"jacob navia" <ja*********@jacob.remcomp.fr> wrote in message news:<bl**********@news-reader5.wanadoo.fr>...

isPow2 = x && !( (x-1) & x );

Algorithm:

If x is a power of two, it doesn't have any bits in common with x-1, since it

consists of a single bit on. Any positive power of two is a single bit, using

binary integer representation.

This means that we test if x-1 and x doesn't share bits with the and operator.

Of course, if x is zero (not a power of two) this doesn't hold, so we add an

explicit test for zero with xx && expression.

Few Mathematicians say any positive integer raised to the power minus

infinity is 0. That is,

n (pow) (-infinity) = 0

IOW, 2 (pow) 3 = 8, 2 (pow) 2 = 4, 2 (pow) 1 = 2, 2 (pow) 0 = 1,

and, 2 (pow) (-infinity) = 0.

Because of this religious reason, I would just write my macro as

(without 0 check)

#define ISPOWOF2( n ) ( ! ( n & (n-1) ) )

But, someone here in CLC told me that the above macro won't work all

the time. I know, it won't work for float; I know the necessity of

additional paranthesis for n. But, I couldn't see any other reasons.

Anybody have any good comments? TIA

---

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