Question

I’m reading through ‘the ruby programming language’ and have come across the following piece of code.

a.inject do | sum, x |
    sum + x
end

I understand that x contains one element from a that will be passed to the bock but the ‘sum’ part is confusing me, is it some kind of operator or function or is it just another variable like x. I understand that the x variable could be called f, or v but if thats the case how does ruby know that x is that variable to pass the element to the iterator in, surly it could just as easily pass the variable into sum (if it is even a variable).

I hope that makes sense, as always I’m finding it had to put my confusion into works as if I understood the nature of the problem better I would be more likley to be able to find my own solution.

Thanks!

Answer 1

sum is the accumulator object, for example the following code:

(1..10).inject(0) do |sum, num| #sum is initialized to 0 and passed in as the accumulator
    sum + num #since sum is the accumulator, the result of this addition is stored in sum
end

The above code is the same as:

sum = 0
(1..10).each do 
    |num|
    sum = sum + num
end

You can give the accumulator object any valid variable name, of course better with a meaningful name in the context.