From: Luther <lutherfire@...>
Date: 2006-11-15T16:13:43+09:00
Subject: Re: Design problem with 'inject'

--- Gary Boone <dr@garyboone.com> wrote:
> Ruby's inject has a design that can lead to hard to find bugs. The 
> problem is that you don't have to specify what is summed; the value
> of 
> the block is summed. That means 'next' can lead to a bug. Here's an
> 
> example:
> 
> No problem:
> 
>    arr.inject(0) do |sum, i|
>                     sum += i
>                  done

You should use a '+' instead of a '+='. The return value of the block
will be assigned to the next iteration's value for 'sum', so it
doesn't make sense to modify 'sum' inside the block.

> But suppose you need to skip some elements, so you add a 'next' 
> statement.
> Problem:
> 
>    arr.inject(0) do |sum, i|
>                    next if (i==3)
>                    sum += i
>                  done

Try it like this:

arr.inject(0) do |sum, i|
  if i == 3
    sum
  else
    sum + i
  end
end

Any time you want to skip an element, simply return 'sum'. This will
carry the value from the last iteration on over to the next
iteration, doing nothing with the current element. As far as I can
see, you should never have to use 'next' in an inject block, unless
you really want the intermediate result to be nil.

Luther Thompson


 
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