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

my $sum=0;
foreach my $num (1..1000)
{
   if ($num % 2)       #odd
   {
      $sum += $num
   }
   else                #even
   {
      $sum -= $num;
   }
}

print $sum;
[download]

• Abusive one-liner:

  my $sum = 0;
  $sum += (-1)**($_+1) * $_ for 1 .. 1000;
  print "$sum\n";
  [download]

• More logical:

  my $sum = 0;
  my $sign = 0;
  for my $i (1 .. 1000) {
      $sum += ($sign ? -1 : 1) * $i;
      $sign = !$sign;
  }
  print $sum;
  [download]

• or:

  my $sum = 0;
  my $sign = 0;
  for my $i (1 .. 1000) {
      $sum += ($sign++ % 2 ? -1 : 1) * $i;
  }
  print $sum;
  [download]

• Smart:

  my $sum = 0.5*(1 + 999)*( (999-1)/2 + 1 ) - 0.5*(2 + 1000)*( (1000-2)/
  +2 + 1 );
  print $sum;
  [download]

Of course, "Smart" can yield round-off issues...

Why use a boolean to represent a sign?

my $sum = 0;
my $sign = 1;
for my $i (1 .. 1000) {
    $sum += $sign * $i;
    $sum*=-1;
}
print $sum;
[download]

Rule One: "Do not act incautiously when confronting a little bald wrinkly smiling man."

$ perl -le' $_ = join "+", 1 .. 10; print; s/\+(\d+(?:\+|$))/-$1/g; pr
+int; print eval'
1+2+3+4+5+6+7+8+9+10
1-2+3-4+5-6+7-8+9-10
-5
[download]

sub sum_alternating {-1 * 500}
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