Question

I have a hash that maps an array of integers to an integer. For some reason the hash has one key mapped to multiple values like:

{[1,2]=>3, [1,2]=> 4}

How can I prevent this from happening? Running

for key, value in map
    puts key.inspect + "=>" + value.inspect + ":" + key.hash.inspect
end

gives me

[1, 2]=>11:11
[0, 4, 6, 8, 9]=>10:253
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]=>15:11189
[0, 3, 4, 6, 7, 8, 9]=>13:981
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]=>14:11189
[0, 1, 2, 4, 5, 6, 7, 8, 9]=>12:4661

Answer 1

I can think of two cases. The first has the obscure compare_by_identity switched on. Don’t know what to do about it. Don’t switch it on? Copy everything to a normal hash?

h={}
h.compare_by_identity
h[[1,2]]=2
h[[1,2]]=3
p h #=> {[1, 2]=>2, [1, 2]=>3}

The second case is more plausible: the key gets changed after being put in the hash.

h={}
h[[1,2,3]]=2
h[[1,2]]=3
h.keys.first.pop # assuming ruby 1.9
p h #=> {[1, 2]=>2, [1, 2]=>3}

That’s easy to remedy (but also easy to forget):

h.rehash
p h #=> {[1, 2]=>3}