array[4,0] returns [], but array[5,0] returns nil... why? [duplicate]

Thi开发者_开发问答s question already has answers here: Closed 11 years ago.

Possible Duplicate:

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?

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The two arguments version of the method behaves a little bit different than expected, you can get a full explanation by Gary Wright here.

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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.

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array[4,0]:

len=0 and beg=4
beg > RARRAY_LEN   => false
len == 0           => return ary_new(klass, 0) => []

array[5,0]:

len=0 and beg=5
beg > RARRAY_LEN   => true => return Qnil => nil

array.c:rb_ary_subseq

VALUE
rb_ary_subseq(VALUE ary, long beg, long len)
{
     VALUE klass;

     if (beg > RARRAY_LEN(ary)) return Qnil;
     if (beg < 0 || len < 0) return Qnil;

     if (RARRAY_LEN(ary) < len || RARRAY_LEN(ary) < beg + len) {
        len = RARRAY_LEN(ary) - beg;
     }
     klass = rb_obj_class(ary);
     if (len == 0) return ary_new(klass, 0);