From: Eric Hodel <drbrain@...7.net>
Date: 2009-04-04T07:25:55+09:00
Subject: Re: Array#insert with negative index

On Apr 3, 2009, at 15:09, matt neuburg wrote:
> Eric Hodel <drbrain@segment7.net> wrote:
>> On Apr 3, 2009, at 12:49, matt neuburg wrote:
>>
>>> p [1,2,3].insert(-1, "howdy")
>>> #=> [1, 2, 3, "howdy"]
>>>
>>> Docs: "Inserts the given values before the element with the given
>>> index
>>> (which may be negative)."
>>>
>>> The element with index -1 is 3, the last item. Before is before. I
>>> have
>>> a feeling, though, that what's meant here is that when the index is
>>> negative, before means after (perhaps because we're looking at the
>>> array
>>> from the other end, as it were). Is that right?
>>
>> array: [  1,  2,  3  ]
>> index:   0   1   2  3
>>         -4  -3  -2 -1
>>
>> So -1 is "one before the end", etc.
>
> No, I don't follow.  I don't get what your diagram is a diagram of. In
> an array...
>
> arr = [1,2,3]
>
> ...the "3" is arr[2]. It is also arr[-1]. There is no arr[3] or  
> arr[-4].

Yes there is:

irb(main):001:0> [1, 2, 3].insert -4, 'x'
=> ["x", 1, 2, 3]
irb(main):002:0> [1, 2, 3].insert 3, 'x'
=> [1, 2, 3, "x"]

> The indices don't designate spaces between the items; they designate  
> the
> items. m.

They indicate the insertion point for Array#insert