Re: Seeing if two numbers have the same sign

[demerphq]

You are doing more work than you need to. Remember that mathematical comparisons return booleans, and booleans can be directly compared.

if ($x<0 == $y<0) {
  print "$x and $y have the same sign";
}

(($x ^ $y) < 0) would probably also work assuming they are integers. I think the trick with abs or the comparison operator would probably actually be less efficient than doing a comparison. Using multiplication is going to be slow as well.

BTW the sign of zero isnt a theological debate, it is an implementation detail of the machine you are working on and the data types you are using. :-)

[WoodyWeaver]

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

x	y	xorNeg()	spaceship()	mult()	anchor()
1	1		1	1	1
-1	1				0
1	-1				0
-1	-1		1	1	1

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 square	spaceship	multiply	logic
spaceship	--------	worse 22.28%	worse 10.40%
multiply	better 22.28%	--------	better 13.26%
logic	better 10.40%	worse 13.26%	--------

and

signed int	spaceship	multiply	logic
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)

Read more... (4 kB)

[demerphq]

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. :-)

[hipowls]

Neat (wish I'd thought of it) but it implicitly assumes that zero is a positive number, given 0 and -1 the test returns 0, given 0 and +1 it returns 1. If the test was ( $x<=0 == $y<=0 ) then zero is treated as both positive and negative and the two previous examples return 1. Which is consistent but might not be what you want.

[graff]

Maybe I'm just old-fashioned (primitive?), but my understanding of the term "same sign", at least as it relates to programming, has always been: when the sign bit of two values is the same, those values have "the same sign". In that interpretation, zero is always included among the set of "non-negative" values: it is the same sign as "1", and not the same sign as "-1".

[girarde]

I like your method better than mine (snarls enviously).

Mathematically, the sign of zero is undefined. Just as in football, you can't say which side of center the center lines up on.