Finding the bigger number using bitwise operation in C

There are multiple issues in the code:

• the function big() does not work if the numbers have the same number of significant bits, eg: if you enter 1 and 0 you will get 0, which is incorrect, but the same for 3 and 2, 6 and 4 etc.

• your method would work if you shift the values after the test and if you first mask off bits that are identical in both values.

• the tests in big and small are redundant, you can remove the first if.

• testing x < 0 in the main function is moot: x is an unsigned int thus cannot be negative. You might want to use a different parsing method.

Here is a modified version:

#include <stdio.h>

unsigned int big(unsigned int x, unsigned int y)
{
    unsigned int xx = x & ~y;
    unsigned int yy = y & ~x;
    for (;; xx >>= 1, yy >>= 1) {
        if (xx == 0)
            return y;
        if (yy == 0)
            return x;
    }
}

int main(void)
{
    for (;;) {
        unsigned int x, y;
        printf("Give me a number: ");
        if (scanf("%u", &x) != 1) { printf ("error\n"); break; };
        printf("Give me another number: ");
        if (scanf("%u", &y) != 1) { printf ("error\n"); break; };
        printf("The bigger number is %u\n\n", big(x, y));
    }
    return 1;
}

Here is an input function that is more restrictive:

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

int get_unsigned(const char *prompt, unsigned int *dest) {
    for (;;) {
        char buf[128];
        printf("%s: ");
        fflush(stdout);
        if (!fgets(buf, sizeof buf, stdin)) {
            fprintf(stderr, "unexpected end of file\n");
            return 0;
        }
        int len = 0;
        sscanf(buf, " %*[0-9] %n", &len);
        if (!len || buf[len] != '\0') {
            printf("invalid input: %s\n", buf);
            continue;
        }
        errno = 0;
        unsigned long v = strtoul(buf, NULL, 0);
        if (errno || v > UINT_MAX) {
            printf("number too large: %s\n", buf);
            continue;
        }
        *dest = (unsigned int) v;
        return 1;
    }
}

int main(void) {
    for (;;) {
        unsigned int x, y;
        if (!get_unsigned("Give me a number", &x)
        ||  !get_unsigned("Give me another number", &y))
            break;
        printf("The bigger number is %u\n\n", big(x, y));
    }
    return 1;
}

Here is a simpler way to obtain the larger value without loops or tests, but using subtraction (-) which strictly speaking is not a bitwise operation:

#include <limits.h>

unsigned int big(unsigned int x, unsigned int y) {
    // compute mask as -1 if (x > y) and 0 otherwise
    unsigned mask = (y - x) >> (sizeof(x) * CHAR_BIT - 1);
    return (x & mask) | (y & ~mask);
}

Based on the feedback provided by other users it appears the code in the question needs to be corrected by following the steps mentioned below -

1. In the main() the variables x and y should be declared as int instead of uint . In this way negative input values can be manually rejected and the variables can then be passed on to big() by typecasting them into unsigned int . This is a beginner level solution it needs to be noted as typecasting is not liked very much by the advanced level C programmers.

2. In the big() the temporary variables need to be initialized as unsigned int big = x & ~y, small = ~x & y; instead of unsigned int big = x, small = y; . Doing this will make sure that the programme gives the correct answer when both x and y happen to have the same highest significant bit.

3. Conditions need to be checked at the end of the loop instead of at the beginning. This can be achieved by using a do-while loop.

    #include <stdio.h>
   
    unsigned int big (unsigned int x, unsigned int y)
    {
        unsigned int big = x & ~y, small = ~x & y;
        do {
            if (!big) { return y; }
            if (!small) { return x; }
            big >>= 1; small >>= 1;
        } while (1);
    }
   
    int main (void)
    {
        for (;;)
        {
            int x, y;
            printf ("Give me a number: ");
            if ((scanf ("%d", &x) != 1) || (x < 0)) { printf ("error\n"); break; }
            printf("Give me another number: ");
            if ((scanf ("%d", &y) != 1) || (y < 0)) { printf ("error\n"); break; }
            printf ("The bigger number is %u\n\n", big ((unsigned int)x, (unsigned int)y));
        }
        return 1;
    }