Ruby inject with initial being a hash

Question

Can any one tell me why the following:

['a', 'b'].inject({}) {|m,e| m[e] = e }

throws the error:

IndexError: string not matched
        from (irb):11:in `[]='
        from (irb):11:in `block in irb_binding'
        from (irb):11:in `each'
        from (irb):11:in `inject'
        from (irb):11
        from C:/Ruby192/bin/irb:12:in `<main>'

whereas the following works?

a = {}
a["str"] = "str"

Answer 1

Your block needs to return the accumulating hash:

['a', 'b'].inject({}) {|m,e| m[e] = e; m }

Instead, it's returning the string 'a' after the first pass, which becomes m in the next pass and you end up calling the string's []= method.

Answer 2

The block must return the accumulator (the hash), as @Rob said. Some alternatives:

With Hash#update :

hash = ['a', 'b'].inject({}) { |m, e| m.update(e => e) }

With Enumerable#each_with_object :

hash = ['a', 'b'].each_with_object({}) { |e, m| m[e] = e }

With Hash#[] :

hash = Hash[['a', 'b'].map { |e| [e, e] }]

With Array#to_h (Ruby >= 2.1):

hash = ['a', 'b'].map { |e| [e, e] }.to_h

With Enumerable#mash from Facets:

require 'facets'
hash = ['a', 'b'].mash { |e| [e, e] }

Answer 3

Rather than using inject, you should look into Enumerable#each_with_object .

Where inject requires you to return the object being accumulated into, each_with_object does it automatically.

From the docs:

Iterates the given block for each element with an arbitrary object given, and returns the initially given object.

If no block is given, returns an enumerator.

eg:

evens = (1..10).each_with_object([]) {|i, a| a << i*2 }
#=> [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

So, closer to your question:

[1] pry(main)> %w[a b].each_with_object({}) { |e,m| m[e] = e }
=> {"a"=>"a", "b"=>"b"}

Notice that inject and each_with_object reverse the order of the yielded parameters.