base_2.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <assert.h>

char *digits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

char *base(int nro, int base, char *buff){
	int length;
	int mod;
	int i;

	if (nro == 0){
		strcpy(buff, "0");
		return buff;
	}
	else{
		// Calculo cuantos dígitos son necesarios
		length = log(nro) / log(base) + 1;
	}
	buff[length] = '\0'; // Terminación del string
	i = length - 1; // Lo cargamos al revés

	while (nro > 0) {
		mod = nro % base;
		nro = nro / base;
		buff[i] = digits[mod];
		i--; // Decremento hasta llegar a cero
	}
	buff[i] = 0;
	return buff;
}

int main(){
	char str[256];
	printf("%s\n", base(50, 2, str));
	printf("%s\n", base(50, 3, str));
	printf("%s\n", base(50, 10, str));
	printf("%s\n", base(50, 16, str));
	printf("%s\n", base(50, 36, str));
	return EXIT_SUCCESS;
}