Add First N Numbers in C Using a Function

Written: August 14, 2026

Moving the sum into a function separates calculation from input/output—good GTU style.

This walkthrough keeps the logic small and readable so you can type it, run it, and then change one input at a time to see what happens.

What you will build

sum_n(n) loops from 1 to n accumulating a total, then main prints the returned value.

• Write int sum_n(int n)
• Accumulate total from 1..n
• Return total
• Read n in main and print sum_n(n)

Working C example

#include <stdio.h>

int sum_n(int n) {
    int i, total = 0;
    for (i = 1; i <= n; i++) total += i;
    return total;
}

int main(void) {
    int n;
    printf("Enter N: ");
    if (scanf("%d", &n) != 1 || n < 1) return 1;
    printf("Sum = %d\n", sum_n(n));
    return 0;
}

How the logic works

Read the program top to bottom: includes and main first, then the statements that change variables, then the print that proves the result.

If your output looks wrong, print the variables before and after the critical lines. That single habit catches most beginner bugs faster than rewriting the whole file.

Compare your loop result with the formula n*(n+1)/2 for a quick correctness check.

Common mistakes

Forgetting a semicolon, using the wrong format specifier in printf/scanf, and mixing up assignment (=) with comparison (==) are the usual culprits.

Compile with warnings enabled (`gcc -Wall`) so the compiler points at risky casts and unused variables before you chase them by hand.

Try this next

Change the sample inputs, add a second test case, and briefly note what stayed the same. Teaching yourself with tiny experiments sticks better than copying a longer program you never run.

When you can explain every line without looking, you are ready for the next exercise in the series.