Why does this array slice return an empty array instead of nil?

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Array slicing in Ruby: looking for explanation for illogical behaviour (taken from Rubykoans.com)

I've been playing around with array slicing in ruby but I don't understand the last 2 results below:

a = [1,2,3]
a[2,1]      # output is [3]
a[3,1]      # output is []    ...why??
a[4,1]      # output is nil   ...according to the docs this makes sense

Why would a[3,1] be an empty array while a[4,1] is nil?

If anything, I would expect a[3,1] to return nil as well. According to the ruby docs an array split should return nil if the start index is out of range. So why is it that a[3,1] is not returning nil?

Note: This is on ruby 1.9.2

score 5 · Accepted Answer · answered Apr 07 '11 at 16:37

5

You're asking for the end of the array, which is []. Look at it this way: the Array [1,2,3] can be considered to be constructed from cons cells as such: (1, (2, (3, ())), or 1:2:3:[]. The 3rd index (4th item) is then clearly [].

answered Apr 07 '11 at 16:37

Rein Henrichs

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score 2 · Answer 2 · edited May 23 '17 at 12:04

2

" Returns nil if the index (or starting index) are out of range."

a[3,1] is a special case according to the example in the same link.

more info can be seen here by the way: Array slicing in Ruby: looking for explanation for illogical behaviour (taken from Rubykoans.com)

edited May 23 '17 at 12:04

Community

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answered Apr 07 '11 at 16:28

corroded

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oh wait hmmm yeah that is strange lol – corroded Apr 07 '11 at 16:29
hehe yeah. Doing a[3] returns nil so I know it's out of range. – Dty Apr 07 '11 at 16:31
it does have the same example as yours, look at a[5,1], also returns empty array – corroded Apr 07 '11 at 16:31
I'm using 1.9.2 and for a[5,1] I get nil. – Dty Apr 07 '11 at 16:33
it says that it is a special case apparently. using a[a.size,1] will return an empty array – corroded Apr 07 '11 at 16:34
+1 for the good link. Some answers there were convincing. – sawa Apr 07 '11 at 18:01

score 0 · Answer 3 · answered Apr 07 '11 at 16:37

0

It's easier to understand why if you imagine the slice on the left side of an assignment.

>> (b = a.clone)[2,1] = :a; p b
[1, 2, :a]
>> (b = a.clone)[2,0] = :b; p b
[1, 2, :b, 3]
>> (b = a.clone)[3,1] = :c; p b
[1, 2, 3, :c]
>> (b = a.clone)[3,0] = :d; p b
[1, 2, 3, :d]

answered Apr 07 '11 at 16:37

DigitalRoss

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Is this just an alternate way to push to the array? – Dty Apr 07 '11 at 16:52
do you know of a situation when something like this would be useful? Right now I'm more curious about the WHY than the HOW. – Dty Apr 19 '11 at 05:09

Why does this array slice return an empty array instead of nil?

3 Answers3