Overwrite a string in C - c

Question

I'm trying to create a code that converts a decimal into any base between 2 and 16. But I don't know how to write a new value in my string.
void base_conversion(char s[], int x, int b) {
int j, y;
j = 0;
// your code here
while(b > 1 && b < 17) {
if (x < 0) {
x = (-x);
}else if(x == 0) {
s[j] = '\0';
}
x = (x/b);
while(x > 0) {
y = (x%b);
if (y == 10) {
s[j] = 'A';
}else if(y == 11) {
s[j] = 'B';
}else if(y == 12) {
s[j] = 'C';
}else if(y == 13) {
s[j] = 'D';
}else if(y == 14) {
s[j] = 'E';
}else if(y == 15) {
s[j] = 'F';
}else{
s[j] = y;
}
}
}j = j + 1;
}

Answer 1

You were almost there, although several mistakes, so I have "improved" your code. The infinite loop testing the base which needed to be done once only. The while() loops weren't quite organised right - x/b being done outside the digit extraction loop. Another change I made was to use a lookup array to convert each digit to a character, which saves a lot of laborious testing. I also returned the string passed as the function value - might as well add a tad more functionality. In the case of passing a bad base value, I could have returned NULL instead of an empty string. Note also I update j in the same statements where I use it as an index, which makes the code a little more fluent.
#include <stdio.h>
char *base_conversion (char *s, int x, int b) {
char charr[] = "0123456789ABCDEF";
int i, j = 0, len, digit, neg = 0;
*s = 0; // terminate the string passed
if (b < 2 || b > 16) // check the base
return s; // return an empty string
if (x < 0) {
x = -x; // adjust for negative input
neg = 1;
}
do {
digit = x % b; // extract each l.s. digit
s[j++] = charr [digit]; // convert to character
} while (x /= b); // implicitly test for 0
if (neg) // negative input
s[j++] = '-'; // append a minus sign
s[j] = 0; // terminate the string
// reverse the string
len = j;
for (i=0; i<len/2; i++) {
digit = s[i];
s[i] = s[--j]; // pre-decrement j to next char index
s[j] = digit;
}
return s;
}
int main () {
int n;
char strig[65];
for (n=1000; n>=-1000; n-=2000) {
printf ("Binary %d: %s\n", n, base_conversion (strig, n, 2));
printf ("Ternary %d: %s\n", n, base_conversion (strig, n, 3));
printf ("Octal %d: %s\n", n, base_conversion (strig, n, 8));
printf ("Decimal %d: %s\n", n, base_conversion (strig, n, 10));
printf ("Hexadecimal %d: %s\n", n, base_conversion (strig, n, 16));
}
return 0;
}
Program output:
Binary 1000: 1111101000
Ternary 1000: 1101001
Octal 1000: 1750
Decimal 1000: 1000
Hexadecimal 1000: 3E8
Binary -1000: -1111101000
Ternary -1000: -1101001
Octal -1000: -1750
Decimal -1000: -1000
Hexadecimal -1000: -3E8