CODEWITHSUNDEEP
rubyhashmap
meandre
meandre

Reputation: 2329

What are values mapped to?

I don't understand how Ruby hashes work.

I expect these:

a = 'a'
{a => 1}[a] # => 1
{a: 1}[:a] # => 1
{2 => 1}[2] # => 1

How does this work?

{'a' => 1}['a'] # => 1

The first string 'a' is not the same object as the second string 'a'.

Upvotes: 0

Views: 58

Answers (3)

steenslag
steenslag

Reputation: 80075

As a footnote to other answers, you can let a hash behave like you expected:

h = {'a'=> 1}
p h['a'] #=> 1
h.compare_by_identity
p h['a'] #=> nil ; not the same object

Upvotes: 3

Frederick Cheung
Frederick Cheung

Reputation: 84132

Ruby doesn't use object equality (equal?) for comparing hash keys. It wouldn't be very useful if it did after all.

Instead it uses eql?, which for strings is the same as ==

Upvotes: 3

Sergio Tulentsev
Sergio Tulentsev

Reputation: 230461

some_hash[k] = v

Basically, when you do this, what is stored is not a direct association k => v. Instead of that, k is asked for a hash code, which is then used to map to v.

Equal values yield equal hash codes. That's why your last example works the way it does.

A couple of examples:

1.9.3p0 :001 > s = 'string'
 => "string" 
1.9.3p0 :002 > 'string'.hash 
 => -895223107629439507 
1.9.3p0 :003 > 'string'.hash == s.hash
 => true 
1.9.3p0 :004 > 2.hash
 => 2271355725836199018 
1.9.3p0 :005 > nil.hash
 => 2199521878082658865

Upvotes: 1

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