重み付き乱択アルゴリズム ref: http://qiita.com/items/6d20c2571dac55e4de4c

make_random_picker.rb

#! ruby
class DUP
    def initialize(name, age)
        @name  = name
        @age = age
    end
    attr_accessor :name,:age
end

def make_random_picker(dup)
    dup.sort{|a,b| a.age <=> b.age}
    age_sum = 0
    dup.each do |d|
        age_sum +=  d.age
    end
    dup.each do |d|
        d.age /= age_sum
    end
    r = rand()
    sum = 0
    dup.each do |d|
        sum += d.age
        return d if r < sum
    end
    return dup[dup.length - 1]
end

20.times do
    p make_random_picker([DUP.new("digiko", 10.0),
                          DUP.new("usada",  14.0),
                          DUP.new("puchiko", 5.0),
                          DUP.new("piyoko", 7.0),
                          DUP.new("rinna", 10.0),
                          DUP.new("mike", 10.0)]).name
end

result.sh

    274 "usada"
    179 "digiko"
    171 "rinna"
    171 "mike"
    130 "piyoko"
     75 "puchiko"