Bitwise operations

Bitwise Operator quizzes

• C Bitwise Operators Quiz
• 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: 
  a: int,
  pos: 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>
#include <stdbool.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\n", 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

延伸閱讀: Bitwise Operators in C

• Uses of Bitwise Operations or Why to Study Bits
  • Compression
  • Set operations
  • Encryption
• The Magic of XOR

  x ^ y == (~x & y) | (x & ~y)

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Applications

• 2018Q1 考古題 / 考古題參考解答
• 2018Q3 考古題 / 考古題參考解答
• 2019Q1 考古題 / 考古題參考解答 / 延伸閱讀