From: Josh Cheek <josh.cheek@...>
Date: 2010-06-15T23:35:38+09:00
Subject: Re: Unexpected behavior in inject

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On Tue, Jun 15, 2010 at 9:21 AM, Alex Fort <imphasing@gmail.com> wrote:

> I ran across something that puzzled me today, and I thought I'd ask here
> and
> see what you guys/girls think. I'm by no means a ruby guru, so if this is
> obvious to everyone but me, I'll take the learning experience.
>
>
> Here's the simple script I was testing out:
>
> [1,2,3].inject(0) do |acc, num|
>  acc + num
> end
>
>
Let me give you a slightly different example to show how it works:  You pass
in the zero, and it goes into acc in the block. Whatever the block returns
is then fed back into acc, for the next iteration. After the last iteration,
whatever the block returns is returned by the inject method. I think of it
as a number passing through an Enumerable and coming out the other side
modified.

(5..10).inject 0 do |sum,num|
  sum + num
end

This should return 45 in the following manner:
The first time the block is passed 0 , 5 and it returns 5
The second time the block is passed 5 , 6 and it returns 11
The third time the block is passed 11 , 7 and it returns 18
The fourth time the block is passed 18 , 8 and it returns 26
The fourth time the block is passed 26 , 9 and it returns 35
The fourth time the block is passed 35 , 10 and it returns 45
The method then returns 45



So why doesn't this work?

[1,2,3].inject(0) do |acc, num|
>  acc + num
>  puts acc
> end
>

Because the puts method returns nil. It is also the last thing in the block,
so the block returns nil. So nil is fed back into acc. Then nil + acc raises
the NoMethodError.

To resolve this, think about where you could put the puts method that won't
cause the block to return nil.


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If you are interested, here is a functional version (takes the enumerable as
a parameter) of inject that I wrote, except I call ti "passthrough" because
that is more meaningful for me.

def passthrough( enumerable , to_pass )
  enumerable.each do |element|
    to_pass = yield to_pass , element
  end
  to_pass
end

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