Question

How to reverse the bits of a number in C

Answer 1

#include <stdio.h>

void print_bits(unsigned int n, unsigned int size) {
    for (unsigned int i = 1 << (size - 1); i > 0; i = i / 2) {
        (n & i) ? printf("1") : printf("0");
    }
}

unsigned int reverseBits(unsigned int num) {
    unsigned int total_bits = sizeof(num) * 8;
    unsigned int reversed_bits = 0;

    for (unsigned int i = 0; i < total_bits; i++) {
        if ((num & (1 << i))) {
            reversed_bits |= 1 << ((total_bits - 1) - i);
        }
    }

    return reversed_bits;
}

int main()
{
    unsigned int num = 42;
    print_bits(num, 32);
    printf("\n");
    num = reverseBits(num);
    print_bits(num, 32);
    printf("\n\n");
    
    num = 19;
    print_bits(num, 32);
    printf("\n");
    num = reverseBits(num);
    print_bits(num, 32);
    
    return 0;
}




/*
run:

00000000000000000000000000101010
01010100000000000000000000000000

00000000000000000000000000010011
11001000000000000000000000000000

*/

 

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