base.c

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

char *digits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

char *invertir(char *str){
	int last = strlen(str) - 1;
	char swp;
	for (int i = 0, j = last; i < j; i++, j--){
		swp = str[i];
		str[i] = str[j];
		str[j] = swp;
	}
	return str;
}

char *base(int nro, int base, char *buff){
	int mod;
	int i = 0;

	if (nro == 0){
		strcpy(buff, "0");
		return buff;
	}
	while (nro > 0) {
		mod = nro % base;
		nro = nro / base;
		buff[i] = digits[mod];
		i++;
	}
	buff[i] = 0;
	return invertir(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;
}