What''s the quickest way to find if a given number is a power of 2?

Find out if log_2 is an integer.

log_2 ???

Yes, the base 2 logarithm.

log without subscript means base 10 logarithm.

ln means base e logarithm.

log_n (where _ denotes subscript) means base n logarithm.

So log_2 is a base 2 logarithm.

ok but how do I do this in the C language? The only functions I saw was log() and log10()...

I'd do:

int mask;int n = 0;for( mask = 1; mask != 0x80000000; mask <<= 1 ){    if( mask & nSomeInt )        n++;}if( n == 1 )    nSomeInt is a power of 2else    its not  

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[edited by - Enselic on March 13, 2003 1:24:13 PM]

If you want a log base 2 function here ya go:

double log2( double x ){   return ( log(x) / log( 2 ) );} 

The log of base N is defined as the natrual log of your number x divided by the natural log of the base you are looking for. In this case 2.

-SirKnight

I haven''t tested but I bet that my method is faster than using log...

Or for a more general log funtion:

double logn( double x, int base ){    return ( log( x ) / log( base ) );} 

Then you can just go:

double blah = logn( 15, 2 ); 

Or whatever.

-SirKnight

You''re probably right but I just wanted to throw that way of doing it in there to show how the log of whatever base is defined.

-SirKnight