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Difference between while and for loop in C

While Loop

The syntax that can be used for the ‘while’ loop in the C programming language is mentioned below:

while (test expression or termination condition) 
{
  // the main body of the ‘while’ loop is written here}

Here, we will let you know about the working of the ‘while’ loop in C:

Initially, we use the parentheses for storing the termination condition of the While loop. Then, there can be two outcomes True or False.

Firstly, if the result is true, then the control will automatically execute the body of the loop. It will continue to evaluate again and again until the termination condition is false. Secondly, if the result is false, then the control will definitely come out of the While loop and will dismiss the While Loop.

Let’s understand the entire process of while loop with the help of an example:

Example 01: This program will print numbers from 10 to 20 with the help of Less Than comparison operation in the ‘while’ loop:

/* Printing integers from 10 to 20 */
#include <stdio.h>
int main () 
{
  int a = 10; // declaration and initialization of the variables
  while (a < 20) // while (test expression)
  {
    printf ("the value of ‘a’ is %d\n", a); // main body of loop having print command 
    ++a; // modifying statement for the further evaluation
  }
  return 0;
}

Output:

the value of ‘a’ is 10
the value of ‘a’ is 11
the value of ‘a’ is 12
the value of ‘a’ is 13
the value of ‘a’ is 14
the value of ‘a’ is 15
the value of ‘a’ is 16
the value of ‘a’ is 17
the value of ‘a’ is 18
the value of ‘a’ is 19
the value of ‘a’ is 20


Explanation:

Firstly, we initialized the value of the integer variable, which we took as a. Then, we applied a termination condition (while (a < 20) to avoid the output running to infinity.

After that, we implemented the main body of the while loop just like the following statement:

[Printf ("the value of ‘a’ is %d\n", a);].

This statement prints the desired output that we want (see output). In the main body, we placed an increment statement (++a;). Through this statement, we can get positive integers starting with 10 and ending with 20. It performs the increment command and will add 1 to the previous value of a.

This evaluation will perform several times until the value of a becomes 20.

When the value of a becomes 20, then it will terminate the entire process because the termination condition gets false as the value of ‘a’ is greater than 19. So, the loop only executes to the values less than 20.

Example 02: This program will print numbers from 25 to 15 with the help of greater than or equal to comparison operation in the ‘while’ loop:

/* Printing integers from 25 to 15 */
#include <stdio.h>
int main() 
{
  int a = 25; // declaration and initialization of the variables  
while (a >= 15) // while (test expression)
  {
    printf ("the value of ‘a’ is %d\n", a); // main body of loop having print command 
   --a; // modifying statement for the further evaluation
  }
  return 0;
}

Output:

the value of ‘a’ is 25
the value of ‘a’ is 24
the value of ‘a’ is 23
the value of ‘a’ is 22
the value of ‘a’ is 21
the value of ‘a’ is 20
the value of ‘a’ is 19
the value of ‘a’ is 18
the value of ‘a’ is 17
the value of ‘a’ is 16
the value of ‘a’ is 15

Explanation:

Firstly, we initialized the value of the integer variable, which we took as a. Then, we applied a termination condition (while (a >= 15) to avoid the output running to negative infinity.

After that, we implemented the main body of the while loop just like the following statement:

[Printf ("the value of ‘a’ is %d\n", a);].

This statement prints the desired output that we want (see output). In the main body, we placed a decrement statement (--a;). Through this statement, we can get positive integers starting with 25 and ending with 15. It performs the decrement command and will subtract 1 from the previous value of a.

This evaluation will perform several times until the value of a becomes 14.

When the value of a becomes 14, then it will terminate the entire process because the termination condition gets false as the value of ‘a’ is less than 15. So, the loop only executes to the values greater than 15 or equals to 15.

For Loop

The syntax of ‘for’ loop in C programming language is given below:

for (initialization statement; test expression; update statement)
{
    /* main body of the ‘for’ loop */
}

How does the ‘for’ loop work?

In for loop, the initialization command is implemented only once.

After that, it checks the test expression of the loop and finds whether the test expression is false or true. And, for loop is dismissed if the test expression is false.   
If the test expression of the 'F' loop becomes true, then the program line provided in the F loop’s body is executed, and the update expression is changed accordingly.
Again, the entire process of evaluation is to be done. The loop executes its body until and unless the value of the test expression in for loop becomes false. The loop automatically terminates as soon as the test expression result becomes false.

Example 01: This program will print counting from 33 to 42 with the help of less than comparison operation:

// Code to print numbers from 33 to 42
#include <stdio.h>
int main()
{
    int a; // declaring variables
    for (a = 33; a < 42; ++a) /* (initialization statement, termination condition, increment or decrement statement) */
    {
        printf(" Current value of ‘a’ is %d \n", a); // printing the output
    }
    return 0;
}

Output:

Current value of ‘a’ is 34
Current value of ‘a’ is 35
Current value of ‘a’ is 36
Current value of ‘a’ is 37
Current value of ‘a’ is 38
Current value of ‘a’ is 39
Current value of ‘a’ is 40
Current value of ‘a’ is 41
Current value of ‘a’ is 42

Explanation:
Here, we initialized the value of a to 33, and the loop starts its evaluation from 33. In the for statement, the termination condition is implemented as a<42.
The value of 'a' is smaller than 42, so the condition is true, and the statement inside the 'FOR' loop is implemented. As a result, 33 will be printed as the final value on the output screen. After that, the altering statement (++a) is implemented for increment. Due to increment, the value of variable ‘a’ will become 34.
Again, the termination condition is checked for true or false, and if it is found to be true, then the body of FOR-LOOP is implemented once again. This time the value is displayed as 34 on the output.
This process will continue to check and evaluate until the value of ‘a’ becomes 42. When the value of 'a' becomes 42, then the a<42 condition automatically becomes false, and the for loop terminates.

Example 02: This program will print the values from 10 to 2 with the help of greater than comparison operation:

// to print numbers from 10 to 2
#include <stdio.h>
int main ()
{
    int a; // declaring variables
    for (a = 10; a > 1; --a) /* (initialization statement, termination condition, increment or decrement statement) */
    {
        printf(" Current value of ‘a’ is %d \n", a); // printing the output
    }
    return 0;
}

Output:

Current value of ‘a’ is 10
Current value of ‘a’ is 9
Current value of ‘a’ is 8
Current value of ‘a’ is 7
Current value of ‘a’ is 6
Current value of ‘a’ is 5
Current value of ‘a’ is 4
Current value of ‘a’ is 3
Current value of ‘a’ is 2

Explanation:

Here, we initialized the value of a to 10, and the loop starts its evaluation from 10. In the for statement, the termination condition is implemented as a>1.
The value of 'a' is greater than 1, so the condition is true, and the statement inside the 'FOR' loop is implemented. As a result, 10 will be printed as the final value on the output screen. After that, the altering statement (--a) is implemented for decrease the value of a. Due to decrement, the value of variable ‘a’ will become 9.
Again, the termination condition is checked for true or false, and if it is found to be true, then the body of FOR-LOOP is implemented once again. This time the value is displayed as 9 on the output.
This process will continue to check and evaluate until the value of ‘a’ becomes 1. When the value of 'a' becomes 1, then the a>1 condition automatically becomes false, and the for loop terminates.