in reply to Weighted random numbers generator

Hi, I usually try to establish some kind of function (stepped or non-stepped) on probability. This can be quite an involved process if I'm, for instance, trying for a normal distribution. So (bear with my ideosyncrasies)- 
my @x = @_;
my $y = 0;
my $i = 0;
until ($i > $#x) {
 $y = $y + $x[$i];
 $i = $i + 1;
}
$x[0] = $x[0] / $y;
$i = 1;
until ($i > $#x) {
 $x[$i] = $x[$i - 1] + $x[$i] / $y;
 $i = $i + 1;
}

$y = rand();

# This bit constitutes the "function"

$i = 0;
until ($y < $x[$i]) {
  $i = $i + 1;
}
return $i;

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Re: Re: Weighted random numbers generator
by Anonymous Monk on Mar 14, 2003 at 05:48 UTC
    Woops! Should have placed greater focus on the word _establish_. To speed things up, I furnish the details of the function (in this case, $x$i within the "function" bit at the end), instead of re-evaluating it every time I call a subroutine. So, to clarify, say I use two subroutines, choose_weighted and weight_array. 
    my $etc = 0;

my @x = (1.3, 1.4, 900, .03, 7, $etc, $etc);

@x = weight_array(@x);
my $random_value = choose_weighted(@x);

sub weight array {
 my @x = @_;
 my $y = 0;
 my $i = 0;
 until ($i > $#x) {
  $y = $y + $x[$i];
  $i = $i + 1;
 }
 $x[0] = $x[0] / $y;
 $i = 1;
 until ($i > $#x) {
  $x[$i] = $x[$i - 1] + $x[$i] / $y;
  $i = $i + 1;
 }
 return @x;
}

sub choose_weighted {
 my $y = rand();

 my $i = 0;
 until ($y < $x[$i]) {
  $i = $i + 1;
 }
 return $i;
}

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[d/l]
      Bugger. Spot the derro who forgot to change @x to @_ in choose_weighted.
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