Be Genius

Fork Me
Mugshot

Hi, I'm Bodaniel Jeanes.

I'm a Ruby developer from Brisbane, Australia

I work at Mocra where I hack on awesome code. Follow me, recommend me, and link with me.

Gotcha with Hash.new when providing a default value

For a project I am working on at the moment, I needed a Hash that returned a different default value, i.e. not nil. Specifically, I needed it to return another Hash, and for that internal Hash’s default value to be an Array, like so:

1
2
3
4
hash = # ... p hash.class #=> Hash p hash[:non_existant_key].class #=> Hash p hash[:another][:non_existant_key].class #=> Array

Naïvely, I threw this into my code, and went along my merry way: entries = Hash.new(Hash.new([])). A few days later, I came back to this code to finish it and realised that something was awry with the values I was getting out of the Hash. It took me a while to figure it out because the values were all integers being summed together and I was never looking or setting the values directly.

However, eventually I saw a pattern — many of the values were the same. For instance, given the following assignments, I should expect p entries[153][:monday] #=> [12.hours]:

1
2
3
4
entries[153][:monday] << 12.hours entries[153][:tuesday] << 3.hours entries[87][:monday] << 7.hours entries[87][:tuesday] << 2.5.hours

However, what I found was the following:

1
2
3
4
5
6
7
p entries[153][:monday] #=> [12.hours, 3.hours, 7.hours, 2.5.hours] p entries[153][:tuesday] #=> [12.hours, 3.hours, 7.hours, 2.5.hours] p entries[87][:monday] #=> [12.hours, 3.hours, 7.hours, 2.5.hours] p entries[87][:tuesday] #=> [12.hours, 3.hours, 7.hours, 2.5.hours] # And in fact: p entries[:any][:thing] #=> [12.hours, 3.hours, 7.hours, 2.5.hours]

Let that soak in for a second. Hash.new({}) uses the same instance of the internal Array as the default value for each of the inner-most keys. It also doesn’t set the key you want so p entries still printed {}. In retrospect, it is blatantly obvious that it does this, but the ramifications are still huge.

The way to get the intended effect is of course to use the block syntax of Hash.new which, while much uglier, definitely works as expected:

1
2
3
4
5
6
7
entries = Hash.new {|hash, key| hash[key] = Hash.new {|h, k| h[k] = [] }} entries[153][:monday] << 12.hours entries[153][:tuesday] << 3.hours entries[87][:monday] << 7.hours entries[87][:tuesday] << 2.5.hours p entries #=> {87=>{:monday=>[7.hours], :tuesday=>[2.5.hours]}, 153=>{:monday=>[12.hours], :tuesday=>[3.hours]}}

Wow. Obvious, but good to remember. I am sure most of you have had this issue before, but I think it is still worthy of mention…