Reduce Hash Values

Question

I am having trouble with the syntax for reduce. I have a hash of the following format:

H = {"Key1" => 1, "Key2" => 2}

I would like to use reduce to find the sum of the values in this function.

Something Like

H.reduce(0) {|memo, elem| memo+=elem}

I know this is wrong. I dont understand how I can make elem the value of the hash.

Solution 1

Use Enumerable#reduce, if you're ok with getting nil if the hash happens to be empty:

H.values.reduce(:+) # => 3
Hash.new.values.reduce(:+) # => nil

To safely get 0 when the hash is empty, use:

H.values.reduce(0) { |sum,x| sum + x } # or...
H.reduce(0) { |sum,(key,val)| sum + val } # ...if you need to inspect the key

Here's a quick benchmark, for kicks. Note that it appears to be slightly faster to reduce just the values rather than values from the key/value pairs:

                               user     system      total        real
H.values.reduce(:+)        4.510000   0.080000   4.590000 (  4.595229)
H.values.reduce(0) {...}   4.660000   0.080000   4.740000 (  4.739708)
H.reduce(0) {...}          5.160000   0.070000   5.230000 (  5.241916)

require 'benchmark'

size = 1_000
hash = Hash[* Array.new(size*2) { rand } ]

N=10_000
Benchmark.bm(24) do |x|
  x.report('H.values.reduce(:+)')      { N.times { hash.dup.values.reduce(:+) } }
  x.report('H.values.reduce(0) {...}') { N.times { hash.dup.values.reduce(0) { |sum,x| sum + x } } }
  x.report('H.reduce(0) {...}')        { N.times { hash.dup.reduce(0) { |sum,(_,v)| sum + v } } }
end

Solution 2

You can make elem contain the value by splitting it up in 2 variables:

H.reduce(0) {|memo, (key, val)| memo + val}

Solution 3

Try this:

H.reduce(0) { |memo, elem| memo += elem[1] }

or

H.reduce(0) { |memo, (key, value)| memo += value }

Solution 4

I know I'm excavating this one, but if you happen to use Rails, the .sum method can help:

H = {"Key1" => 1, "Key2" => 2}
=> {"Key1"=>1, "Key2"=>2}
> H.values.sum
=> 3

Advantage is that it returns 0 on empty hashes:

> {}.values.sum
=> 0
> {}.values.reduce(:+)
=> nil

I noticed it was Rails-specific only after typing this answer. I know the OP didn't add the Rails tag, but I figured it might be useful for people stopping by.

Note that as of Ruby 2.4.0, .sum is now available.

View more solutions

Comments

[Josh Bodah]

Note that this doesn't return 0 if a hash is empty. You'd need to use the memo version below

[Ludovic Kuty]

I would have written memo + val to avoid letting people think that updating memo is helpful.

[Ludovic Kuty]

I would have written memo + value to avoid letting people think that updating memo is helpful.

[user3162553]

.sum is now implemented in Ruby 2.4 Enumerable#sum.

[Pierre]

Strongly agreeing with @lkuty, the += mutation of memo is unnecessary and misleading.