3/28/15 - Permutations in Ruby and Python

Right at the end of class on Friday I finished my ruby permutations implementation, but didn't get a chance to blog it.

	def permutations(list)
	return [list] if list.size == 1
	result = []
	list.each do |locked|
	list_copy = list.dup
	list_copy.delete locked
	permutations(list_copy).each do |permutation|
	result << permutation.insert(0, locked)
	end
	end
	result # this implicitly returns result. Initially I forgot to add it in and got weird results until I realized this was missing
	end
	if __FILE__ == $0
	p permutations %w[a b c] # %w is ruby shorthand for an array of strings, this example creates the array: ['a', 'b', 'c']
	p permutations %w[d e f g]
	end

view raw node_permutations_ruby.rb hosted with ❤ by GitHub

Sam also wrote a python version and explained the yield statement to me. I even wrote about it in Thursday's post, but I still don't really understand what is happening here. I'll try to get around to looking a bit more closely at the code and figure out how it works.

	# Sam's python implementation
	def permutations(listt): # apparently 'list' is a reserved keyword in python (or at least is a different color in the editor)
	if len(listt) == 1:
	yield listt
	return
	else:
	for i, o in enumerate(listt):
	listt_copy = listt.copy()
	del listt_copy[i]
	for perm in permutations(listt_copy):
	yield [o] + perm # still not quite clear on how the yield statements work here

view raw node_permutations_python.py hosted with ❤ by GitHub