Bit-wise operations

Operation

Bitwise Operators
線上測驗 (附上解答)

Set a bit

where n is the bit number, and 0 is the least significant bit)

unsigned char a |= (1 << n);

Example:

a               1 0 0 0 0 0 0 0
a |= (1 << 1) = 1 0 0 0 0 0 1 0
a |= (1 << 3) = 1 0 0 0 1 0 0 0
a |= (1 << 5) = 1 0 1 0 0 0 0 0

Clear a bit:

 unsigned char b &= ~(1 << n);

Example 1:

b                1 1 1 1 1 1 1 1
b &= ~(1 << 1) = 1 1 1 1 1 1 0 1
b &= ~(1 << 3) = 1 1 1 1 0 1 1 1
b &= ~(1 << 5) = 1 1 0 1 1 1 1 1

Example 2:
Write a macro with two arguments a and pos, where a is a byte and the pos specifies a bit position of the byte. The macro should clear the particular bit position in the given byte.

#include <stdio.h>

/* bit clear: 
  int,
  bit position to clear */
#define CLEARBIT(a, pos) (a &= ~(1 << pos))

int main()
{
    /* 'z': decimal value 122 (=01111010) */
    char a = 'z';  

    /* clearing the 5th bit */
    char aOut = CLEARBIT(a, 5);

    /* aOut = 'Z': decimal value 90 (=01011010) */
    printf("aOut=%c\n", aOut);

    return 0;
}

Toggle a bit:

 unsigned char c ^= (1 << n);

Example:

c               1 0 0 1 1 0 1 1
c ^= (1 << 1) = 1 0 0 1 1 0 0 1
c ^= (1 << 3) = 1 0 0 1 0 0 1 1
c ^= (1 << 5) = 1 0 1 1 1 0 1 1

Test a bit:

unsigned char e = d & (1 << n); //d has the byte value.

The right/left most byte

assuming 16 bit, 2-byte short integer:

unsigned char right = val & 0xff; /* right most (least significant) byte */
unsigned char left = (val >> 8) & 0xff; /* left most (most significant) byte */

sign bit

assuming 16 bit, 2-byte short integer, two's complement:

bool sign = val & 0x8000; // sign bit

Setting and Clearing a Bit

The code below shows how to set or clear a bit of an integer.

#include <stdio.h>
void binary(unsigned int n) {
    for (int i = 256; i > 0; i /= 2) {
        if (n & i) 
            printf(" 1");
        else
            printf(" 0");
    }
    printf("\n");
}

bool getBit(int n, int index) {
    return ((n & (1 << index)) > 0);
}

int setBit(int n, int index, bool b) {
    if (b)
        return (n | (1 << index));	
    int mask = ~(1 << index);
    return n & mask;
}

int main() {
    int num = 16, index;

    printf("Input\n")
    for (int i = 7; i >= 0; i--) 
        printf("%d ", getBit(num,i));
    printf("\n");

    /* set bit */
    index = 6;
    printf("Setting %d-th bit\n", index);
    num = setBit(num, index, true);
    for (int i = 7; i >= 0; i--) 
        printf("%d ", getBit(num,i));
    printf("\n");

    /* unset (clear) bit */
    index = 4;
    printf("Unsetting (Clearing) %d-th bit", index);
    num = setBit(num, index, false);
    for (int i = 7; i >= 0; i--) 
        printf("%d ", getBit(num,i));
    printf("\n");

    return 0;
}

Output is:

Input
0 0 0 1 0 0 0 0
Setting 6-th bit
0 1 0 1 0 0 0 0
Unsetting (Clearing) 4-th bit
0 1 0 0 0 0 0 0

2018q1 第 1 週測驗題