I'm trying to group by keys in an Array of Hashes in the following form and add their totals. Maybe with #map and #reduce/#inject. I know there is a more compact way to do this.

Given I have

codes_and_totals = [{'1001' => 153, '212' => 153}, {'212' => 1}]

When I run


Then the return value should be

=> {'1001' => 153, '212' => 154}

Currently #process_codes_and_totals looks like this

def process_codes_and_totals(codes_and_totals)
    totals_across_locations = {}
    codes_and_totals.each do |location|
      location.each do |class_code, total|
        totals_across_locations[class_code] ||= 0
        totals_across_locations[class_code] += total

  • 1
    \$\begingroup\$ Please try to edit your title to reflect what your code is doing. Saying you want better code in Code Review is redundant. \$\endgroup\$
    – Marc-Andre
    Commented Aug 1, 2016 at 14:29
  • \$\begingroup\$ Thanks for the feedback @Marc-Andre, let me know if there is anything else I can do to make this clearer/better. \$\endgroup\$
    – David West
    Commented Aug 1, 2016 at 14:40
  • \$\begingroup\$ "Better", "more compact", "faster", etc. are all implied for questions on Code Review. The site standard is to simply state exactly what the code accomplishes. See How to Ask. \$\endgroup\$ Commented Aug 1, 2016 at 14:53
  • \$\begingroup\$ I have rolled back Rev 4 to 3. Please see What to do when someone answers. \$\endgroup\$ Commented Aug 1, 2016 at 17:55
  • \$\begingroup\$ As a note, some people recommend using #each_with_object and a blank hash as the argument. More info is here technology.customink.com/blog/2014/10/14/…. \$\endgroup\$
    – David West
    Commented Aug 1, 2016 at 22:54

1 Answer 1


One way might be to use #reduce (as you suggest) and Hash#merge with a block:

totals = codes_and_totals.reduce({}) do |sums, location|
  sums.merge(location) { |_, a, b| a + b }

The merge-block is only invoked if a key exists in both the hashes being merged, so we don't need to guard against nil or start with a zero value. If the block runs, it's because there are two number values that need to be merged (i.e. added, in this case).

  • \$\begingroup\$ What is the _ for as the first block argument to the merge? \$\endgroup\$
    – David West
    Commented Aug 1, 2016 at 14:31
  • 4
    \$\begingroup\$ _ is the conventional name for a "throw-away" variable whose value you don't care about. \$\endgroup\$ Commented Aug 1, 2016 at 14:32
  • \$\begingroup\$ Is there a way to do this for nested hashes? \$\endgroup\$
    – Kelseydh
    Commented Feb 28, 2023 at 2:22

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