# Bit Manipulation

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:

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.

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