Program to find the Sum of numbers in a given range in C

March 20, 2025

Find the Sum of Numbers in a Given Range in C

Given two integer inputs num1 and num2, the objective is to write a code to Find the Sum of Numbers in a Given Range in C. To do so we’ll keep iterating and adding the numbers from num1 until num2 to the Sum variable.

Example
Input : 2 4
Output : 2 + 3 + 4 = 9
sum of numbers in given range

Find the Sum of Numbers in a Given Interval in C

Given two integer inputs the objective is to Find the Sum of all the integers that lay within the given interval input by writing a code in C Language. Some Methods for solving the above problem are mentioned below,

  • Method 1 : Using Brute Force
  • Method 2 : Using the Formula
  • Method 3 : Using Recursion

We’ll discuss about the above mentioned methods in the sections below in detail.

Greatest of two numbers

Greatest of the Three numbers

Prime number

Prime number within a given range

Leap year or not

Method 1 : Using Brute Force

Algorithm and Working

The algorithm and working of the above mentioned problem is mentioned below,

  • Include the required Header files using include keyword.
  • Initialize the required variables.
  • Create a variable sum = 0
  • Run for loop in iterations of (I) in between [a, b]
    • In each iteration add I to sum value
    • Print sum value

Now let’s try and implement the algorithm in C language.

C Code

#include <stdio.h>

int main()
{
    int a = 5;
    int b = 10;
    
    int sum = 0;
    
    for (int i = a; i <= b; i++)
        sum = sum + i;
    
    printf("%d",sum);
    
    return 0;
}
// Time complexity : O(N)
// Space complexity : O(1)

Output

45

Method 2 : Using the Formula

Algorithm and Working

This method uses a concept that the sum of first n natural numbers can be found using direct formulae – n(n+1)/2

For sum between [a, b] we can simply –

  • Create a variable sum = 0
  • sum = b*(b+1)/2 – a*(a+1)/2 + a
  • Print the sum

Note – An extra ‘a’ is added at the end for offset (check last part of the formula)

Example – Assume you had to calculate sum b/w [3, 5]. Doing b*(b+1)/2 – a*(a+1)/2 what we have done is subtracted (1, 2, 3) from (1, 2, 3, 4, 5). Thus only got the sum of (4, 5). So extra 3 i.e. a is added in the formula

C Code

#include <stdio.h>

int main()
{
    int a = 5;
    int b = 10;
    
    int sum = b*(b+1)/2 - a*(a+1)/2 + a;
    
    printf("%d",sum);
    
    return 0;
}
// Time complexity : O(1)
// Space complexity : O(1)

Output

45

Method 3 : Using Recursion

Working and Algorithm

Uses recursion starting with a (lower number) but only going till b (upper number)

For sum between [a, b] we can simply –

  • Include the Required header files using include keyword.
  • Initialize the required variables.
  • Create a variable sum = 0
  • Call function getSum(0, a, b)
    • In each recursion, call return i + getSum(sum, i + 1, b)
    • In base Case i.e. when recursion call goes beyond upper limit return sum

Let’s try and implement the above algorithm in C Language.

C Code

#include <stdio.h>

int getSum(int sum, int i, int b){
        
    // stop when any recursion call tries to go over b (larger number)
    if (i > b)
        return sum;

    return i + getSum(sum, i + 1, b);
}

int main()
{
    int a = 5;
    int b = 10;
    
    int sum = getSum(0, a, b);
    
    printf("%d",sum);
    
    return 0;
}
// Time complexity : O(N)
// Space complexity : O(1)
// Auxilary Space complexity : O(1)
// Because of function call stack

Output

45

Getting Started

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