I have a hash that maps a set of things to their "popularity". In this example, there are ten animals, each of which has received a certain number of votes. 
{ alligator => 100, bear => 90, cat => 80, ... jellyfish => 10 }
total_votes = 550

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I want to be able to randomly select from this set with three different biases: favoring the popular, impartial, and favoring the unpopular.

The impartial case is simple; choose the Nth item where 0 <= N <= 9.

To favor the popular, you choose a random number N from 0 to 549, then traverse the set of animals, subtracting each one's popularity from N until N goes negative, and that's the animal you choose.

But I can't figure out how to favor those with the least popularity. (Admittedly, this is more of an algorithm question than a Perl question, and I'll gladly take it to a different forum if it's inappropriate for PM.)

Here's my demo code; 

#!/usr/bin/env perl
use 5.010;
use warnings;
use strict;

# Initialize %votes: map animals to their number of votes.
my @animals
    = qw/alligator bear cat dog elephant fox giraffe hippo iguana jell
+yfish/;
my $votes   = 100;            # start with 100 votes for first word
my $decr    = int( $votes / (@animals) ); # evenly decrease # of votes
my $sum     = 0;              # grand total of votes for all words
my %votes = map
    {my $n = $votes; $sum += $n; $votes -= $decr; ( $_, $n )}
    @animals;

# Display animals and votes
print '===> ';
printf "%s:%3d ", $_, $votes{$_} for sort keys %votes;
print "  Total votes=$sum";

# Get random selections
my %popular;
my %impartial;
my %unpopular;
for ( 1 .. $sum ) {
    $popular{ popular() }++;       # should tend toward the popular
    $impartial{ impartial() }++;   # should be equally distributed
    $unpopular{ unpopular() }++;   # should tend toward the unpopular
}

# Display distribution of votes from each method
print "\nPOP: ";
printf "%s:%3d ", $_, $popular{$_}//0
    for sort keys %votes;

print "\nIMP: ";
printf "%s:%3d ", $_, $impartial{$_}//0
    for sort keys %votes;

print "\nUNP: ";
printf "%s:%3d ", $_, $unpopular{$_}//0
    for sort keys %votes;
say '';

# Method 1: choose randomly with preference for higher vote-getters
sub popular {
    my $rand = int( rand $sum );
    for my $key (
        # sort { ($votes{$b} <=> $votes{$a}) } #unnecessary
        keys %votes ) {
        return $key if ( $rand -= $votes{$key} ) <= 0;
    }
    die 'popular bug';
}

# Method 2: choose randomly without regard for popularity
sub impartial {
    return ( keys %votes )[ rand keys %votes ];
}

# Method 3: choose randomly with preference for lower vote-getters
sub unpopular {
    return '?';
}

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In reply to Randomly choosing from a set of alternatives with varying popularity by ibm1620

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