Forum: Ruby find previous & next in array

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John G. (Guest)
on 2009-05-14 07:44
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?
Boby Selamet Hartono (Guest)
on 2009-05-14 07:49
(Received via mailing list)
2009/5/14 John G. <removed_email_address@domain.invalid>

> flash
>
> any ideas?
> --
> Posted via http://www.ruby-forum.com/.
>
>
a[a.index(:c)-1] or a[a.index(:c)+1]?
Joshua C. (Guest)
on 2009-05-14 10:55
(Received via mailing list)
Boby had it almost right!

instead of:

a[a.index(:c)+1]

it is:

a[a.index('c')+1]

:-)

On Wed, May 13, 2009 at 11:48 PM, Boby Selamet Hartono
Boby Selamet Hartono (Guest)
on 2009-05-14 11:35
(Received via mailing list)
2009/5/14 Joshua C. <removed_email_address@domain.invalid>

> :-)
> > > a = ['a','b','c','d']
> > > --
> > > Posted via http://www.ruby-forum.com/.
> > >
> > >
> > a[a.index(:c)-1] or a[a.index(:c)+1]?
> >
> > --
> > Tidak ada yang lebih baik dari kembali ke asal
> > Nothing can be better than back to the roots
> >
>

It was a copy from my irb. I'm use symbols  as objects in my array. I've
never use index method but since I figure out that method returns an
Integer
I think it would be something like that (thanks to irb). I'm a newbie
and
just joining this ML yesterday :-).
Robert K. (Guest)
on 2009-05-14 11:47
(Received via mailing list)
2009/5/14 John G. <removed_email_address@domain.invalid>:
> 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
John G. (Guest)
on 2009-05-14 19:38
> 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

thanks Robert, i was trying to build up a little function to find the
prev/next elements in an array, thanks to your help it's sorted;
couldn't find a solution online. appreciate it all.


John.
Joshua C. (Guest)
on 2009-05-14 21:35
(Received via mailing list)
Bobby, I am new too, so I might be missing something as well.

I tried your method first, and when I tried to do the symbol in irb it
gave
an error. Maybe you have settings I do not?
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