List comprehensions: Difference between revisions

There is no elegant way to comprise a list from multiple variables. Pythagorean triplets need three variables, with 1..n giving x, then x..n giving y, then y..n giving z. A less than elegant way is to nest three blocks.

=== Ruby 1.8.6 ===

{{works with|Ruby|1.8.6 or later}}

<lang ruby>n = 20

unless Enumerable.method_defined? :flat_map

module Enumerable

def flat_map

inject([]) { |a, x| a.concat yield(x) }

end

end

end

unless Array.method_defined? :keep_if

class Array

def keep_if

delete_if { |x| not yield(x) }

end

end

end

# select Pythagorean triplets

r = ((1..n).flat_map { |x|

(x..n).flat_map { |y|

(y..n).flat_map { |z|

[[x, y, z]].keep_if { x * x + y * y == z * z }}}})

p r # print the array _r_</lang>

This is the exact same code, except that it now defines Enumerable#flat_map and Array#keep_if when those methods are missing. It now works with older versions of Ruby, like 1.8.6.