From: Robert Klemme <shortcutter@...>
Date: 2009-05-14T16:46:03+09:00
Subject: Re: find previous & next in array

2009/5/14 John Griffiths <indiehead@gmail.com>:
> hi, anyone know how to do this?
>
> basically i've got an array of items
>
> a = ['a','b','c','d']
>
> now given i can find the position of say 'c' using .index('c'), how do i
> get the positions of the ones previous & next to it.
>
> a recursive routine would probably do it, but looking for something more
> flash
>
> any ideas?

Depends on what you need to do.  Do you need those indexes or do you
only need access to those elements?

There are a few variants, but the nice ones aren't really safe against
edge cases (first and last element):

irb(main):004:0> a = ['a','b','c','d']
=> ["a", "b", "c", "d"]
irb(main):005:0> i = a.index('c') and a[i-1 .. i+1]
=> ["b", "c", "d"]
irb(main):006:0> a[a.index('c') - 1, 3] rescue nil
=> ["b", "c", "d"]
irb(main):007:0> i = a.index('a') and a[i-1 .. i+1]
=> []
irb(main):008:0> a[a.index('a') - 1, 3] rescue nil
=> ["d"]

The safest is probably

i = a.index('c')

if i
  predecessor = i > 0 ? a[i -1] : nil
  successor = a[i + 1] # safe!
end

Or as a variant

i = a.index('c') and begin
  predecessor = i > 0 ? a[i -1] : nil
  successor = a[i + 1] # safe!
end

Kind regards

robert

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