CODEWITHSUNDEEP
rubyarrayshash
Xiao Jia
Xiao Jia

Reputation: 4259

How do I map an array of hashes?

I have an array of hashes:

arr = [ {:a => 1, :b => 2}, {:a => 3, :b => 4} ]

What I want to achieve is:

arr.map{|x| x[:a]}.reduce(:+)

but I think it's a bit ugly, or at least not that elegant as:

arr.map(&:a).reduce(:+)

The later one is wrong because there is no method called a in the hashes.

Are there any better ways to write map{|x| x[:a]}?

Upvotes: 7

Views: 9940

Answers (4)

Jimbali
Jimbali

Reputation: 2518

Ruby 2.7 added a shorthand block syntax where you can use _1, _2 etc to access the params passed to a block, so you can do something like this:

arr.map { _1[:a] }.reduce(:+)

Edit:

In Ruby 3.4 (which is in preview at the time of writing) you can use it instead of _1:

arr.map { it[:a] }.reduce(:+)

https://bugs.ruby-lang.org/issues/18980

Upvotes: 2

wscourge
wscourge

Reputation: 11281

There's a way, extending the Symbol.

lib/core_extensions/symbol.rb (credit goes here)

# frozen_string_literal: true

class Symbol
  def with(*args, &)
    ->(caller, *rest) { caller.send(self, *rest, *args, &) }
  end
end

Then, given:

arr = [ {:a => 1, :b => 2}, {:a => 3, :b => 4} ]

you can do this:

arr.map(&:[].with(:a)).reduce(:+)

Explanation: to access hash value under any key, you call Hash#[] method. When passed as a :[] (extended) symbol to the Array#map, you can then call .with(*args) on this symbol, effectively passing the parameter (hash key) down to the :[] method. Enjoy.

Upvotes: 0

Andrew Marshall
Andrew Marshall

Reputation: 96914

You could make actual Objects, possibly with a Struct:

MyClass = Struct.new :a, :b
arr = [MyClass.new(1, 2), MyClass.new(3, 4)]
arr.map(&:a).reduce(:+)  #=> 4

Or for more flexibility, an OpenStruct:

require 'ostruct'
arr = [OpenStruct.new(a: 1, b: 2), OpenStruct.new(a: 3, b: 4)]
arr.map(&:a).reduce(:+)  #=> 4

Of course either of these can be constructed from existing hashes:

arr = [{ :a => 1, :b => 2 }, { :a => 3, :b => 4 }]

ss = arr.map { |h| h.values_at :a, :b }.map { |attrs| MyClass.new(*attrs) }
ss.map(&:a).reduce(:+)  #=> 4

oss = arr.map { |attrs| OpenStruct.new attrs }
oss.map(&:a).reduce(:+)  #=> 4

Or, for a more creative, functional approach:

def hash_accessor attr; ->(hash) { hash[attr] }; end
arr = [{ :a => 1, :b => 2 }, { :a => 3, :b => 4 }]
arr.map(&hash_accessor(:a)).reduce(:+)  #=> 4

Upvotes: 4

Sergey Bolgov
Sergey Bolgov

Reputation: 806

It is unclear what you mean as "better" and why you think the correct version is ugly.

Do you like this "better"?

arr.inject(0) { |sum, h| sum + h[:a] }

Upvotes: 0

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