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

Question

Slicing and indexing are two different operations, and inferring the behaviour of one from the other is where your problem lies.

The first argument in slice identifies not the element but the places between elements, defining spans (and not elements themselves):

  :peanut   :butter   :and   :jelly
0         1         2      3        4

4 is still within the array, just barely; if you request 0 elements, you get the empty end of the array. But there is no index 5, so you can't slice from there.

When you do index (like array[4]), you are pointing at elements themselves, so the indices only go from 0 to 3.

this has to do with the fact that slice returns an array, relevant source documentation from Array#slice:

 *  call-seq:
 *     array[index]                -> obj      or nil
 *     array[start, length]        -> an_array or nil
 *     array[range]                -> an_array or nil
 *     array.slice(index)          -> obj      or nil
 *     array.slice(start, length)  -> an_array or nil
 *     array.slice(range)          -> an_array or nil

which suggests to me that if you give the start that is out of bounds, it will return nil, thus in your example array[4,0] asks for the 4th element that exists, but asks to return an array of zero elements. While array[5,0] asks for an index out of bounds so it returns nil. This perhaps makes more sense if you remember that the slice method is returning a new array, not altering the original data structure.

EDIT:

After reviewing the comments I decided to edit this answer. Slice calls the following code snippet when the arg value is two:

if (argc == 2) {
    if (SYMBOL_P(argv[0])) {
        rb_raise(rb_eTypeError, "Symbol as array index");
    }
    beg = NUM2LONG(argv[0]);
    len = NUM2LONG(argv[1]);
    if (beg < 0) {
        beg += RARRAY(ary)->len;
    }
    return rb_ary_subseq(ary, beg, len);
}

if you look in the array.c class where the rb_ary_subseq method is defined, you see that it is returning nil if the length is out of bounds, not the index:

if (beg > RARRAY_LEN(ary)) return Qnil;

In this case this is what is happening when 4 is passed in, it checks that there are 4 elements and thus does not trigger the nil return. It then goes on and returns an empty array if the second arg is set to zero. while if 5 is passed in, there are not 5 elements in the array, so it returns nil before the zero arg is evaluated. code here at line 944.

I believe this to be a bug, or at least unpredictable and not the 'Principle of Least Surprise'. When I get a few minutes I will a least submit a failing test patch to ruby core.

At least note that the behavior is consistent. From 5 on up everything acts the same; the weirdness only occurs at [4,N].

Maybe this pattern helps, or maybe I'm just tired and it doesn't help at all.

array[0,4] => [:peanut, :butter, :and, :jelly]
array[1,3] => [:butter, :and, :jelly]
array[2,2] => [:and, :jelly]
array[3,1] => [:jelly]
array[4,0] => []

At [4,0], we catch the end of the array. I'd actually find it rather odd, as far as beauty in patterns go, if the last one returned nil. Because of a context like this, 4 is an acceptable option for the first parameter so that the empty array can be returned. Once we hit 5 and up, though, the method likely exits immediately by nature of being totally and completely out of bounds.

This makes sense when you consider than an array slice can be a valid lvalue, not just an rvalue:

array = [:peanut, :butter, :and, :jelly]
# replace 0 elements starting at index 5 (insert at end or array):
array[4,0] = [:sandwich]
# replace 0 elements starting at index 0 (insert at head of array):
array[0,0] = [:make, :me, :a]
# array is [:make, :me, :a, :peanut, :butter, :and, :jelly, :sandwich]

# this is just like replacing existing elements:
array[3, 4] = [:grilled, :cheese]
# array is [:make, :me, :a, :grilled, :cheese, :sandwich]

This wouldn't be possible if array[4,0] returned nil instead of []. However, array[5,0] returns nil because it's out of bounds (inserting after the 4th element of a 4-element array is meaningful, but inserting after the 5th element of a 4 element array is not).

Read the slice syntax array[x,y] as "starting after x elements in array, select up to y elements". This is only meaningful if array has at least x elements.