From: "David A. Black" <dblack@...>
Date: 2008-03-25T12:55:35+09:00
Subject: Re: hash of arrays - appending to one of the arrays

Hi --

On Tue, 25 Mar 2008, Adam Akhtar wrote:

> Hi Ive done a search on this and there have been quite a few posts which
> have incresed my knowlege somewhat but still have a few questions.
>
> Im creating a hash of arrays. The keys and values are added within a
> loop. For each key there may be multiple values hence the need for an
> array as a value. Values for a key are found over interations of a loop.
> I therefore have to append a new found value to the existing values in
> an array.
>
> I tried doing this
>
> something.each do |blah|
> if (test something)
>    hash[key] = hash[key] + value
> end
> end
>
> but the compliler complains about there being no method for nil class.
>
> I read the basic way to do this would be
>
> hash[key] = [] #create blank array
> hash[key] = hash[key] + value
>
> but as im in a loop that would require me to do a check to see if the
> key exists  or not. If im processing lots of informatino that would slow
> me down a bit.
>
> IS there a slicker way of doing this?

The most common idiom is:

   (hash[key] ||= []) << value

You can also make a hash that has a default behavior, for non-existent
keys, of putting an array at that key:

irb(main):004:0> hash = Hash.new {|h,k| h[k] = [] }
=> {}
irb(main):005:0> hash[1] << 2
=> [2]
irb(main):006:0> hash
=> {1=>[2]}


David

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