Ruby 1.9.3p125 Array#slice weird behaviour

Question

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

Say you have an array

a = [1,2,3]

why the a.slice(3,6) returns [] while the a.slice(4,6) returns nil?

score 4 · Answer 1 · answered Mar 29 '12 at 15:35

4

The documentation lists special cases for when the start index equal to the length of the array:

a = [ "a", "b", "c", "d", "e" ]

# special cases
a[5, 1]                #=> []
a[5..10]               #=> []

from: http://www.ruby-doc.org/core-1.9.3/Array.html#method-i-slice

So this appears to be the built-in functionality, since the start index is the length of the array the slice method is supposed to return an [], but when you pass the length of the array you get nil. This is probably due to how Ruby is defining ranges within an array.

answered Mar 29 '12 at 15:35

Hunter McMillen

59,865
24
119
170

Your answer is correct but this doesn't mean the same for the behaviour of `slice`. `slice` should literally slice (if we want to name it that way) the array, starting from the first_index and ending at the second_index, and thus returning nothing (which is `nil` in Ruby) in both cases. – Gerry Mar 29 '12 at 16:12
if an empty array was the same as nil, then the behavior above wouldn't have happened. – Hunter McMillen Mar 29 '12 at 16:43
Yes, it's built-in functionality, but the OP seemed to be asking "Why?" and the answer is: *to generalize assignment.* – DigitalRoss Mar 29 '12 at 17:40

DigitalRoss · Accepted Answer · 2012-03-29T18:30:37.527

Because it makes assignment more general

The mechanism is designed this way so slices can work in a highly generalized way on the left-hand side of assignment operators.

It doesn't really matter for #slice exactly because that result cannot be assigned but the same interpretation applies to x[3, 6] and those expressions can be assigned.

It's best to look at the array indices as identifying the spaces between elements, rather than the elements themselves.

This interpretation creates a consistent and useful interface ... for example, code can be written that will handle replacing elements or appending to zero length or populated Arrays, and all without needing special-case tests.

Ruby 1.9.3p125 Array#slice weird behaviour

2 Answers2

Because it makes assignment more general