Well, if you could portably ask whether the mantissa of the number was represented with all 0 bits (since the leading 1 bit is implied and so doesn't appear)... 

#!/usr/bin/perl -w
use strict;

my $mask;
BEGIN {
    my $max= 2;
    $max *= 2   until  2*$max+1 == 2*$max;
    $mask= pack("d",$max) ^ pack("d",2*$max-1);
}

sub power2 {
    my( $n )= @_;
    return   if  ! $n;
    return(  ( $mask & pack "d", $n ) !~ /[^\0]/  );
}

for(  split " ", do { local($/); <DATA> }  ) {
    print "$_: ", power2($_)?1:0, "\n";
}
__END__
0 1 2 3 4 5 6 7 8 9 0.5 0.2 0.25 0.125 -1 -2 -3 -.25

[download]
produces 
0: 0
1: 1
2: 1
3: 0
4: 1
5: 0
6: 0
7: 0
8: 1
9: 0
0.5: 1
0.2: 0
0.25: 1
0.125: 1
-1: 1
-2: 1
-3: 0
-.25: 1

[download]
which has the advantage of catching negative-powers of 2 and negative powers-of-2. (:

        - tye (but my friends call me "Tye")

In reply to (tye)Re: How can I tell if a number is a power of 2? by tye
in thread How can I tell if a number is a power of 2? by larryk

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