Bit Manipulation

June 6, 2019

Bit Manipulation is a technique to manipulate the binary using Bitwise Operation.

Before we get started, I want to introduce ASCII to you. ASCII stands for American Standard Code for Information Interchange. Every character represents a number to ASCII. Such as "a" represent 01100001 in binary or 97 in decimal, "A" represent 01000001 in binary or 65 in decimal, and so on. It is a character encoding standard for electronic communication.

So, every character has a binary number, and the skill to manipulate it is a must for every software engineer. The technique is simple, using bitwise operation.

Here are the bitwise operators:

Bitwise Operators
Symbol	Name	Example
~	NOT	NOT 0111 (decimal 7) = 1000 (decimal 8)
&	AND	0101 (decimal 5) AND 0011 (decimal 3) = 0001 (decimal 1)
❘	OR	0101 (decimal 5) OR 0011 (decimal 3) = 0111 (decimal 7)
^	XOR	0101 (decimal 5) XOR 0011 (decimal 3) = 0110 (decimal 6)
»	SHIFT RIGHT	10010111 (decimal −105) SHIFT RIGHT BY 1 = 11001011 (decimal −53)
«	SHIFT LEFT	00010111 (decimal +23) SHIFT LEFT BY 1 = 00101110 (decimal +46)

I hope you can understand my short examples above. If not, let me explain it once again.

NOT 1 equals to 0.
1 AND 1 equals to 1. 1 AND 0 equals to 0.
1 OR 1 equals to 1. 1 OR 0 equals to 1.
1 XOR 1 equals to 0. 1 XOR 0 equals to 1.
0010 (2) SHIFT RIGHT by 1 equals to 0001 (1).
0010 (2) SHIFT LEFT by 1 equals to 0100 (4).

void print_bits(int number) {
    unsigned int check_bit = 1 << (sizeof(int) * 8 - 1);
    
    while (check_bit) {
        int result = number & check_bit;
    
        result == check_bit ? printf("1 ") : printf("0 ");
    
        check_bit >>= 1;
    }
}

Printing Bits. One of the most questions in a programming interview about Bit Manipulation. I use unsigned integer here because I only need a positive number. Integer has a size of 4 bytes. 1 byte is equal to 8 bits. Decrease one for the current position. I use "&" (AND) to check a bit. Print 1 if the result is the same with check_bit. Then, shift right by one for every loop.

int count_bits(int number) {
    int count = 0;
    
    while (number) {
        number &= (number - 1);

        count++;
    }
    
    return count;
}

Counting Bits. Count how many 1s in bits.

int reverse_bits(int number) {
    int reversed_number = 0;
    unsigned int count = sizeof(int) * 8 - 1;
    
    while (count) {
        int last_bit = number & 1;
        reversed_number |= last_bit;
        reversed_number <<= 1;
        number >>= 1;
        count--;
    }
    
    reversed_number <<= count;
    
    return reversed_number;
}

Reverse Bits. One of the most questions in a programming interview. Shift right the original number and shift left the reversed number. Extract the rightmost bit then add it to the rightmost position of the result.

References:

Pointer

Bit Manipulation

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