C++ write first program

Table of Contents

Earlier you learned about C++ program structure. In this article, you will learn to write about your first C++ program and program elements in detail.

The meat of the C++ is in main function, and before main we need to declare the header files.

Simple C++ Program

Let us consider an example program and then we discuss the program elements in detail. The program declare a header file required for standard input and output operations called $iostream$ .

#include <iostream.h>
void main(void)
{
         //Variable declaration
        int number;
        // Reading input values
        cout << "Enter the a number:" ;
        cin >> number;
       // if block
       if (number > 100)
       {
           cout << "Number is greater than 100;
       }
}

The above program prints only when the number is greater than $100$ . That is, when the user input a number that is more than $100$ .

Variable Declaration

The main program begins with variable declaration. In C++ , you must declare your variable first and the syntax to declare the variable is:

<date_type> <indentifier 1>, <identifier 2>, ... <identifier N>;

For example,

int A;                         // variable declared as integer type
char ID1, ID2,ID3;             // variable declared as character type
float Number;                  // variable declared as floating type

Statements

A statement in C++ program perform some action. Every statement ends with a semi colon ( $;$ ). The C++ statements can be divided into following category.

Simple statements
Expressions
Statement blocks

Simple Statements – This type of statement are instruction to get input, print output or any one line statement that ends with a semi-colon.

For example,

cout<<  " Hello World ! ";
int number;

Sometimes a simple expression can also be a simple statement.

Expressions – An expression is a mathematical expression or an assignment that evaluates to a single value and ends with semi-colon. This type of statements use C++ operators.

For example,

result = number1 + number2;
area_of_triangle = ( 1/2 * base) * height;

We can write any type of complex expressions which depends on the problem we are solving.

Statement blocks – In the above program, if block is an example of statement block which encloses many simple statements.

{
     if ( x == 0)
     {
          area = 0;
     }
     else
     {
          area = x * x;
     }
     cout << "Area = " <<  " " << area ;
}

For example,

The example above has two blocks – if block and else block. The if block contains only one statement – an expession and the else block contains two statements.

This the advantage of grouping the statements into blocks, so that you can execute then as part of a task. The else block above computes the area and prints the result. Two actions were performed within a block.

Comments

C++ comments are lines ignored by the compiler. The compiler understands that the comments are not to be executed. The comments are of two types

Single line comment ( $\slash \slash$ )
Multiline comment ( $\slash\ast \cdots \ast\slash$ )

Single Line Comment – You can write a single line of text as comment after ( $\slash \slash$ ) symbol. See the program above.

For example,

// this is a comment ignored by the compiler
// this is another line

Multiline Comment – Inherited from C language, C++ allows to add multiple lines as comments. Anything in between $\slash \ast$ and $\ast \slash$ is ignored. The multiple line comment is used for describing the program details. There is not restriction on white space or new line for multiline comments.

For example,

/*
Program Name: Tax Calculator
Program Author: Notesformsc.org
Program Description: The program receives inputs such as yearly income, tax rates and computes total tax payable by the employees.
Version: 1.0
*/

Common Mistakes While Writing C++ Programs

These are some common mistakes by beginners of C++ programming language.

Forget to add necessary header files.
Spelling mistakes
Not initializing the variables
Not following a naming convention for variables.
Forget to add semi-colon at the end of the statement.
Block is not closed.

If you avoid the above mistakes, then you will have a clean and error free programs.

C++ Write First Program

Simple C++ Program

Variable Declaration

Statements

Comments

Common Mistakes While Writing C++ Programs