Swap bits in a number in C [duplicate]

Answer 1

There is no "correct answer" to this kind of interview question. There are several ways to do this (lookup tables, anyone?) and the tradeoffs between each way (readability vs. performance vs. portability vs. maintainability) would need to be discussed.

The question is just an opening gambit to get you discussing some of the above issues, and to determine how 'deeply' you can discuss such problems.

Answer 2

Are you looking for something more clever than standard bit-shifting?

(assuming a is an 8-bit type)

a = ((a >> 4) & 0xF)  + ((a << 4) &0xF0)

Answer 3

unsigned char b;
b = (b << 4) | (b >> 4);

Answer 4

If you haven't seen or done much bit twiddling, a good resource to study is:

• Bit Twiddling Hacks

Answer 5

x86 assembly:

asm{
  mov AL, 10111110b
  rol AL
  rol AL
  rol AL
  rol AL
}

http://www.geocities.com/SiliconValley/Park/3230/x86asm/asml1005.html

Answer 6

/*swaping four bits*/
#include<stdio.h>

void printb(char a) {
    int i;
    for( i = 7; i >= 0; i--)
        printf("%d", (1 &  (a >> i)));
    printf("\n");
}

int swap4b(char a) {
    return ( ((a & 0xf0) >> 4) | ((a & 0x0f) << 4) );
}


int main()
{
    char a = 10;
    printb(a);
    a = swap4b(a);
    printb(a);
    return 0;
}