Loop statements in C language

During writing programs, sometimes it is necessary to execute the statement(s) several times. And, execute those statement a specified number of times until a condition is met.

Behavior of loop

C language has many different variants of loop statements.

  • while
  • do...while
  • for

while loop

The while loop executed the block of statement(s) till the condition is true.

Syntax:

while (condition)
{
    // statements
}

Example:

#include <stdio.h>
int main()
{
  int age = 15;

  while (age < 18)
  {
      printf("You still have %d years to reach voting age.\n", (18 - age));
      ++age;
  }

  printf("Congratulations! You are able to vote now.\n");
  return 0;
}

// output
You still have 3 years to reach voting age.
You still have 2 years to reach voting age.
You still have 1 years to reach voting age.
Congratulations! You are able to vote now.
In the following examples, just replace the content of 'main' body with the given one.

do…while loop

It is similar to while loop with one important difference. The body of do...while loop is executed first and condition is checked later.

Body of 'do...while' loop get executed at least once even though the condition is false.

Syntax:

do
{
   // statements
}
while (condition);

Example:

int age = 18;

do
{
  printf("You still have %d years to reach voting age.\n", (18 - age));
  ++age;
} while (age < 18);

printf("Congratulations! You are able to vote now.\n");
return 0;

// output
You still have 0 years to reach voting age.
Congratulations! You are able to vote now.

NOTE: Check the difference by updating the value of age to 18 in while loop. So, in above case the output is not expected and hence, we need to use while loop to implement that logic.

for loop

A for loop helps to efficiently write a loop that will execute block of statement(s) a specific number of times.

Syntax:

for (initialization; conditional; increment)
{
    // statements inside the body of loop
}

Where

  • The initialization statement is executed only once which is generally used to initialize the loop counter.
  • The condition is evaluated. If the condition is evaluated to true, the statements inside the body is executed. If the condition is evaluated to false, then the loop terminates.
  • Then, increment statement is executed to update the loop counter which will be used in condition.

Example:

int age;

for (age = 15; age < 18; age++)
{
  printf("You still have %d years to reach voting age.\n", (18 - age));
}

printf("Congratulations! You are able to vote now.\n");
return 0;

// output
You still have 3 years to reach voting age.
You still have 2 years to reach voting age.
You still have 1 years to reach voting age.
Congratulations! You are able to vote now.

break statement

break statement is used to end the loop immediately when it is encountered.

It is almost always used with if...else statement inside the loop.

Example:

// Program to calculate the sum of 5 numbers
#include <stdio.h>

int main() {
  int i;
  float num, sum = 0.0;

  printf("--- Enter 5 numbers\n");
  for (i = 1; i <= 5; ++i) {
    printf("Enter number #%d: ", i);
    scanf("%f", &num);

    // break the loop if user enter negative number
    if (num < 0.0) {
      printf("You entered negative number. Exiting ...\n");
      break;
    }

    sum += num;
  }

  printf("Sum = %.2lf", sum);

  return 0;
}

// output
--- Enter 5 numbers
Enter number #1: 10
Enter number #2: 01
Enter number #3: -1
You entered negative number. Exiting ...
Sum = 11.00

continue statement

The continue statement is used to skip the current iteration of the loop and continues with the next iteration.

It is almost always used with if...else statement inside the loop.

Example:

// Program to calculate the sum of 5 numbers
#include <stdio.h>

int main() {
  int i;
  float num, sum = 0.0;

  printf("--- Enter 10 numbers\n");
  for (i = 1; i <= 10; ++i) {
    printf("Enter number #%d: ", i);
    scanf("%f", &num);

    // break the loop if user enter negative number
    if (num < 0.0) {
      printf("You entered negative number. Skipping adding this number \n");
      continue;
    }

    sum += num;
  }

  printf("Sum = %.2lf", sum);

  return 0;
}

// output
--- Enter 5 numbers
Enter number #1: 10
Enter number #2: 20
Enter number #3: -5
You entered negative number. Skipping adding this number
Enter number #4: 30
Enter number #5: 40
Sum = 100.00

Examples

  • Print the value of number from 1 to 15.

    #include <stdio.h>
int main()
{
  int count;

  for(count = 1; count <= 15; count++)
  {
    printf("%d ", count);
  }

  return 0;
}

// output
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

  • Calculate the sum of first 15 natural numbers

    #include <stdio.h>
int main()
{
  int n, sum = 0;

  for(n = 1; n <= 15; n++)
  {
    sum += n;
  }

  printf("Sum of first %d numbers = %d\n", n, sum);

  return 0;
}

// output
Sum of first 16 numbers = 120
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