How do I convert a comma-separated string into an array?

Use the split method to do it:

"one,two,three,four".split(',')
# ["one","two","three","four"]

If you want to ignore leading / trailing whitespace use:

"one , two , three , four".split(/\s*,\s*/)
# ["one", "two", "three", "four"]

If you want to parse multiple lines (i.e. a CSV file) into separate arrays:

require "csv"
CSV.parse("one,two\nthree,four")
# [["one","two"],["three","four"]]