Converting Number Bases (Binary, Hex)

This lesson covers converting number bases in C — binary, hex, and more — so you understand how a computer represents numbers internally. It's for anyone searching "C number base conversion implementation."

Converting to hexadecimal is easy with printf("%x", value), but there's no dedicated format specifier for binary, so you need to implement it yourself using division and remainders.

The example's toBinary() function repeatedly divides n by 2, taking the remainder each time, and builds the result as characters. Notice the care needed in the order the string gets assembled.

A common early mistake is not knowing binary conversion has no standard function. Many languages have a dedicated conversion function, so it can be surprising that C requires you to build the algorithm yourself.

Binary is commonly used inside a computer, and hexadecimal is commonly used for things like color codes — worthwhile foundational knowledge to remember.

📖 Reference code

#include <stdio.h>
#include <stdlib.h>

void toBinary(int n, char *buf) {
    int i = 0;
    char tmp[33];
    if (n == 0) {
        buf[0] = '0';
        buf[1] = '\0';
        return;
    }
    while (n > 0) {
        tmp[i++] = (n % 2) + '0';
        n /= 2;
    }
    int j = 0;
    while (i > 0) {
        buf[j++] = tmp[--i];
    }
    buf[j] = '\0';
}

int main() {
    int num = 42;
    char buf[33];
    toBinary(num, buf);
    printf("%s\n", buf);
    printf("%x\n", num);
    printf("%ld\n", strtol("101010", NULL, 2));
    return 0;
}

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🧪 This site can't compile or run C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).