Ruby: Array slice out of range?

Rails
1

Hello,

I am noodling with the Ruby Koans and I ran across an Array slice
behavior that I just can’t quite wrap my brain around:

ruby-1.9.2-p180 :102 > array = [:peanut, :butter, :and, :jelly]
=> [:peanut, :butter, :and, :jelly]
ruby-1.9.2-p180 :103 > array[4,0]
=> []
ruby-1.9.2-p180 :104 > array[5,0]
=> nil

from class Array - RDoc Documentation
“Returns nil if the index (or starting index) are out of range.”

I don’t understand why the 4th element is an empty array, whereas the
5th element is out of range. I would expect the 4th element to be out
of range as ‘array’ contains elements 0-3.

Can someone plz explain?
Thanks!
-Tk

2

On 11 Aug 2011, at 17:03, viciousfish wrote:

from class Array - RDoc Documentation
“Returns nil if the index (or starting index) are out of range.”

I don’t understand why the 4th element is an empty array, whereas the
5th element is out of range. I would expect the 4th element to be out
of range as ‘array’ contains elements 0-3.

You’re right – this behaviour is a bit strange, and doesn’t seem to
match that documentation. I was about to say this looks like a bug in
Ruby, but then I checked the 3rd edition of Programming Ruby, which has
this to say about the #slice method:

“Returns nil if the index of the first element selected is greater than
the array size. If the start index equals the array size and a length or
range parameter is given, an empty array is returned.”

This matches the behaviour you see, so it seems somebody is at least
aware of this, and it’s perhaps even intentional. Might be worth posting
the query to the Ruby mailing list, to see if anyone knows why #slice is
designed this way.

Chris

3

viciousfish wrote in post #1016211:

Hello,

I am noodling with the Ruby Koans and I ran across an Array slice
behavior that I just can’t quite wrap my brain around:

ruby-1.9.2-p180 :102 > array = [:peanut, :butter, :and, :jelly]
=> [:peanut, :butter, :and, :jelly]
ruby-1.9.2-p180 :103 > array[4,0]
=> []
ruby-1.9.2-p180 :104 > array[5,0]
=> nil

from class Array - RDoc Documentation
“Returns nil if the index (or starting index) are out of range.”

I don’t understand why the 4th element is an empty array, whereas the
5th element is out of range. I would expect the 4th element to be out
of range as ‘array’ contains elements 0-3.

Can someone plz explain?

Slices are different than single indexes: array[4, 0] and array[4] point
to two different spots in the array. The spot that array[4,0] points to
is just inbounds, while the spot that array[4] points to is out of
bounds. See here:

http://www.ruby-forum.com/topic/1393096

4

On 12 Aug 2011, at 01:07, 7stud – wrote:

=> nil
is just inbounds, while array[4] is out of bounds. See here:

String/array slices - Ruby - Ruby-Forum

Thanks for the link – a very useful explanation, and I’d somehow
managed to miss this point despite years of Rubying!

I wonder if it might be worth a little patch to the Ruby API docs to
clarify the semantics of array range access. May take this conversation
to the Ruby list.

Chris