Read the answers in the duplicate questions for the meaning and usage of &:.... In this case, entries is an array, and there are three methods map, sort_by, and map chained. sort_by(&:last) is equivalent to sort_by{|x| x.last}. map(&:first) is the same as map{|x| x.first}. The reason the first map does not use &:... is because (i) the receiver of accept_entry is not e, and (ii) it takes an argument e.
