ft_itoa — Integer to String

Allowed functions: malloc

Write a function that takes an int and converts it to a null-terminated string. The function returns the result in a char array that you must allocate.

char	*ft_itoa(int nbr);

Solution

Download ft_itoa.c

#include <stdlib.h>

int numlen(int n) {
	int len = 0;
	if (n <= 0)
		len = 1;

	while (n != 0) {
		n = n / 10;
		len++;
	}
	return len;
}

char *ft_itoa(int nbr) {

	long n = nbr;
	int length = numlen(nbr);

	char *str = malloc(sizeof(char) * (length + 1));
	if (!str) {
		return NULL;
	}

	if (n < 0) {
		n = -n;
		str[0] = '-';
	}

	if (n == 0)
		str[0] = '0';

	while (n > 0) {
		length--;
		str[length] = n % 10 + '0';

		n = n / 10;
	}

	str[length] = '\0';
	return str;
}

How It Works

Goal: Convert an integer to a dynamically allocated null-terminated string.

Approach: Count the number of digits, allocate a string of that size, then fill it from the end by extracting digits with modulo.

Step by step:

1. numlen counts digits — it starts at 1 for zero or negative numbers (to account for '0' or '-'), then divides by 10 until the number is 0.
2. Allocate a char array of length + 1 (for the null terminator).
3. If the number is negative, store '-' at index 0 and negate the value (using long to safely handle INT_MIN).
4. Fill digits from the end: str[--length] = n % 10 + '0' extracts the last digit, then n /= 10 removes it.

Key concept: Digit extraction using modulo and division, handling negative numbers and zero as edge cases.