Question

this should be pretty easy for ruby wizards here. I’m having a problem with inject. This is it simply :

a = Resource.all
a.inject({ :wood => 0 }) { |res, el| res[:wood] + el.cost(1)[:wood] }
TypeError: can't convert Symbol into Integer

a is a collection and i would like to create a sum of all the wood resources of this collection. The el.cost(1)[:wood] works fine and gets an integer (resources value). So this part is correct. It seems that i have a problem with initializing my new hash with the :wood symbol and setting that value in each iteration, but i can’t really find the problem.

Any ideas ?

Answer 1

inject works like this:

1. take initialization value, pass it into the lambda with the first element in the list. Use the result of the lambda as the new accumulator
2. pass that new accumulator into the lambda together with the next element in the list. Use the result of the lambda as the new accumulator
3. And so on…

So what you have to do in the lambda is:

1. Take the hash in res.
2. Modify it.
3. Return the hash.

You fail to do 2 and 3, that’s why that code doesn’t work. Try the following:

a.inject({ :wood => 0 }) { |res, el| res[:wood] += el.cost(1)[:wood]; res }

This is however a bit redundant. You can easily accumulate the integers first and then create a hash:

{ :wood => a.map { |el| el.cost(1)[:wood] }.reduce(0, :+) }