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c++cbitwise-operators
MuhsinFatih
MuhsinFatih

Reputation: 2231

Invert (flip) last n bits of a number with only bitwise operations

Given a binary integer, how can I invert (flip) last n bits using only bitwise operations in c/c++?
For example:

// flip last 2 bits
0110 -> 0101
0011 -> 0000
1000 -> 1011

Upvotes: 2

Views: 2639

Answers (1)

Mehmet Emin Kartal
Mehmet Emin Kartal

Reputation: 177

You can flip last n bits of your number with

#define flipBits(n,b) ((n)^((1u<<(b))-1))

for example flipBits(0x32, 4) will flip the last 4 bits and result will be 0x3d

this works because if you think how XOR works

 0 ^ 0 => 0
 1 ^ 0 => 1

bits aren't flipped

0 ^ 1 => 1
1 ^ 1 => 0

bits are flipped

 (1<<b)-1

this part gets you the last n bits for example, if b is 4 then 1<<4 is 0b10000 and if we remove 1 we get our mask which is 0b1111 then we can use this to xor with our number to get the desired output.

works for C and C++

Upvotes: 6

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