Re: How can I tell if a number is a power of 2?

[cLive ;-)]

Obviously no mathematicians in the house :)

sub is_power_of_2 {
  return 0 unless $_[0]>0; # to avoid log error
  log($_[0])/log(2) - int(log($_[0])/log(2)) ? 0 : 1 
}

cLive ;-)

Update: - Cine (not linked because of very irritating JS on home node) proves me wrong while I'm typing up :)

Update: - Damn you tye... This falls down as far as computing accuracy goes for 2 to the power of 29, 31, 39, 47 and probably a few more. Where's that posix math module when you need it :)

Ah well, here's another:

sub is_power_of_2 {
  local $_ = $_[0];
  $_/=2 while ($_ > 1);
  return if ($_ - int($_));
  1;
}

[tye]

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

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

which has the advantage of catching negative-powers of 2 and negative powers-of-2. (:
        - tye (but my friends call me "Tye")