get absolute value without using abs function nor if statement

c++ c bit-manipulation
shanwu picture shanwu · Mar 19, 2012 · Viewed 86.8k times · Source

I was thinking how to get the absolute value of an integer without using if statement nor abs(). At first I was using shift bits left (<<), trying to get negative sign out of the range, then shift bits right back to where it be, but unfortunately it doesn't work for me. Please let me know why it isn't working and other alternatives ways to do it.

Answer

Hasturkun picture Hasturkun · Mar 19, 2012

From Bit Twiddling Hacks:

int v;           // we want to find the absolute value of v
unsigned int r;  // the result goes here 
int const mask = v >> sizeof(int) * CHAR_BIT - 1;

r = (v + mask) ^ mask;

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