Transform 2D array into hash

Question

I have a sorted 2D array as follows:

a = [[1, 2011], [3, 1999], [4, 2014], [6, 1998], ...]

How can I transform this into a hash with the key being the year and the value being the number in front?

{2011 => 1, 1999 => 3, 2014 => 4, 1998 => 6, ...}

Answer 1

[[1, 2011], [3, 1999], [4, 2014], [6, 1998]].map(&:reverse).to_h
  # => {2011=>1, 1999=>3, 2014=>4, 1998=>6}

For older versions of Ruby, you could use:

Hash[a.map(&:reverse)]

Answer 2

Here are a couple of other ways that should be 1.8.7-friendly:

a = [[1, 2011], [3, 1999], [4, 2014], [6, 1998]]

v, y = a.transpose
Hash[y.zip(v)]
  #=> {2011=>1, 1999=>3, 2014=>4, 1998=>6}

a.reduce({}) { |h,(v,y)| h.update({ y=>v }) }
  #=> {2011=>1, 1999=>3, 2014=>4, 1998=>6}

Hash#update (aka merge!) could be replaced with Hash#merge.

Answer 3

Here is one more way.

a.each.with_object({}) {|(v,k), h| h[k] = v}