Question

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

I’m following Ruby Koans and I’ve gotten to a part that deals with an array that looks like this:

array = [:peanut, :butter, :and, :jelly]

One of the tests focuses on what array[4,0] returns, and another focuses on what array[5,0] returns.

There are only 4 elements in this array, meaning it goes up to array[3], correct? So why is array[4,0] returning a blank array while array[5,0] returns nil?

Answer 1

The [i, n] form is identifying substring boundaries and not characters

The short answer is that you are defining a substring to either return or replace.

There is a zero-length string at the beginning and at the end that needs to be identifiable.

In the two-argument index, the positions are really the individual boundaries between the characters, and there is one such boundary after the last character.