Ruby: Creating a hash key and value from a variable in Ruby

Question

I have a variable id and I want to use it as a key in a hash so that the value assigned to the variable is used as key of the hash.

For instance, if I have the variable id = 1 the desired resulting hash would be { 1: 'foo' }.

I've tried creating the hash with,

{
  id: 'foo'
}

But that doesn't work, instead resulting in a hash with the symbol :id to 'foo'.

I could have sworn I've done this before but I am completely drawing a blank.

Solution

If you want to populate a new hash with certain values, you can pass them to Hash::[]:

Hash["a", 100, "b", 200]             #=> {"a"=>100, "b"=>200}
Hash[ [ ["a", 100], ["b", 200] ] ]   #=> {"a"=>100, "b"=>200}
Hash["a" => 100, "b" => 200]         #=> {"a"=>100, "b"=>200}

So in your case:

Hash[id, 'foo']
Hash[[[id, 'foo']]]
Hash[id => 'foo']

The last syntax id => 'foo' can also be used with {}:

{ id => 'foo' }

Otherwise, if the hash already exists, use Hash#=[]:

h = {}
h[id] = 'foo'

Comments

[Arup Rakshit]

I was confused with question itself, now this answer made me double confused. :( Could you tell me what was OP asking?

[Gumbo]

@ArupRakshit OP wants to use the value of id as key. { id: 'foo' } doesn’t work as id: 'foo' is equivalent to :id => 'foo'.

[James McMahon]

Ah so the old style hash syntax works, but not the new style. Thank you.

[mu is too short]

@James: The JavaScript style only works with a limited set of symbols and only works in Hash literals. You can't use the JavaScript style if the key not a symbol (with a limited format).

[Ray Toal]

What's really cool is mixing the styles: x = :a; puts ({y: 3, x => 5})

[Fabrizio Bertoglio]

in Ecmascript 6 you can build hashes using variables like a = {"a": 100} and b = {"b": 200} then {a, b} will be => {"a": 100, "b": 200} but I don't believe this feauture exist in ruby

[Promise Preston]

Also, If you want to use a key and value that are variables you can use: MyHash = { id => foo, id2 => foo2 } OR MyHash = Hash[id, foo, id2, foo2]. If only the key is a variable then use: MyHash = { id => 'foo', id2 => 'foo2' } OR MyHash = Hash[id, 'foo', id2, 'foo2']

[pmrotule]

You can also write it with string interpolation: h = { "#{id}" => 'foo' }