box-o-sand/cookbook/007/02.rb

require 'pp'

def call_twice
  puts "Calling your block."
  ret1 = yield("very first")
  puts "The value of your block: #{ret1}"

  puts "Calling your block again."
  ret2 = yield("second")
  puts "The value of your block: #{ret2}"
end

call_twice do |which_time|
  puts "I'm a code block, called for the #{which_time} time."
  which_time == "very first" ? 1 : 2
end


class Hash
  def find_all
    new_hash = Hash.new
    each { |k,v| new_hash[k] = v if yield(k, v) }
    new_hash
  end
end

squares = {0=>0, 1=>1, 2=>4, 3=>9}
print "squares = "
pp squares
(squares.find_all { |key, value| key > 1 }).each { |k,v| puts "#{k}: #{v}" }