CODEWITHSUNDEEP
c
Maria
Maria

Reputation: 11

Converting from one base to another

When you have to convert a number from base 16 to base 2, what is the difference between lowercase and uppercase letters? (ex: 0Xa5B)

I thought there was no difference, but I'm not sure.

Upvotes: 1

Views: 241

Answers (1)

chqrlie
chqrlie

Reputation: 144695

There is no difference between upper and lower case in this context. The C language grammar and the library functions strtol() and similar accept both upper and lower case letters for the 0x or 0X prefix and the digits greater than 9 in any combination.

You can convert the string to an unsigned long with strtoul() and you will need a custom function to convert the resulting integer to its binary representation as a string.

Here is an example:

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

/* Convert an unsigned long to a string of binary digits.
 * if the destination array is too short, truncate the high order bits;
 * leading zeroes are omitted (except the last one for the value 0).
 */
char *convert_to_binary(char *dest, size_t size, unsigned long value) {
    if (size > 0) {
        size_t i = size;
        dest[--i] = '\0';
        while (i > 0) {
           dest[--i] = '0' + (value & 1);
           value >>= 1;
           if (value == 0) /* remove this test if you want leading zeroes */
               break;
        }
        if (i > 0) {
            memmove(dest, dest + i, size - i);
        }
    }
    return dest;
}

int main() {
    char buf[sizeof(unsigned long) * CHAR_BIT + 1];
    unsigned long value;
    const char *str = "0Xa5B";
    char *p;

    /* convert the hex string to a number */
    errno = 0;
    value = strtoul(str, &p, 16);  /* 0x prefix is ignored for base 16 */
    /* check for conversion errors */
    if (p == str) {
        fprintf(stderr, "not a number: %s\n", str);
        return 1;
    }
    if (*p != '\0') {
        fprintf(stderr, "extra characters: %s\n", str);
        return 1;
    }
    if (errno != 0) {
        /* range error */
        fprintf(stderr, "conversion error: %s: %s\n", str, strerror(errno));
        return 1;
    }
    convert_to_binary(buf, sizeof buf, value);
    printf("%s -> %s\n", str, buf);
    return 0;
}

Upvotes: 1

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