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Re^2: Seeing if two numbers have the same sign

by WoodyWeaver (Scribe)
on Jan 10, 2008 at 23:18 UTC ( #661753=note: print w/ replies, xml ) Need Help??


in reply to Re: Seeing if two numbers have the same sign
in thread Seeing if two numbers have the same sign

I don't understand your (($x^$y)<0).

Using

sub andNeg { return (($_[0]^$_[1]) < 0); }
and test harness
print "<table border>\n<tr><td>", join("</td><td>", "x", "y", "andNeg( +)", "spaceship()", "mult()", "anchor()"), "</td></tr>\n"; for (0..3) { my $x = 1 - 2 * ($_ % 2); my $y = 1 - 2 * int($_ / 2); print "<tr><td>", join("</td><td>", $x, $y, andNeg($x, $y), spaceshi +p($x, $y), mult($x, $y), anchor($x, $y)), "</td></tr>\n"; } print "</table>\n";
I get
xyxorNeg()spaceship()mult()anchor()
11111
-110
1-10
-1-1111
As for spaceship, multiplication, and logic, implemented as
sub spaceship { (0 <=> $_[0]) == (0<=> $_[1]); } sub mult { $_[0]*$_[1] > 0; } sub anchor { if ($_[0] > 0) { if ($_[1] > 0) { # both pos return 1 } } elsif ($_[0] < 0) { if ($_[1] < 0) { # both neg return 1 } } return 0; }
I get
unit squarespaceshipmultiplylogic
spaceship--------worse 22.28%worse 10.40%
multiply better 22.28%--------better 13.26%
logic better 10.40%worse 13.26%--------
and
signed intspaceshipmultiplylogic
spaceship--------worse 21.43%worse 7.65%
multiply better 21.43%--------better 14.92%
logic better 7.65%worse 14.92%--------
that is, if you take points (x,y) uniformly chosen from the unit disk [-1,1]x[-1,1], then spaceship is 22% worse than testing multiplication, multiplication is 13% better than using logic, etc; and if you take them as random unsigned ints, multiplication is (surprisingly!) even better.

full test program (don't use this use Benchmark; instead)

#!perl -w # use Time::HiRes qw(time); print "<table border>\n<tr><td>", join("</td><td>", "x", "y", "andNeg( +)", "spaceship()", "mult()", "anchor()"), "</td></tr>\n"; for (0..3) { my $x = 1 - 2 * ($_ % 2); my $y = 1 - 2 * int($_ / 2); print "<tr><td>", join("</td><td>", $x, $y, andNeg($x, $y), spaceshi +p($x, $y), mult($x, $y), anchor($x, $y)), "</td></tr>\n"; } print "</table>\n"; my $count = 1000000; for ('unit square', 'signed int') { doTimeTest($count, $_); } sub doTimeTest { my $count = shift; my $type = shift; my @coordinates = primeCoordinates($count, $type ); my $time0 = Time::HiRes::time(); foreach (@coordinates) { spaceship($_->[0],$_->[1]); } my $time1 = Time::HiRes::time(); foreach (@coordinates) { mult($_->[0], $_->[1]); } my $time2 = Time::HiRes::time(); foreach (@coordinates) { anchor($_->[0], $_->[1]); } my $time3 = Time::HiRes::time; my $spaceship = $time1 - $time0; my $multiple = $time2 - $time1; my $logic = $time3 - $time2; print join("\t", $type, "spaceship", "multiply", "logic"), "\n"; print join("\t", "spaceship", '-'x8, pretty($spaceship,$multiple), p +retty($spaceship,$logic)), "\n"; print join("\t", "multiply ", pretty($multiple,$spaceship), '-'x8, p +retty($multiple,$logic)), "\n"; print join("\t", "logic ", pretty($logic,$spaceship), pretty($log +ic,$multiple), '-'x8), "\n"; print "\n"; } sub pretty { my ($one, $two) = @_; if ($one > $two) { return sprintf("worse %.2f%%", ($one - $two) / $one * 100); } else { return sprintf("better %.2f%%", ($two - $one) / $two * 100); } } sub spaceship { (0 <=> $_[0]) == (0<=> $_[1]); } sub mult { $_[0]*$_[1] > 0; } sub anchor { if ($_[0] > 0) { if ($_[1] > 0) { # both pos return 1 } } elsif ($_[0] < 0) { if ($_[1] < 0) { # both neg return 1 } } return 0; } sub andNeg { return (($_[0]^$_[1]) < 0); } sub primeCoordinates { my $count = shift; my $type = shift; my @retval; if ($type eq 'unit square') { for (0..$count-1) { $retval[$_] = [ 1 - 2 * rand(), 1 - 2 * rand()]; } } elsif ($type eq 'signed int') { use POSIX; for (0..$count-1) { $retval[$_] = [ rand() > 0.5 ? rand(INT_MAX) : -1 * rand(INT_MAX +), rand() > 0.5 ? rand(INT_MAX) : -1 * rand(INT_MAX +)]; } } else { die "Blue. No yel-- Auuuuuuuugh!\n"; } return @retval; }


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Re^3: Seeing if two numbers have the same sign
by demerphq (Chancellor) on Jan 11, 2008 at 19:23 UTC

    Hmm. I didnt actually test it and had assumed that the result of binary xor was going to be signed, but of course its unsigned, hence the xor ideas doesnt work as written. My bad. Sorry. Id just use the $x<0 == $y<0 approach anyway. :-)

    ---
    $world=~s/war/peace/g

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