C Language Practice: Binary to Hexagonal Numbers

Convert Binary to Hexagonal

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

int btod(char *b)
{
    int i;
    int decimal = 0;
    int place = 0, temp = 0;

    for(i = strlen(b) - 1; i >= 0; i--)
    {
        if ((b[i] == '0') || (b[i] == '1'))
        {
            temp = b[i] - '0';
        }
        else
        {
            printf("error: not a valid binary number\n");
            exit(0);
        }
        decimal += temp * pow(2, place);
        place++;
    }

    return decimal;
}

int main(void)
{
    char b[100];
    printf("please input a binary number: ");
    scanf("%s", b);
    char h[100];

    int q, r, i = 0, j;
    q = btod(b);
    printf("the decimal number for this binary number is: %d\n", q);

    while(q != 0)
    {
        r = q % 16;
        if ((r >= 0) && (r <= 9))
        {
            h[i++] = r + '0';
        }
        else
        {
            h[i++] = r - 10 + 'a';
        }
        q = q / 16;
    }

    printf("the hexagonal number for this binary number is: ");
    for(j = i - 1; j >= 0; j--)
    {
        printf("%c", h[j]);
    }
    printf("\n");

    return 0;
}