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Tip of the Day
Home » Tip Bank » C++
Language: C
Expertise: Advanced
Aug 15, 2003

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Fast Check for 2^N (2 power N) Numbers

To test if an integer is a power of 2 (1,2,4,8,16...256...), use the following code:

if (!(x & (x-1)) then ...
Because 2^N-1 numbers have no common bits with 2^N in binary, they require only 1 logical AND and 1 SUBTRACT. Note that this only works with positive numbers:

2^N   : 100000000000.....
2^N-1 : 011111111111.....
Julien Olivier
 
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