test whether a number is a power of 2

[Matt]

Give a one-line C expression to test whether a number is a power of 2.
No loops allowed.

[Ed Morton]

On 9/28/2003 1:21 PM, Matt wrote:

Give a one-line C expression to test whether a number is a power of 2.
No loops allowed.

if (!(x%2)) {
puts("x is a power of 2");
}

Regards,

Ed.

[Jirka Klaue]

Ed Morton wrote:

On 9/28/2003 1:21 PM, Matt wrote:

Give a one-line C expression to test whether a number is a power of 2.
No loops allowed.

if (!(x%2)) {
puts("x is a power of 2");
}

Are you implying that 42 is a power of 2 and 43 is not?
BTW: It's a three-liner and no C expression at all. ;-)

Jirka

[Ed Morton]

On 9/29/2003 10:44 AM, Jirka Klaue wrote:

Ed Morton wrote:

On 9/28/2003 1:21 PM, Matt wrote:

Give a one-line C expression to test whether a number is a power of 2.
No loops allowed.

if (!(x%2)) {
puts("x is a power of 2");
}

Are you implying that 42 is a power of 2 and 43 is not?
BTW: It's a three-liner and no C expression at all. ;-)

Jirka

I had intended !(x%2) to be the expression, the rest was demonstrating using it.
I'll go back to sleep now.... wake me next time you want to know if a number is
a multiple of 2 rather than a power of 2. Sorry 'bout that.

Ed.

[lawrence.jones]

Joona I Palaste <pa*****@cc.hel sinki.fi> wrote:

((((x&&!(x&(x-!!x)))+x)-x)^x)^x

What do I win?

Nothing. What if x is negative? What if it's not an integer? What if
the original poster provided a complete specification of the problem? :-)

-Larry Jones

I kind of resent the manufacturer's implicit assumption
that this would amuse me. -- Calvin

[Peter Nilsson]

Eric Sosman <Er*********@su n.com> wrote in message news:<3F******* ********@sun.co m>...

Matt wrote:

Give a one-line C expression to test whether a number is a power of 2.
No loops allowed.

true_or_false = a_number > 0;
(For example, 3.14159 ~= 2 ** 1.65149.)

Why stop there? C99 has complex types. ;)

--
Peter

[CBFalconer]

Ed Morton wrote:

On 9/28/2003 1:21 PM, Matt wrote:

Give a one-line C expression to test whether a number is a
power of 2. No loops allowed.

if (!(x%2)) {
puts("x is a power of 2");
}

^^^^^^^^^^^^
even

--
Chuck F (cb********@yah oo.com) (cb********@wor ldnet.att.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net> USE worldnet address!

[Aishwarya]

Da*****@cern.ch (Dan Pop) wrote in message news:<bl******* ***@sunnews.cer n.ch>...
...

Now, !!x is an obfuscated way of writing 1, when you know that x cannot
be zero (it was already tested)

this is exactly what i got stuck in :-) i should have guessed it ...
thanks for the explanation though :-)

[R. Rajesh Jeba Anbiah]

"jacob navia" <ja*********@ja cob.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

---
"Silence is the only right answer for many wrong questions" --
G.K.Moopanaar, Indian Politician
http://guideme.itgo.com/atozofc/ - "A to Z of C" Project
Email: rrjanbiah-at-Y!com