From: murphy <murphy@...>
Date: 2007-11-15T18:49:17+09:00
Subject: Re: Proposal: Array#walker

Wolfgang N��dasi-Donner wrote:
> I wanted to have a Method of class Array, which "walks" through an Array
> object like "each" without using any index at all, and presents the left
> and the right context of the actual element of the "each like"
> processing inside the block.
I know :) I wanted to propose a more general method that would still
help you to simpify / speed up your code. there currently is no method
to split an array into two with one call based on indices.

even a method that splits an array into two at a given index seems to
specialized for me. that's why split_at should work as the opposite of
Array#slice/Array#[], which has well-known semantics. (a syntax like
a]i[ would be funny, but please ignore it ^_^)

I'm not sure about the name though..."at" sounds like it always takes an
index. however, here's a version that does just that:

VALUE
rb_ary_split_at(VALUE ary, VALUE pos)
{
    long index = NUM2LONG(pos);

    if (RARRAY_LEN(ary) == 0) {
	return rb_assoc_new(rb_ary_new2(0), rb_ary_new2(0));
    }

    if (index < 0) {
	index += RARRAY_LEN(ary);
    }

    /* special cases */
    if (index < 0) {
	return rb_assoc_new(rb_ary_new2(0), rb_ary_dup(ary));
    }
    if (index >= RARRAY_LEN(ary)) {
	return rb_assoc_new(rb_ary_dup(ary), rb_ary_new2(0));
    }

    return rb_assoc_new(
	rb_ary_subseq(ary, 0, index),
	rb_ary_subseq(ary, index + 1, RARRAY_LEN(ary) - (index + 1)) );
}

irb(main):005:0> a = Array(1..10); a.each_with_index { |el,i|
  pre, post = a.split_at(i); p [pre, el, post] }
[[], 1, [2, 3, 4, 5, 6, 7, 8, 9, 10]]
[[1], 2, [3, 4, 5, 6, 7, 8, 9, 10]]
[[1, 2], 3, [4, 5, 6, 7, 8, 9, 10]]
[[1, 2, 3], 4, [5, 6, 7, 8, 9, 10]]
[[1, 2, 3, 4], 5, [6, 7, 8, 9, 10]]
[[1, 2, 3, 4, 5], 6, [7, 8, 9, 10]]
[[1, 2, 3, 4, 5, 6], 7, [8, 9, 10]]
[[1, 2, 3, 4, 5, 6, 7], 8, [9, 10]]
[[1, 2, 3, 4, 5, 6, 7, 8], 9, [10]]
[[1, 2, 3, 4, 5, 6, 7, 8, 9], 10, []]
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

[murphy]