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Loop Programs in C

Print sum of first N odd numbers

Introduction

Odd numbers are numbers that are not divisible by 2, such as 1, 3, 5, 7, 9, etc.
The formula for the sum of first N odd numbers is:

Sum=N2

However, we’ll calculate it using a loop, to understand how summation works step by step.

 

C Program: Print sum of first N odd numbers

NOTE: The Program is written using for, while and do..while loop.
Method 1 is for loop, Method 2 is while loop and Method 3 is do..while loop

Method 1: Using for loop

C 
#include <stdio.h>


 


int main() {


    int N, i, sum = 0;


 


    // Input


    printf("Enter the value of N: ");


    scanf("%d", &N);


 


    // Loop to calculate sum of first N odd numbers


    for (i = 1; i <= N; i++) {


        sum += 2 * i - 1;  // ith odd number = 2*i - 1


    }


 


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


 


    return 0;


}

Output

 
OUTPUT 1 :
Enter the value of N: 5
Sum of first 5 odd numbers = 25

OUTPUT 2 :
Enter the value of N: 7
Sum of first 7 odd numbers = 49

Explanation

  1. The user inputs the value of N.
  2. The for loop runs from 1 to N.
  3. Each iteration adds the i-th odd number (2*i - 1) to sum.
  4. After the loop ends, the program prints the total sum.

 

C Program: Print sum of first N odd numbers

Method 2: Using while loop

C 
#include <stdio.h>


 


int main() {


    int N, i = 1, sum = 0;


 


    // Input


    printf("Enter the value of N: ");


    scanf("%d", &N);


 


    // While loop to calculate sum


    while (i <= N) {


        sum += 2 * i - 1;  // Add ith odd number


        i++;               // Increment counter


    }


 


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


 


    return 0;


}

Output

 
OUTPUT 1 :
Enter the value of N: 5
Sum of first 5 odd numbers = 25

OUTPUT 2 :
Enter the value of N: 8
Sum of first 8 odd numbers = 64

Explanation

  1. Start with i = 1 and sum = 0.
  2. The while loop executes while i <= N.
  3. In each iteration, 2 * i - 1 gives the current odd number.
  4. Add it to sum and increment i.
  5. Print the total sum after the loop ends.

 

C Program: Print sum of first N odd numbers

Method 3: Using do..while loop

C 
#include <stdio.h>


 


int main() {


    int N, i = 1, sum = 0;


 


    // Input


    printf("Enter the value of N: ");


    scanf("%d", &N);


 


    // Do-while loop to calculate sum


    do {


        sum += 2 * i - 1;  // Add the ith odd number


        i++;               // Increment counter


    } while (i <= N);


 


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


 


    return 0;


}

Output

 
OUTPUT 1 :
Enter the value of N: 5
Sum of first 5 odd numbers = 25

OUTPUT 2 :
Enter the value of N: 7
Sum of first 7 odd numbers = 49

Explanation

  1. The user enters the number N.
  2. The loop starts with i = 1.
  3. Inside the loop:
    • Calculates the odd number using 2 * i - 1.
    • Adds it to sum.
    • Increments i.
  4. The condition (i <= N) is checked after each iteration.
  5. Prints the final sum when the loop ends.

 

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