ruby - How does negative index work with `Array#[]=`?

Question

I tried to see how Array#[]= works, and played around:

enum[int] = obj → obj
enum[start, length] = obj → obj
enum[range] = obj → obj

Question 1

I have one array b holding nil at its 0 index.

b = []
b[0]   # => nil

I tried to replace nil with integer 10 in the code below.

b[-1] = 10 # => IndexError: index -1 too small for array; minimum: 0

Why doesn't the code above work, but the ones below do? In case of an array with size 1, why are the indices 0 and -1 treated differently?

b[0] = 5   # => 5
b[-1] = 10 # => 10

Question 2

I created an array of size 2, and did the following:

a = [1,2]

a[-3] = 3       # => IndexError: index -3 too small for array; minimum: -2
a[-3] = [3]     # => IndexError: index -3 too small for array; minimum: -2
a[-3..-4] = [3] # => RangeError: -3..-4 out of range

I believe that negative index never increases the size of an array, but I don't know why. Why did the code below succeed?

a[-2..-3] = [3,4] #=> [3, 4]

Answer 1

I would suggest you to take a look at the first para in Array documentation. It surprisingly says: “A negative index is assumed to be relative to the end of the array—that is, an index of -1 indicates the last element of the array, -2 is the next to last element in the array, and so on.”

That means, that you may set a[-N]th element if and only |N| <= a.size. That’s why a = [1,2] ; a[-3] = 3 fails (3 > 2).

On the other hand, there is [likely] not documented feature for ruby arrays: a[INBOUNDS_IDX..NONSENSE_IDX]=SMTH will insert SMTH before INBOUNDS_IDX index:

a=[1,2]
a[2..0]='a'
a[2..1]='b'
a[2..-100]='c'
# ? [1, 2, "c", "b", "a"]
a[2..-1]='q'
# ? [1, 2, "q"]

Nonsense here means “less than INBOUNDS_IDX, and not treatable as index in an negative notation” (that’s why a[2..-1] in the example above is treated as a[2..(a.size - 1)].)