Find LCM in C

What You’ll Learn

The least common multiple is the smallest shared positive multiple of two integers. This tutorial covers the gcd–lcm identity, a safe C formula with long long, a brute multiple scan for intuition, a live preview, algorithm steps, worked C examples, edge cases, and complexity.

📝 Problem & Approach

Given nonnegative integers a and b, compute lcm(a,b). Prefer the gcd formula; optionally show a brute multiple scan for the same inputs.

/* a = 12, b = 18
 * gcd = 6
 * lcm = (12 / 6) * 18 = 2 * 18 = 36
 *
 * Multiples of 12: 12, 24, 36, ...
 * Multiples of 18: 18, 36, ...
 * First common positive multiple: 36
 */

Inputs & Outputs

Minimal workflow

function gcd(a, b):  // nonnegative
    while b != 0:
        (a, b) = (b, a mod b)
    return a

function lcm(a, b):
    if a = 0 or b = 0:
        return 0
    g = gcd(a, b)
    return (a / g) * b

Method comparison

🔮 Live Preview

Enter two nonnegative integers and see gcd and lcm (same identity as the C examples).

Press “Compute lcm”.

Examples Gallery

Two complete C programs — lcm from gcd with long long, and a brute multiple scan. Both use 12 and 18 → 36. Click View Output to reveal sample console results.

#include <stdio.h>

int find_gcd(int num1, int num2) {
    while (num2 != 0) {
        int temp = num2;
        num2 = num1 % num2;
        num1 = temp;
    }
    return num1;
}

long long find_lcm_ll(int num1, int num2) {
    int g;

    if (num1 == 0 || num2 == 0) {
        return 0LL;
    }

    g = find_gcd(num1, num2);
    return (long long)num1 / g * (long long)num2;
}

int main(void) {
    int number1 = 12;
    int number2 = 18;
    long long lcm = find_lcm_ll(number1, number2);

    printf("LCM of %d and %d is: %lld\n", number1, number2, lcm);
    return 0;
}

LCM of 12 and 18 is: 36

#include <stdio.h>

int lcm_scan_positive(int a, int b) {
    int step;
    int m;

    if (a <= 0 || b <= 0) {
        return 0;
    }

    step = a > b ? a : b;
    m = step;

    while (m % a != 0 || m % b != 0) {
        m += step;
    }

    return m;
}

int main(void) {
    int number1 = 12;
    int number2 = 18;

    printf("LCM of %d and %d is: %d\n", number1, number2,
           lcm_scan_positive(number1, number2));
    return 0;
}

LCM of 12 and 18 is: 36

🔎 Worked Walkthrough — 12 and 18

Trace Euclid, then the divide-before-multiply formula.

Check: 12 · 18 = 216 and 216 / 6 = 36 — same lcm, but the formula avoids forming 216 in a narrow int first.

🔄 Sample Values

Known results for common interview inputs.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

The gcd-based method dominates in practice.

💡 Best Practices