLE





Programming is a process of problem solving .Different people use different techniques to solve different programs.some techniques are easy and common and widly used.

In programming envoirnment the problem solving  process  requires the following steps.

step 1.    Analyze the problem , outline the problem and its solution requirements and design an algorithm to solve the problem.

step 2.    implement the algorithm(a step by step problem solving process in which a solution is arrived at finite time) in a programming language such as c++ and verify that the algorithm work.

step 3.   Maintain the program by using and modifying it if the problem domain changes.



summary of these three steps


                                                                   PROBLEM

                                                                   ANALYSIS

                                                                  ALGORITH DESIGN

                                                                  CODING

                                                                  PREPROCESSOR

                                                                 COMPILER

                                                                 NO ERROR
LIBRARY
                                                                 LINKER

                                                                 LOADER

                                                                 EXECUTION

                                                                 NO ERROR

                                                                RESULTS

This is a problem analysing coding execution cycle.

Creating a c++ program


Creating a c++ program

A c++ program is a collection of functions one of which is the function main . Therefore  if a c++ program has or consists of only one function then it will be the function main.Moreover a function is a set of instructions designed to accomplish a specific and special task.

The syntax of the function main is

int main (   )

{

 statement 1
     .
     .
     .
statement n
return 0;

}


in the syntax of function main each statement is usually either a declarative statement or an executable statement.The statement return 0; should be included in the function main and should be the last statement.Source code must be saved in file with the file extention .cpp.

HOW TO LEARN PROGRAMMING


HOW TO LEARN PROGRAMMING .(TIP).



Learning programming language is like learning to become a chef  or learning to become a or play a musical instrument .All these requres tools but they also required practice.You cannot become a good chef  or even a poor chef only by reading recipes .similarly you can not become a good player by reading books about musical instrument .The same is true for programming  .You must have fundamental knowledge of programming and you should have to practice of computer.

Programming is a process of planning and creating a program.

Programming language :   A set of rules ,symbols ,and special words

A SIMPLE SAMPLE OF C++ PROGRAM


A SIMPLE C++ PROGRAM.

Consider the following c++ program

#include <iostream>
using namespace std;
int main ( )
{
  cout << " The sum of 4 and 7 is  =" << 11<<endl;
return o;
}

Description of this program

The symbol # is called preprocessor directives iostream is header file.
The body of the c++ program  is

#include <iostream>
using namespace std;
int main ( )
{

return 0;
}

in c++ program every statement end is with semicolon ;

THE BODY OF C++ PROGRAM


THE BODY OF C++ PROGRAM;


#include <iostream>
using namespace std;
int main ( )
{

return 0;
}

in c++ program every statement end is with semicolon ;
; is used to terminate(end) the statement.
# is called preprocessor directives

iostream is called header file.

in c++ using and namespace are reserved words

return 0 is code and it complete the programming main part and goes back compiler

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TYPE CONVERSION (CASTING) IN C++


For type conversion ( casting ):

when a value of one data type is automatically changed to another data type then implicit type coercion occured.
To avoid implicit coercion , c++ provides cast operator used for convertion.

For converting a integer value into floating type value then we have to use this conversion

static_cast <double> (12) evaluates to  12.0

For converting a double type value into integer value then we have to use this conversion

static_cast <int> ( 9.7 ) evaluates to 9.

for this method we can conclude all values.    

Difference between assignment operator(=) and equality operator (= =) :


Difference between assignment operator(=) and equality operator (= =) :
Assignment operator and equality operator are different but they are similar in reading . = operator is used to assign value and = = operator is used to said equal.
Be careful! The operator = (one equal sign) is not the same as the operator = = (two equal signs), the first one is an assignment operator (assigns the value at its right to the variable at its left) and the other one (= =) is the equality operator that compares whether both expressions in the two sides of it are equal to each other. Thus, in the last expression ((b=3) = = a), we first assigned the value 3 to b and then we compared it to a, that also stores the value 3, so the result of the operation is true.

BINARY UNITS


 LANGUAGE OF COMPUTER 
The language of computer is called machine language,which is a sequence of 0s and 1s.The  digit 0 or 1 is called a binary digit or bit. Sometimes a sequence of 0 and 1 is considered as a binary code or a binary number.
Bit. A binary digit 0 or 1.
A sequnce of eight bits is called a byte.

Byte.                        8 bits

Kilobyte.   KB          1O24bytes

Megabyte. MB         1024KB =1048576 bytes

Gigabyte. GB          1024MB =1073741824 bytes

Terabyte.  TB          1024GB =1099511627776 bytes

Petabyte.  PB          1024TB =1125899906842624 bytes

Exabyte.   EB          1024PB =1152921504606846976 bytes

Zettabyte. ZB          1024EB =1180591620717411303424 bytes

American standard code for information interchange(ASCII) is a seven bit code and consists of 128 chracters numbered 0 through 127.

CONSTANTS


ConstantsConstants are expressions with a fixed value.

Literals

Literals are the most obvious kind of constants. They are used to express particular values within the source code of a program. We have already used these previously to give concrete values to variables or to express messages we wanted our programs to print out, for example, when we wrote:

a = 5;


the 5 in this piece of code was a literal constant.

Literal constants can be divided in Integer Numerals, Floating-Point Numerals, Characters, Strings and Boolean Values.

Integer Numerals


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1776
707
-273


They are numerical constants that identify integer decimal values. Notice that to express a numerical constant we do not have to write quotes (") nor any special character. There is no doubt that it is a constant: whenever we write 1776 in a program, we will be referring to the value 1776.

In addition to decimal numbers (those that all of us are used to using every day), C++ allows the use of octal numbers (base 8) and hexadecimal numbers (base 16) as literal constants. If we want to express an octal number we have to precede it with a 0 (a zero character). And in order to express a hexadecimal number we have to precede it with the characters 0x (zero, x). For example, the following literal constants are all equivalent to each other:

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75         // decimal
0113       // octal
0x4b       // hexadecimal 


All of these represent the same number: 75 (seventy-five) expressed as a base-10 numeral, octal numeral and hexadecimal numeral, respectively.

Literal constants, like variables, are considered to have a specific data type. By default, integer literals are of type int. However, we can force them to either be unsigned by appending the u character to it, or long by appending l:

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75         // int
75u        // unsigned int
75l        // long
75ul       // unsigned long 


In both cases, the suffix can be specified using either upper or lowercase letters.

Floating Point Numbers

They express numbers with decimals and/or exponents. They can include either a decimal point, an e character (that expresses "by ten at the Xth height", where X is an integer value that follows the e character), or both a decimal point and an e character:

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3.14159    // 3.14159
6.02e23    // 6.02 x 10^23
1.6e-19    // 1.6 x 10^-19
3.0        // 3.0 


These are four valid numbers with decimals expressed in C++. The first number is PI, the second one is the number of Avogadro, the third is the electric charge of an electron (an extremely small number) -all of them approximated- and the last one is the number three expressed as a floating-point numeric literal.

The default type for floating point literals is double. If you explicitly want to express a float or a long double numerical literal, you can use the f or l suffixes respectively:

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3.14159L   // long double
6.02e23f   // float 


Any of the letters that can be part of a floating-point numerical constant (e, f, l) can be written using either lower or uppercase letters without any difference in their meanings.

Character and string literals

There also exist non-numerical constants, like:

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'z'
'p'
"Hello world"
"How do you do?" 


The first two expressions represent single character constants, and the following two represent string literals composed of several characters. Notice that to represent a single character we enclose it between single quotes (') and to express a string (which generally consists of more than one character) we enclose it between double quotes (").

When writing both single character and string literals, it is necessary to put the quotation marks surrounding them to distinguish them from possible variable identifiers or reserved keywords. Notice the difference between these two expressions:

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x
'x'


x alone would refer to a variable whose identifier is x, whereas 'x' (enclosed within single quotation marks) would refer to the character constant 'x'.

Character and string literals have certain peculiarities, like the escape codes. These are special characters that are difficult or impossible to express otherwise in the source code of a program, like newline (\n) or tab (\t). All of them are preceded by a backslash (\). Here you have a list of some of such escape codes:

\nnewline
\rcarriage return
\ttab
\vvertical tab
\bbackspace
\fform feed (page feed)
\aalert (beep)
\'single quote (')
\"double quote (")
\?question mark (?)
\\backslash (\)

For example:

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'\n'
'\t'
"Left \t Right"
"one\ntwo\nthree" 


Additionally, you can express any character by its numerical ASCII code by writing a backslash character (\) followed by the ASCII code expressed as an octal (base-8) or hexadecimal (base-16) number. In the first case (octal) the digits must immediately follow the backslash (for example \23 or \40), in the second case (hexadecimal), an x character must be written before the digits themselves (for example \x20 or \x4A).

String literals can extend to more than a single line of code by putting a backslash sign (\) at the end of each unfinished line.

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"string expressed in \
two lines" 


You can also concatenate several string constants separating them by one or several blank spaces, tabulators, newline or any other valid blank character:

"this forms" "a single" "string" "of characters"


Finally, if we want the string literal to be explicitly made of wide characters (wchar_t type), instead of narrow characters (char type), we can precede the constant with the L prefix:

L"This is a wide character string"


Wide characters are used mainly to represent non-English or exotic character sets.

Boolean literals

There are only two valid Boolean values: true and false. These can be expressed in C++ as values of type bool by using the Boolean literals true and false.

Defined constants (#define)

You can define your own names for constants that you use very often without having to resort to memory-consuming variables, simply by using the #define preprocessor directive. Its format is:

#define identifier value
For example:

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#define PI 3.14159
#define NEWLINE '\n' 


This defines two new constants: PI and NEWLINE. Once they are defined, you can use them in the rest of the code as if they were any other regular constant, for example:

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// defined constants: calculate circumference

#include <iostream>
using namespace std;

#define PI 3.14159
#define NEWLINE '\n'

int main ()
{
  double r=5.0;               // radius
  double circle;

  circle = 2 * PI * r;
  cout << circle;
  cout << NEWLINE;

  return 0;
}
31.4159


In fact the only thing that the compiler preprocessor does when it encounters #define directives is to literally replace any occurrence of their identifier (in the previous example, these were PI and NEWLINE) by the code to which they have been defined (3.14159 and '\n' respectively).

The #define directive is not a C++ statement but a directive for the preprocessor; therefore it assumes the entire line as the directive and does not require a semicolon (;) at its end. If you append a semicolon character (;) at the end, it will also be appended in all occurrences of the identifier within the body of the program that the preprocessor replaces.

Declared constants (const)

With the const prefix you can declare constants with a specific type in the same way as you would do with a variable:

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const int pathwidth = 100;
const char tabulator = '\t';


Here, pathwidth and tabulator are two typed constants. They are treated just like regular variables except that their values cannot be modified after their definition.

BASIC INPUT/OUTPUT



BASIC INPUT/OUTPUT


C++ uses a convenient abstraction called streams to perform input and output operations in sequential media such as the screen or the keyboard. A stream is an object where a program can either insert or extract characters to/from it. We do not really need to care about many specifications about the physical media associated with the stream - we only need to know it will accept or provide characters sequentially.





The standard C++ library includes the header file iostream, where the standard input and output stream objects are declared.

Standard Output (cout)

By default, the standard output of a program is the screen, and the C++ stream object defined to access it is cout

means console output

cout is used in conjunction with the insertion operator, which is written as << (two "less than" signs).

The << operator inserts the data that follows it into the stream preceding it.


 cout << "Hello";       // prints Hello  
cout << Hello;        // prints the content of Hello variable



The insertion operator (<<) may be used more than once in a single statement:


cout << "Hello, " << "I am " << "a C++ statement";


This last statement would print the message Hello, I am a C++ statement on the screen. The utility of repeating the insertion operator (<<) is demonstrated when we want to print out a combination of variables and constants or more than one variable:


cout << "Hello, I am " << age << " years old and my code is " << code;


If we assume the age variable to contain the value 24 and the code variable to contain 90064 the output of the previous statement would be:

Hello, I am 24 years old and my code is 90064

It is important to notice that cout does not add a line break after its output unless we explicitly indicate it, therefore, the following statements:








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will be shown on the screen one following the other without any line break between them:





This is a sentence.This is another sentence.


even though we had written them in two different insertions into cout. In order to perform a line break on the output we must explicitly insert a new-line character into cout. In C++ a new-line character can be specified as \n (backslash, n):








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This produces the following output:





First sentence.
Second sentence.
Third sentence.


Additionally, to add a new-line, you may also use the endl manipulator. For example:








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would print out:





First sentence.
Second sentence.


The endl manipulator produces a newline character, exactly as the insertion of '\n' does, but it also has an additional behavior when it is used with buffered streams: the buffer is flushed. Anyway, cout will be an unbuffered stream in most cases, so you can generally use both the \n escape character and the endl manipulator in order to specify a new line without any difference in its behavior.






Standard Input (cin).

The standard input device is usually the keyboard. Handling the standard input in C++ is done by applying the overloaded operator of extraction (>>) on the cin stream. The operator must be followed by the variable that will store the data that is going to be extracted from the stream. For example:








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The first statement declares a variable of type int called age, and the second one waits for an input from cin (the keyboard) in order to store it in this integer variable.





cin can only process the input from the keyboard once the RETURN key has been pressed. Therefore, even if you request a single character, the extraction from cin will not process the input until the user presses RETURN after the character has been introduced.





You must always consider the type of the variable that you are using as a container with cin extractions. If you request an integer you will get an integer, if you request a character you will get a character and if you request a string of characters you will get a string of characters.








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14 // i/o example #include <iostream> using namespace std; int main () { int i; cout << "Please enter an integer value: "; cin >> i; cout << "The value you entered is " << i; cout << " and its double is " << i*2 << ".\n"; return 0; }
Please enter an integer value: 702 The value you entered is 702 and its double is 1404.









The user of a program may be one of the factors that generate errors even in the simplest programs that use cin (like the one we have just seen). Since if you request an integer value and the user introduces a name (which generally is a string of characters), the result may cause your program to misoperate since it is not what we were expecting from the user. So when you use the data input provided by cin extractions you will have to trust that the user of your program will be cooperative and that he/she will not introduce his/her name or something similar when an integer value is requested. A little ahead, when we see the stringstreamclass we will see a possible solution for the errors that can be caused by this type of user input.





You can also use cin to request more than one datum input from the user:








cin >> a >> b;









is equivalent to:








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In both cases the user must give two data, one for variable a and another one for variable b that may be separated by any valid blank separator: a space, a tab character or a newline.






cin and strings

We can use cin to get strings with the extraction operator (>>) as we do with fundamental data type variables:








cin >> mystring;









However, as it has been said, cin extraction stops reading as soon as if finds any blank space character, so in this case we will be able to get just one word for each extraction. This behavior may or may not be what we want; for example if we want to get a sentence from the user, this extraction operation would not be useful.





In order to get entire lines, we can use the function getline, which is the more recommendable way to get user input with cin:








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16 // cin with strings #include <iostream> #include <string> using namespace std; int main () { string mystr; cout << "What's your name? "; getline (cin, mystr); cout << "Hello " << mystr << ".\n"; cout << "What is your favorite team? "; getline (cin, mystr); cout << "I like " << mystr << " too!\n"; return 0; }
What's your name? Juan Soulie Hello Juan Soulie. What is your favorite team? The Isotopes I like The Isotopes too!









Notice how in both calls to getline we used the same string identifier (mystr). What the program does in the second call is simply to replace the previous content by the new one that is introduced.






stringstream

The standard header file <sstream> defines a class called stringstream that allows a string-based object to be treated as a stream. This way we can perform extraction or insertion operations from/to strings, which is especially useful to convert strings to numerical values and vice versa. For example, if we want to extract an integer from a string we can write:








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3 string mystr ("1204"); int myint; stringstream(mystr) >> myint;









This declares a string object with a value of "1204", and an int object. Then we use stringstream's constructor to construct an object of this type from the string object. Because we can use stringstream objects as if they were streams, we can extract an integer from it as we would have done on cin by applying the extractor operator (>>) on it followed by a variable of type int.





After this piece of code, the variable myint will contain the numerical value 1204.








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21 // stringstreams #include <iostream> #include <string> #include <sstream> using namespace std; int main () { string mystr; float price=0; int quantity=0; cout << "Enter price: "; getline (cin,mystr); stringstream(mystr) >> price; cout << "Enter quantity: "; getline (cin,mystr); stringstream(mystr) >> quantity; cout << "Total price: " << price*quantity << endl; return 0; }
Enter price: 22.25 Enter quantity: 7 Total price: 155.75









In this example, we acquire numeric values from the standard input indirectly. Instead of extracting numeric values directly from the standard input, we get lines from the standard input (cin) into a string object (mystr), and then we extract the integer values from this string into a variable of type int (quantity).





Using this method, instead of direct extractions of integer values, we have more control over what happens with the input of numeric values from the user, since we are separating the process of obtaining input from the user (we now simply ask for lines) with the interpretation of that input. Therefore, this method is usually preferred to get numerical values from the user in all programs that are intensive in user input.

OPERATORS



OPERATORS
Assignment operators
Arithmetic operators
Compound statements
Relational operators
Equality operators
Increase and decrease operators
Increment and decrement operators
Logical operators
Conditional operators
Comma operators
Bitwise operators
Other operators
All these operators are explained in this stage





Once we know of the existence of variables and constants, we can begin to operate with them. For that purpose, C++ integrates operators. Unlike other languages whose operators are mainly keywords, operators in C++ are mostly made of signs that are not part of the alphabet but are available in all keyboards. This makes C++ code shorter and more international, since it relies less on English words, but requires a little of learning effort in the beginning.

Assignment (=)

The assignment operator assigns a value to a variable.

a = 4;

This statement assigns the integer value 4 to the variable a. The part at the left of the assignment operator (=) is known as thelvalue (left value) and the right one as the rvalue (right value). The value has to be a variable whereas the rvalue can be either a constant, a variable, the result of an operation or any combination of these.
The most important rule when assigning is the right-to-left rule: The assignment operation always takes place from right to left, and never the other way.

a = b;

This statement assigns to variable a (the value) the value contained in variable b (the rvalue). The value that was stored until this moment in a is not considered at all in this operation, and in fact that value is lost.

Consider also that we are only assigning the value of b to a at the moment of the assignment operation. Therefore a later change of b will not affect the new value of a.
Here is an example of assignment operator

 // assignment operator

 #include<iostream>
 using namespace std;
 int main ()

int a, b; // a:?, b:? a = 10; // a:10, b:? b = 4; // a:10, b:4 a = b; // a:4, b:4 b = 7; // a:4, b:7 cout << "a:"; cout << a; cout << " b:";
 cout << b;
 return 0;
}
a:4
 b:7

This code will give us as result that the value contained in a is 4 and the one contained in b is 7. Notice how a was not affected by the final modification of b, even though we declared a = b earlier (that is because of the right-to-left rule).

A property that C++ has over other programming languages is that the assignment operation can be used as the rvalue (or part of an rvalue) for another assignment operation. For example:

a = 3 + (b = 6);
is equivalent to.

1.  b = 6;
2. a = 3 + b;

that means: first assign 6 to variable b and then assign to a the value 3 plus the result of the previous assignment of b (i.e. 6), leaving a with a final value of 9.

The following expression is also valid in C++:


a = b = c = 7;

It assigns 7 to the all three variables: a, b and c.




Arithmetic operators ( +, -, *, /, % )

The five arithmetical operations supported by the C++ language are:


Arithmatic operators which are

Addition    +

Subraction  -

Multiplication  *

Division    /

Modulus operator  or remainder   %

NOTE: You can use the operators + , - , *,and / use both floating point and integral data types.But the modulus operator % is only use with integral data types.

Unary operators : An operator that has only one operand.

Binary operator : An operator that has two operands.

Order of precedence in arithmatic operators

According to order of precedence rule   * , / , %  are at a higher level of precedence than :

+ , - .

for example in the expression

4 + 5 * 2 = 14.

* is evaluated before +

Here is an example for arithmatic operations

#include <iostream>
using namespace std;
int main ( )
{
  cout<<" 3 + 3"<< 3+3<<endl;
cout<<" 3 -3"<<3+3<<endl;
cout<<" 3 *3"<< 3+3<<endl;
cout<<" 3 %3"<< 3+3<<endl;
cout<<" 3 /3"<< 3+3<<endl;

return 0;
}

After execution in machine the results can be shown




Operations of addition, subtraction, multiplication and division literally correspond with their respective mathematical operators. The only one that you might not be so used to see is modulo; whose operator is the percentage sign (%). Modulo is the operation that gives the remainder of a division of two values. For example, if we write:

a = 13 % 3;

the variable a will contain the value 1, since 1 is the remainder from dividing 13 between 3.


Compound assignments:


Compound assignments are given below

 (+=, -=, *=, /=, %=, >>=, <<=, &=, ^=, |=)

When we want to modify the value of a variable by performing an operation on the value currently stored in that variable we can use compound assignment operators:

Expression is equivalent to

value += increase;

value = value + increase;

a -= 5;

a = a - 5;


a /= b;

a = a / b;


price *= units + 1;

price = price * (units + 1);

and the same for all other operators.
 For example:

// compound assignment operators
 #include <iostream>
 using namespace std;
 int main ()
{
 int a, b=5;
 a = b;
 a+=4; // equivalent to a=a+4
 cout << a;
 return 0;
}

The result of this example is  9


Increase and decrease (++, --)

Shortening even more some expressions, the increase operator (++) and the decrease operator (--) increase or reduce by one the value stored in a variable. They are equivalent to +=1 and to -=1, respectively. Thus:

1. c++;
2. c+=1;
3. c=c+1;
are all equivalent in its functionality: the three of them increase by one the value of c.

A characteristic of this operator is that it can be used both as a prefix and as a suffix. That means that it can be written either before the variable identifier (++b) or after it (b++). Although in simple expressions like b++ or ++b both have exactly the same meaning, in other expressions in which the result of the increase or decrease operation is evaluated as a value in an outer expression they may have an important difference in their meaning: In the case that the increase operator is used as a prefix (++b) the value is increased before the result of the expression is evaluated and therefore the increased value is considered in the outer expression; in case that it is used as a suffix (b++) the value stored in a is increased after being evaluated and therefore the value stored before the increase operation is evaluated in the outer expression. Notice the difference:

Example 1.

B=3;
A=++B;
// A contains 4, B contains 4


Example 2.

B=3;
A=B++;
// A contains 3, B contains 4





In Example 1, B is increased before its value is copied to A. While in Example 2, the value of B is copied to A and then B is increased.


Relational and equality operator :


Relational and equality operator ( ==, !=, >, <, >=, <= )


In order to evaluate a comparison between two expressions we can use the relational and equality operators. The result of a relational operation is a Boolean value that can only be true or false, according to its Boolean result.


= =        means   Equal to

!=          means   Not equal to


>           means   Greater than


<          means   Less than


>=          Greater than or equal to


<=          Less than or equal to

Here there are some examples:


 (6 = = 5) // evaluates to false.
 (6 > 4) // evaluates to true.
 (8 != 2) // evaluates to true.
(6 >= 6) // evaluates to true.
 (5 < 5) // evaluates to false.

Of course, instead of using only numeric constants, we can use any valid expression, including variables. Suppose that a=2, b=3 andc=6,

 (a = = 4) // evaluates to false since a is not equal to 4.
 (a*b >= c) // evaluates to true since (2*3 >= 6) is true.
 (b+4 > a*c) // evaluates to false since (3+4 > 2*6) is false.
 ((b=2) == a) // evaluates to true.

Difference between assignment operator(=) and equality operator (= =) :




Assignment operator is different from equality operator and it can be seen clearly.
Be careful! The operator = (one equal sign) is not the same as the operator = = (two equal signs), the first one is an assignment operator (assigns the value at its right to the variable at its left) and the other one (= =) is the equality operator that compares whether both expressions in the two sides of it are equal to each other. Thus, in the last expression ((b=4) = = a), we first assigned the value 4 to b and then we compared it to a, that also stores the value 4, so the result of the operation is true.

Logical operators ( !, &&, || ):OR Boolean operations

The Operator ! is the C++ operator to perform the Boolean operation NOT, it has only one operand, located at its right, and the only thing that it does is to inverse the value of it, producing false if its operand is true and true if its operand is false. Basically, it returns the opposite Boolean value of evaluating its operand. For example:

 !(7 = = 7)              // evaluates to false because the expression at its right (7  = = 7) is true.
 !(8 <= 7)              // evaluates to true because (8 <= 7) would be false. 

  !true // evaluates to false

 !false // evaluates to true.

Logical operator take only logical values  as operands and yield only logical values as results.The operator ! is unary so it has only one operand .so the ! NOT operator is binary and && AND operator and || OR operator are binary operators.
The logical operators && and || are used when evaluating two expressions to obtain a single relational result. The operator &&corresponds with Boolean logical operation AND. This operation results true if both its two operands are true, and false otherwise.When you use the ! operator !true means false and !false means true .
The operator || corresponds with Boolean logical operation OR. This operation results true if either one of its two operands is true, thus being false only when both operands are false themselves. Here are the possible results of a || b:

For example:

 ( (7 = = 7) && (2 > 5) )          // evaluates to false ( true && false ).

 ( (7 = = 7) || (2 > 5) )            // evaluates to true ( true || false ).


When using the logical operators, C++ only evaluates what is necessary from left to right to come up with the combined relational result. Therefore, in this last example (( 7= =7 ) || (2>5)), C++ would evaluate first whether 7= =7 is true, and if so, it would never check whether 2 > 5 is true or not. This is known as short-circuit evaluation, and works like this for these operators:

operator

short-circuit

&&
if the left-hand side expression is false, the combined result is false (right-hand side expression not evaluated).

||
if the left-hand side expression is true, the combined result is true (right-hand side expression not evaluated).

This is mostly important when the right-hand expression has side effects, such as altering values:

if ((i<8)&&(++i<n)) { /*...*/ }

This combined conditional expression increases i by one, but only if the condition on the left of && is true, since otherwise  right-hand expression (++i<n)  never evaluated.

Conditional operator ( ? )

The conditional operator evaluates an expression returning a value if that expression is true and a different one if the expression is evaluated as false. Its format is:

condition ? result1 : result2
If condition is true the expression will return result1, if it is not it will return result2.


 7= =6 ? 4 : 3 // returns 3, since 7 is not equal to 5. 7= =5+2 ? 4 : 3 // returns 4, since 7 is equal to 5+2. 5>3 ? a : b // returns the value of a, since 5 is greater than 3. a>b ? a : b // returns whichever is greater, a or b.


 // conditional operator
 #include <iostream>
 using namespace std; 
int main ()
 { 
int a,b,c;
 a=2;
 b=7; 
c = (a>b) ? a : b; 
cout << c;
 return 0;
 }

sample result is 7

In this example a is 2 and b is 7, so the expression being evaluated (a>b) was not true, thus the first value specified after the question mark is discarded in favor of the second value (the one after the colon) which is b, with a value of 7.




Comma operator ( , )
Comma operator is used to evaluate or separate two or more than two expressions
The comma operator (,) is used to separate two or more expressions that are included where only one expression is expected. When the set of expressions has to be evaluated for a value, only the rightmost expression is considered.


Here is an example:

a = (b=4, b+3);

Would first assign the value 4 to b, and then assign b+3 to variable a. So, at the end, variable a would contain the value 7 while variable b would contain value 4.




Bitwise Operators ( &, |, ^, ~, <<, >> )

Bitwise operators modify variables considering the bit patterns that represent the values they store.

operator

asm equivalent

description


&

AND

Bitwise AND


|

OR

Bitwise Inclusive OR


^

XOR

Bitwise Exclusive OR


~

NOT

Unary complement (bit inversion)


<<

SHL

Shift Left


>>

SHR

Shift Right




Explicit type casting operator

Type casting operators allow you to convert a datum of a given type to another. There are several ways to do this in C++. The simplest one, which has been inherited from the C language, is to precede the expression to be converted by the new type enclosed between parentheses (()):
Here is an example of & operator. BY this way you can find any bitwise operator question answers.


#include <iostream>
using namespace std;
int main ( )
{
 int a , b;
 a = 23;
b = 13;
int ab=a&b;
cout <<" The result of a&b is << ab <<endl;
return 0;
}

 result is   5







Other operators

Later in these tutorials, we will see a few more operators, like the ones referring to pointers or the specifics for object-oriented programming. Each one is treated in its respective section.




Precedence of operators

When writing complex expressions with several operands, we may have some doubts about which operand is evaluated first and which later. For example, in this expression:








a = 5 + 7 % 2









we may doubt if it really means:








1
2 a = 5 + (7 % 2) // with a result of 6, or a = (5 + 7) % 2 // with a result of 0









The correct answer is the first of the two expressions, with a result of 6. There is an established order with the priority of each operator, and not only the arithmetic ones (those whose preference come from mathematics) but for all the operators which can appear in C++. From greatest to lowest priority, the priority order is as follows:







Level

Operator

Description

Grouping


1

::

scope

Left-to-right


2

() [] . -> ++ -- dynamic_cast static_cast reinterpret_cast const_cast typeid

postfix

Left-to-right


3

++ -- ~ ! sizeof new delete

unary (prefix)

Right-to-left


* &

indirection and reference (pointers)


+ -

unary sign operator


4

(type)

type casting

Right-to-left


5

.* ->*

pointer-to-member

Left-to-right


6

* / %

multiplicative

Left-to-right


7

+ -

additive

Left-to-right


8

<< >>

shift

Left-to-right


9

< > <= >=

relational

Left-to-right


10

== !=

equality

Left-to-right


11

&

bitwise AND

Left-to-right


12

^

bitwise XOR

Left-to-right


13

|

bitwise OR

Left-to-right


14

&&

logical AND

Left-to-right


15

||

logical OR

Left-to-right


16

?:

conditional

Right-to-left


17

= *= /= %= += -= >>= <<= &= ^= |=

assignment

Right-to-left


18

,

comma

Left-to-right





Grouping defines the precedence order in which operators are evaluated in the case that there are several operators of the same level in an expression.





All these precedence levels for operators can be manipulated or become more legible by removing possible ambiguities using parentheses signs ( and ), as in this example:








a = 5 + 7 % 2;









might be written either as:








a = 5 + (7 % 2);






or





a = (5 + 7) % 2;









depending on the operation that we want to perform.





So if you want to write complicated expressions and you are not completely sure of the precedence levels, always include parentheses. It will also make your code easier to read.

CONSTANTS


Constants

Constants are expressions with a fixed value.

Literals

Literals are the most obvious kind of constants. They are used to express particular values within the source code of a program. We have already used these previously to give concrete values to variables or to express messages we wanted our programs to print out, for example, when we wrote:

 
a = 5;


the 5 in this piece of code was a literal constant.

Literal constants can be divided in Integer Numerals, Floating-Point Numerals, Characters, Strings and Boolean Values.

Integer Numerals


1
2
3
1776
707
-273


They are numerical constants that identify integer decimal values. Notice that to express a numerical constant we do not have to write quotes (") nor any special character. There is no doubt that it is a constant: whenever we write 1776 in a program, we will be referring to the value 1776.

In addition to decimal numbers (those that all of us are used to using every day), C++ allows the use of octal numbers (base 8) and hexadecimal numbers (base 16) as literal constants. If we want to express an octal number we have to precede it with a 0 (a zero character). And in order to express a hexadecimal number we have to precede it with the characters 0x (zero, x). For example, the following literal constants are all equivalent to each other:

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2
3
75         // decimal
0113       // octal
0x4b       // hexadecimal 


All of these represent the same number: 75 (seventy-five) expressed as a base-10 numeral, octal numeral and hexadecimal numeral, respectively.

Literal constants, like variables, are considered to have a specific data type. By default, integer literals are of type int. However, we can force them to either be unsigned by appending the u character to it, or long by appending l:

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3
4
75         // int
75u        // unsigned int
75l        // long
75ul       // unsigned long 


In both cases, the suffix can be specified using either upper or lowercase letters.

Floating Point Numbers

They express numbers with decimals and/or exponents. They can include either a decimal point, an e character (that expresses "by ten at the Xth height", where X is an integer value that follows the e character), or both a decimal point and an e character:

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3
4
3.14159    // 3.14159
6.02e23    // 6.02 x 10^23
1.6e-19    // 1.6 x 10^-19
3.0        // 3.0 


These are four valid numbers with decimals expressed in C++. The first number is PI, the second one is the number of Avogadro, the third is the electric charge of an electron (an extremely small number) -all of them approximated- and the last one is the number three expressed as a floating-point numeric literal.

The default type for floating point literals is double. If you explicitly want to express a float or a long double numerical literal, you can use the f or l suffixes respectively:

1
2
3.14159L   // long double
6.02e23f   // float 


Any of the letters that can be part of a floating-point numerical constant (e, f, l) can be written using either lower or uppercase letters without any difference in their meanings.

Character and string literals

There also exist non-numerical constants, like:

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2
3
4
'z'
'p'
"Hello world"
"How do you do?" 


The first two expressions represent single character constants, and the following two represent string literals composed of several characters. Notice that to represent a single character we enclose it between single quotes (') and to express a string (which generally consists of more than one character) we enclose it between double quotes (").

When writing both single character and string literals, it is necessary to put the quotation marks surrounding them to distinguish them from possible variable identifiers or reserved keywords. Notice the difference between these two expressions:

1
2
x
'x'


x alone would refer to a variable whose identifier is x, whereas 'x' (enclosed within single quotation marks) would refer to the character constant 'x'.

Character and string literals have certain peculiarities, like the escape codes. These are special characters that are difficult or impossible to express otherwise in the source code of a program, like newline (\n) or tab (\t). All of them are preceded by a backslash (\). Here you have a list of some of such escape codes:

\nnewline
\rcarriage return
\ttab
\vvertical tab
\bbackspace
\fform feed (page feed)
\aalert (beep)
\'single quote (')
\"double quote (")
\?question mark (?)
\\backslash (\)

For example:

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2
3
4
'\n'
'\t'
"Left \t Right"
"one\ntwo\nthree" 


Additionally, you can express any character by its numerical ASCII code by writing a backslash character (\) followed by the ASCII code expressed as an octal (base-8) or hexadecimal (base-16) number. In the first case (octal) the digits must immediately follow the backslash (for example \23 or \40), in the second case (hexadecimal), an x character must be written before the digits themselves (for example \x20 or \x4A).

String literals can extend to more than a single line of code by putting a backslash sign (\) at the end of each unfinished line.

1
2
"string expressed in \
two lines" 


You can also concatenate several string constants separating them by one or several blank spaces, tabulators, newline or any other valid blank character:

 
"this forms" "a single" "string" "of characters"


Finally, if we want the string literal to be explicitly made of wide characters (wchar_t type), instead of narrow characters (char type), we can precede the constant with the L prefix:

 
L"This is a wide character string"


Wide characters are used mainly to represent non-English or exotic character sets.

Boolean literals

There are only two valid Boolean values: true and false. These can be expressed in C++ as values of type bool by using the Boolean literals true and false.

Defined constants (#define)

You can define your own names for constants that you use very often without having to resort to memory-consuming variables, simply by using the #define preprocessor directive. Its format is:

#define identifier value
For example:

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2
#define PI 3.14159
#define NEWLINE '\n' 


This defines two new constants: PI and NEWLINE. Once they are defined, you can use them in the rest of the code as if they were any other regular constant, for example:

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16
17
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19
// defined constants: calculate circumference

#include <iostream>
using namespace std;

#define PI 3.14159
#define NEWLINE '\n'

int main ()
{
  double r=5.0;               // radius
  double circle;

  circle = 2 * PI * r;
  cout << circle;
  cout << NEWLINE;

  return 0;
}
31.4159


In fact the only thing that the compiler preprocessor does when it encounters #define directives is to literally replace any occurrence of their identifier (in the previous example, these were PI and NEWLINE) by the code to which they have been defined (3.14159 and '\n' respectively).

The #define directive is not a C++ statement but a directive for the preprocessor; therefore it assumes the entire line as the directive and does not require a semicolon (;) at its end. If you append a semicolon character (;) at the end, it will also be appended in all occurrences of the identifier within the body of the program that the preprocessor replaces.

Declared constants (const)

With the const prefix you can declare constants with a specific type in the same way as you would do with a variable:

1
2
const int pathwidth = 100;
const char tabulator = '\t';


Here, pathwidth and tabulator are two typed constants. They are treated just like regular variables except that their values cannot be modified after their definition.

BASICS OF C++ PROGRAM



Probably the best way to start learning a programming language is by writing a program. Therefore, here is our first program:

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// my first program in C++

#include <iostream>
using namespace std;

int main ()
{
  cout << "Hello World!";
  return 0;
}
Sample run of this program is :
Hello World! 


The first panel (in light blue) shows the source code for our first program. The second one (in light gray) shows the result of the program once compiled and executed. To the left, the grey numbers represent the line numbers - these are not part of the program, and are shown here merely for informational purposes.

The way to edit and compile a program depends on the compiler you are using. Depending on whether it has a Development Interface or not and on its version. Consult the compilers section and the manual or help included with your compiler if you have doubts on how to compile a C++ console program.

The previous program is the typical program that programmer apprentices write for the first time, and its result is the printing on screen of the "Hello World!" sentence. It is one of the simplest programs that can be written in C++, but it already contains the fundamental components that every C++ program has. We are going to look line by line at the code we have just written:

// my first program in C++
This is a comment line. All lines beginning with two slash signs (//) are considered comments and do not have any effect on the behavior of the program. The programmer can use them to include short explanations or observations within the source code itself. In this case, the line is a brief description of what our program is.
#include <iostream>
Lines beginning with a hash sign (#) are directives for the preprocessor. They are not regular code lines with expressions but indications for the compiler's preprocessor. In this case the directive #include <iostream> tells the preprocessor to include the iostream standard file. This specific file (iostream) includes the declarations of the basic standard input-output library in C++, and it is included because its functionality is going to be used later in the program.
using namespace std;
All the elements of the standard C++ library are declared within what is called a namespace, the namespace with the name std. So in order to access its functionality we declare with this expression that we will be using these entities. This line is very frequent in C++ programs that use the standard library, and in fact it will be included in most of the source codes included in these tutorials.
int main ()
This line corresponds to the beginning of the definition of the main function. The main function is the point by where all C++ programs start their execution, independently of its location within the source code. It does not matter whether there are other functions with other names defined before or after it - the instructions contained within this function's definition will always be the first ones to be executed in any C++ program. For that same reason, it is essential that all C++ programs have a main function. The word main is followed in the code by a pair of parentheses (()). That is because it is a function declaration: In C++, what differentiates a function declaration from other types of expressions are these parentheses that follow its name. Optionally, these parentheses may enclose a list of parameters within them. Right after these parentheses we can find the body of the main function enclosed in braces ({}). What is contained within these braces is what the function does when it is executed.
cout << "Hello World!";
This line is a C++ statement. A statement is a simple or compound expression that can actually produce some effect. In fact, this statement performs the only action that generates a visible effect in our first program. cout is the name of the standard output stream in C++, and the meaning of the entire statement is to insert a sequence of characters (in this case the Hello World sequence of characters) into the standard output stream (cout, which usually corresponds to the screen). cout is declared in the iostream standard file within the std namespace, so that's why we needed to include that specific file and to declare that we were going to use this specific namespace earlier in our code. Notice that the statement ends with a semicolon character (;). This character is used to mark the end of the statement and in fact it must be included at the end of all expression statements in all C++ programs (one of the most common syntax errors is indeed to forget to include some semicolon after a statement).
return 0;
The return statement causes the main function to finish. return may be followed by a return code (in our example is followed by the return code with a value of zero). A return code of 0 for the main function is generally interpreted as the program worked as expected without any errors during its execution. This is the most usual way to end a C++ console program.

You may have noticed that not all the lines of this program perform actions when the code is executed. There were lines containing only comments (those beginning by //). There were lines with directives for the compiler's preprocessor (those beginning by #). Then there were lines that began the declaration of a function (in this case, the main function) and, finally lines with statements (like the insertion into cout), which were all included within the block delimited by the braces ({}) of the main function.

The program has been structured in different lines in order to be more readable, but in C++, we do not have strict rules on how to separate instructions in different lines. For example, instead of

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2
3
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5
int main ()
{
  cout << " Hello World!";
  return 0;
}


We could have written:

int main () { cout << "Hello World!"; return 0; }


All in just one line and this would have had exactly the same meaning as the previous code.

In C++, the separation between statements is specified with an ending semicolon (;) at the end of each one, so the separation in different code lines does not matter at all for this purpose. We can write many statements per line or write a single statement that takes many code lines. The division of code in different lines serves only to make it more legible and schematic for the humans that may read it.

You can learn the basic elements and concepts of c++ programming language to creat c++ programs.Here is an example which is an introduction of program.
consider the following c++ program:
whithout comments:


#include <iostream>
using namespace std;
int main ( )
{
  cout<<"My first c++ program."<<endl;
return 0;
}


sample run:
My first c++ program.


Let us add an additional instruction to our first program:

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12
// my second program in C++

#include <iostream>

using namespace std;

int main ()
{
  cout << "Hello World! ";
  cout << "I'm a C++ program";
  return 0;
}
Hello World! I'm a C++ program


In this case, we performed two insertions into cout in two different statements. Once again, the separation in different lines of code has been done just to give greater readability to the program, since main could have been perfectly valid defined this way:

int main () { cout << " Hello World! "; cout << " I'm a C++ program "; return 0; } 


We were also free to divide the code into more lines if we considered it more convenient:

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3
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6
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8
int main ()
{
  cout <<
    "Hello World!";
  cout
    << "I'm a C++ program";
  return 0;
}


And the result would again have been exactly the same as in the previous examples.

Preprocessor directives (those that begin by #) are out of this general rule since they are not statements. They are lines read and processed by the preprocessor and do not produce any code by themselves. Preprocessor directives must be specified in their own line and do not have to end with a semicolon (;).

Comments


Comments are parts of the source code disregarded by the compiler. They simply do nothing. Their purpose is only to allow the programmer to insert notes or descriptions embedded within the source code.

C++ supports two ways to insert comments:

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2
// line comment
/* block comment */ 


The first of them, known as line comment, discards everything from where the pair of slash signs (//) is found up to the end of that same line. The second one, known as block comment, discards everything between the /* characters and the first appearance of the */ characters, with the possibility of including more than one line.
We are going to add comments to our second program:

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12
/* my second program in C++
   with more comments */

#include <iostream>
using namespace std;

int main ()
{
  cout << "Hello World! ";     // prints Hello World!
  cout << "I'm a C++ program"; // prints I'm a C++ program
  return 0;
}
Hello World! I'm a C++ program


If you include comments within the source code of your programs without using the comment characters combinations //, /* or */, the compiler will take them as if they were C++ expressions, most likely causing one or several error messages when you compile it.

variable data type
The usefulness of the "Hello World" programs shown in the previous section is quite questionable. We had to write several lines of code, compile them, and then execute the resulting program just to obtain a simple sentence written on the screen as result. It certainly would have been much faster to type the output sentence by ourselves. However, programming is not limited only to printing simple texts on the screen. In order to go a little further on and to become able to write programs that perform useful tasks that really save us work we need to introduce the concept of variable.

Let us think that I ask you to retain the number 5 in your mental memory, and then I ask you to memorize also the number 2 at the same time. You have just stored two different values in your memory. Now, if I ask you to add 1 to the first number I said, you should be retaining the numbers 6 (that is 5+1) and 2 in your memory. Values that we could now for example subtract and obtain 4 as result.

The whole process that you have just done with your mental memory is a simile of what a computer can do with two variables. The same process can be expressed in C++ with the following instruction set:

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4
a = 5;
b = 2;
a = a + 1;
result = a - b;


Obviously, this is a very simple example since we have only used two small integer values, but consider that your computer can store millions of numbers like these at the same time and conduct sophisticated mathematical operations with them.

Therefore, we can define a variable as a portion of memory to store a determined value.

Each variable needs an identifier that distinguishes it from the others. For example, in the previous code the variable identifiers were a, b and result, but we could have called the variables any names we wanted to invent, as long as they were valid identifiers.

Identifiers

A valid identifier is a sequence of one or more letters, digits or underscore characters (_). Neither spaces nor punctuation marks or symbols can be part of an identifier. Only letters, digits and single underscore characters are valid. In addition, variable identifiers always have to begin with a letter. They can also begin with an underline character (_ ), but in some cases these may be reserved for compiler specific keywords or external identifiers, as well as identifiers containing two successive underscore characters anywhere. In no case can they begin with a digit.

Another rule that you have to consider when inventing your own identifiers is that they cannot match any keyword of the C++ language nor your compiler's specific ones, which are reserved keywords. The standard reserved keywords are:

asm, auto, bool, break, case, catch, char, class, const, const_cast, continue, default, delete, do, double, dynamic_cast, else, enum, explicit, export, extern, false, float, for, friend, goto, if, inline, int, long, mutable, namespace, new, operator, private, protected, public, register, reinterpret_cast, return, short, signed, sizeof, static, static_cast, struct, switch, template, this, throw, true, try, typedef, typeid, typename, union, unsigned, using, virtual, void, volatile, wchar_t, while

Additionally, alternative representations for some operators cannot be used as identifiers since they are reserved words under some circumstances:

and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq

Your compiler may also include some additional specific reserved keywords.

Very important: The C++ language is a "case sensitive" language. That means that an identifier written in capital letters is not equivalent to another one with the same name but written in small letters. Thus, for example, the RESULT variable is not the same as the result variable or the Result variable. These are three different variable identifiers.

Fundamental data types

When programming, we store the variables in our computer's memory, but the computer has to know what kind of data we want to store in them, since it is not going to occupy the same amount of memory to store a simple number than to store a single letter or a large number, and they are not going to be interpreted the same way.

The memory in our computers is organized in bytes. A byte is the minimum amount of memory that we can manage in C++. A byte can store a relatively small amount of data: one single character or a small integer (generally an integer between 0 and 255). In addition, the computer can manipulate more complex data types that come from grouping several bytes, such as long numbers or non-integer numbers.

Next you have a summary of the basic fundamental data types in C++, as well as the range of values that can be represented with each one:

NameDescriptionSize*Range*
char Character or small integer. 1byte signed: -128 to 127
unsigned: 0 to 255
short int (short) Short Integer. 2bytes signed: -32768 to 32767
unsigned: 0 to 65535
int Integer. 4bytes signed: -2147483648 to 2147483647
unsigned: 0 to 4294967295
long int (long) Long integer. 4bytes signed: -2147483648 to 2147483647
unsigned: 0 to 4294967295
bool Boolean value. It can take one of two values: true or false. 1byte true or false
float Floating point number. 4bytes +/- 3.4e +/- 38 (~7 digits)
double Double precision floating point number. 8bytes +/- 1.7e +/- 308 (~15 digits)
long double Long double precision floating point number. 8bytes +/- 1.7e +/- 308 (~15 digits)
wchar_t Wide character. 2 or 4 bytes 1 wide character

* The values of the columns Size and Range depend on the system the program is compiled for. The values shown above are those found on most 32-bit systems. But for other systems, the general specification is that int has the natural size suggested by the system architecture (one "word") and the four integer types char, short, int and long must each one be at least as large as the one preceding it, with char being always one byte in size. The same applies to the floating point types float, double and long double, where each one must provide at least as much precision as the preceding one.

Declaration of variables

In order to use a variable in C++, we must first declare it specifying which data type we want it to be. The syntax to declare a new variable is to write the specifier of the desired data type (like int, bool, float...) followed by a valid variable identifier. For example:

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int a;
float mynumber;


These are two valid declarations of variables. The first one declares a variable of type int with the identifier a. The second one declares a variable of type float with the identifier mynumber. Once declared, the variables a and mynumber can be used within the rest of their scope in the program.

If you are going to declare more than one variable of the same type, you can declare all of them in a single statement by separating their identifiers with commas. For example:

int a, b, c;


This declares three variables (a, b and c), all of them of type int, and has exactly the same meaning as:

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int a;
int b;
int c;


The integer data types char, short, long and int can be either signed or unsigned depending on the range of numbers needed to be represented. Signed types can represent both positive and negative values, whereas unsigned types can only represent positive values (and zero). This can be specified by using either the specifier signed or the specifier unsigned before the type name. For example:

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unsigned short int NumberOfSisters;
signed int MyAccountBalance;


By default, if we do not specify either signed or unsigned most compiler settings will assume the type to be signed, therefore instead of the second declaration above we could have written:

int MyAccountBalance;


with exactly the same meaning (with or without the keyword signed)

An exception to this general rule is the char type, which exists by itself and is considered a different fundamental data type from signed char and unsigned char, thought to store characters. You should use either signed or unsigned if you intend to store numerical values in a char-sized variable.

short and long can be used alone as type specifiers. In this case, they refer to their respective integer fundamental types: short is equivalent to short int and long is equivalent to long int. The following two variable declarations are equivalent:

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short Year;
short int Year;


Finally, signed and unsigned may also be used as standalone type specifiers, meaning the same as signed int and unsigned int respectively. The following two declarations are equivalent:

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unsigned NextYear;
unsigned int NextYear;


To see what variable declarations look like in action within a program, we are going to see the C++ code of the example about your mental memory proposed at the beginning of this section:

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// operating with variables

#include <iostream>
using namespace std;

int main ()
{
  // declaring variables:
  int a, b;
  int result;

  // process:
  a = 5;
  b = 2;
  a = a + 1;
  result = a - b;

  // print out the result:
  cout << result;

  // terminate the program:
  return 0;
}
4


Do not worry if something else than the variable declarations themselves looks a bit strange to you. You will see the rest in detail in coming sections.

Scope of variables

All the variables that we intend to use in a program must have been declared with its type specifier in an earlier point in the code, like we did in the previous code at the beginning of the body of the function main when we declared that a, b, and result were of type int.

A variable can be either of global or local scope. A global variable is a variable declared in the main body of the source code, outside all functions, while a local variable is one declared within the body of a function or a block.


Global variables can be referred from anywhere in the code, even inside functions, whenever it is after its declaration.

The scope of local variables is limited to the block enclosed in braces ({}) where they are declared. For example, if they are declared at the beginning of the body of a function (like in function main) their scope is between its declaration point and the end of that function. In the example above, this means that if another function existed in addition to main, the local variables declared in main could not be accessed from the other function and vice versa.

Initialization of variables

When declaring a regular local variable, its value is by default undetermined. But you may want a variable to store a concrete value at the same moment that it is declared. In order to do that, you can initialize the variable. There are two ways to do this in C++:

The first one, known as c-like initialization, is done by appending an equal sign followed by the value to which the variable will be initialized:

type identifier = initial_value ;
For example, if we want to declare an int variable called a initialized with a value of 0 at the moment in which it is declared, we could write:

int a = 0;


The other way to initialize variables, known as constructor initialization, is done by enclosing the initial value between parentheses (()):

type identifier (initial_value) ;
For example:

int a (0); 


Both ways of initializing variables are valid and equivalent in C++.

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// initialization of variables

#include <iostream>
using namespace std;

int main ()
{
  int a=5;               // initial value = 5
  int b(2);              // initial value = 2
  int result;            // initial value undetermined

  a = a + 3;
  result = a - b;
  cout << result;

  return 0;
}
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Introduction to strings

Variables that can store non-numerical values that are longer than one single character are known as strings.

The C++ language library provides support for strings through the standard string class. This is not a fundamental type, but it behaves in a similar way as fundamental types do in its most basic usage.

A first difference with fundamental data types is that in order to declare and use objects (variables) of this type we need to include an additional header file in our source code: <string> and have access to the std namespace (which we already had in all our previous programs thanks to the using namespace statement).

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// my first string
#include <iostream>
#include <string>
using namespace std;

int main ()
{
  string mystring = "This is a string";
  cout << mystring;
  return 0;
}
This is a string


As you may see in the previous example, strings can be initialized with any valid string literal just like numerical type variables can be initialized to any valid numerical literal. Both initialization formats are valid with strings:

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string mystring = "This is a string";
string mystring ("This is a string");


Strings can also perform all the other basic operations that fundamental data types can, like being declared without an initial value and being assigned values during execution:

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// my first string
#include <iostream>
#include <string>
using namespace std;

int main ()
{
  string mystring;
  mystring = "This is the initial string content";
  cout << mystring << endl;
  mystring = "This is a different string content";
  cout << mystring << endl;
  return 0;
}
This is the initial string content
This is a different string content


For more details on C++ strings, you can have a look at the string class reference.