Question

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'.

Answer 1

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