C PROGRAMMING PROGRAM • FUNCTIONS & RECURSION

Find the Sum of Natural Numbers Using Recursion

Learn how to find the sum of natural numbers using recursion using a clear C program.

IntermediateRecursionBase caseNatural numbers

PROBLEM UNDERSTANDING

Input and expected output

Sample input

Sample output

Sum = 55

COMPLETE C PROGRAM

Complete C implementation

#include <stdio.h>

long long sumTo(int number)
{
    if (number == 0) return 0;
    return number + sumTo(number - 1);
}

int main(void)
{
    int number;

    printf("Enter a non-negative integer: ");
    scanf("%d", &number);
    if (number < 0) return 1;
    printf("Sum = %lld\n", sumTo(number));
    return 0;
}

CURRENT STEP

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EXPECTED OUTPUT FOR THE SAMPLE

Sum = 55

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Add N to the recursive sum ending at N−1 until the base case zero.
Display the computed result.

Add N to the recursive sum ending at N−1 until the base case zero.

EFFICIENCY

Time and space complexity

Time complexity

O(n)

Auxiliary space

O(n)

DEBUGGING CHECKLIST

Common mistakes

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Use the correct format specifier for every variable.

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Initialize variables before using their values.

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Check braces, semicolons and input order carefully.

Try it yourself

Practice: Run the program with the sample input, predict its output, and then test one boundary case of your own.