Home » Ruby » Rails mapping array of hashes onto single hash

Rails mapping array of hashes onto single hash

Posted by: admin November 30, 2017 Leave a comment


I have an array of hashes like so:

 [{"testPARAM1"=>"testVAL1"}, {"testPARAM2"=>"testVAL2"}]

And I’m trying to map this onto single hash like this:

{"testPARAM2"=>"testVAL2", "testPARAM1"=>"testVAL1"}

I have achieved it using

  mitem["params"].each { |h| h.each {|k,v| par[k]=v} } 

But I was wondering if it’s possible to do this in a more idiomatic way (preferably without using a local variable).

How can I do this?


You could compose Enumerable#reduce and Hash#merge to accomplish what you want.

input = [{"testPARAM1"=>"testVAL1"}, {"testPARAM2"=>"testVAL2"}]
input.reduce({}, :merge)
  is {"testPARAM2"=>"testVAL2", "testPARAM1"=>"testVAL1"}

Reducing an array sort of like sticking a method call between each element of it.

For example [1, 2, 3].reduce(0, :+) is like saying 0 + 1 + 2 + 3 and gives 6.

In our case we do something similar, but with the merge function, which merges two hashes.

[{:a => 1}, {:b => 2}, {:c => 3}].reduce({}, :merge)
  is {}.merge({:a => 1}.merge({:b => 2}.merge({:c => 3})))
  is {:a => 1, :b => 2, :c => 3}


How about:

h = [{"testPARAM1"=>"testVAL1"}, {"testPARAM2"=>"testVAL2"}]
r = h.inject(:merge)


Use #inject

hashes = [{"testPARAM1"=>"testVAL1"}, {"testPARAM2"=>"testVAL2"}]
merged = hashes.inject({}) { |aggregate, hash| aggregate.merge hash }
merged # => {"testPARAM1"=>"testVAL1", "testPARAM2"=>"testVAL2"}