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Loops in C

Loops allow us to execute a statement multiple times. Suppose that you want to print Hello World 5 times on the screen. Using loops, you don't have to write printf("Hello World") 5 times. Let's take a look at the below code. 

    #include <stdio.h>
    void main()
    {
        for(int i = 0 ; i < 5 ; i++)
        {
        printf("Hello World\n");
        }
    }

Output

    Hello World
    Hello World
    Hello World
    Hello World
    Hello World
Did you see the magic? The statement on line number 4 is called a for loop. Using it, we were able to print Hello World 5 times without writing it 5 times. Today, we will learn how to use loops.

What is a loop ?

Syntax of for loop

The syntax of a for loop is as follows:

    for(Initialization ; Termination Condition ; Updation)
    {
        // Statement(s)
    }

Working of for loop

The body of a for loop is divided into three parts :
      Initialization - This statement is executed only once when the loop starts. It involves the declaration and initialization of a variable.
      Termination Condition - The loop must not run infinite number of times. Hence, we specify a condition to terminate the loop. This condition is tested at every iteration of the loop. An iteration means a cycle of the loop. If the condition is true, the loop executes the statements inside it otherwise, the control comes out of the loop.
      Updation - It determines how to update the value of the variable declared in the first part of the loop. We can choose any method like increment by one, decrement by one, or increment by 2, etc. Updation is required at every iteration in order to reach the termination condition.

Note that if you forgot to write the updation statement or if the termination condition can never be reached(become true), then the loop keeps on running forever. Such a loop is called an infinite loop and should be avoided by taking special care while writing all the conditions.
We can skip the curly brackets around the for loop if there is only one statement inside the for loop.

Example 1 - Print the table of 10


    #include <stdio.h>
    void main()
    {   
     for(int i = 1 ; i <= 10 ; i++)
     {
    printf("10 X %d = %d \n",i,10*i);
     }
    }

Output

    10 X 1 = 10 
    10 X 2 = 20 
    10 X 3 = 30 
    10 X 4 = 40 
    10 X 5 = 50 
    10 X 6 = 60 
    10 X 7 = 70 
    10 X 8 = 80 
    10 X 9 = 90 
    10 X 10 = 100

Example 2 - Print even numbers in a given range


    #include <stdio.h>
    void main()
    {   
        int a,b;
        printf("Enter range : ");
        scanf("%d %d",&a,&b);
        for(int i = a ; i <= b ; i++)
        {
        if(i%2 == 0)      // if i is divisible by 2, it is even.
        printf("%d ",i);
        }
    }

Output

    Enter range : 1 10
    Even numbers from 1 to 10 are -
    2 4 6 8 10