C Programs

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Loop Programs in C

Print sum of first N natural numbers

Introduction

The sum of first N natural numbers can be calculated in two ways:

1. Using a formula:

1. Using a loop to add numbers from 1 to N.

This program will use both methods to demonstrate.

C Program: Print sum of first N natural 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

#include <stdio.h>
 
int main() {
    int N, i, sum = 0;
 
    // Input
    printf("Enter a positive integer N: ");
    scanf("%d", &N);
 
    // Using for loop
    for (i = 1; i <= N; i++) {
        sum += i;  // sum = sum + i
    }
 
    printf("Sum of first %d natural numbers (using loop) = %d\n", N, sum);
 
    // Using formula
    int sum_formula = N * (N + 1) / 2;
    printf("Sum of first %d natural numbers (using formula) = %d\n", N, sum_formula);
 
    return 0;
}

 
OUTPUT 1 :
Enter a positive integer N: 10
Sum of first 10 natural numbers (using loop) = 55
Sum of first 10 natural numbers (using formula) = 55

OUTPUT 2 :
Enter a positive integer N: 5
Sum of first 5 natural numbers (using loop) = 15
Sum of first 5 natural numbers (using formula) = 15

Explanation

1. User enters N.
2. Loop method:
   • Start sum = 0.
   • Add each number from 1 to N.
3. Formula method:
   • Directly calculate using N*(N+1)/2.
4. Both results are displayed for verification.

C Program: Print sum of first N natural numbers

Method 2: Using while loop

The while loop can be used to iteratively add numbers from 1 to N.
This approach is useful for beginners to understand loops and accumulators.

#include <stdio.h>
 
int main() {
    int N, i = 1, sum = 0;
 
    // Input
    printf("Enter a positive integer N: ");
    scanf("%d", &N);
 
    // While loop to calculate sum
    while (i <= N) {
        sum += i;  // sum = sum + i
        i++;       // increment counter
    }
 
    printf("Sum of first %d natural numbers = %d\n", N, sum);
 
    return 0;
}

 
OUTPUT 1 :
Enter a positive integer N: 8
Sum of first 8 natural numbers = 36

OUTPUT 2 :
Enter a positive integer N: 5
Sum of first 5 natural numbers = 15

Explanation

1. Initialize i = 1 and sum = 0.
2. Loop continues while i <= N.
3. Each iteration: add i to sum and increment i.
4. Loop stops when i > N.
5. Print the total sum.

C Program: Print sum of first N natural numbers

Method 3: Using do..while loop

A do-while loop executes the loop body at least once, making it suitable for calculating sums where N ≥ 1.
We’ll iteratively add numbers from 1 to N using this loop.

#include <stdio.h>
 
int main() {
    int N, i = 1, sum = 0;
 
    // Input
    printf("Enter a positive integer N: ");
    scanf("%d", &N);
 
    // Do-while loop to calculate sum
    do {
        sum += i;  // Add current number to sum
        i++;       // Increment counter
    } while (i <= N);
 
    printf("Sum of first %d natural numbers = %d\n", N, sum);
 
    return 0;
}

 
OUTPUT 1 :
Enter a positive integer N: 7
Sum of first 7 natural numbers = 28


OUTPUT 2 :
Enter a positive integer N: 5
Sum of first 5 natural numbers = 15

Explanation

1. Initialize i = 1 and sum = 0.
2. The do block executes first — adds i to sum.
3. Increment i by 1.
4. The loop continues while i <= N.
5. Print the final sum after the loop ends.