From: "alexeymuranov (Alexey Muranov)" <redmine@...>
Date: 2012-08-14T03:48:17+09:00
Subject: [ruby-core:47158] [ruby-trunk - Feature #6852] [].transpose should behave specially


Issue #6852 has been updated by alexeymuranov (Alexey Muranov).


marcandre (Marc-Andre Lafortune) wrote:
> 
> The later would fail for an empty set of exams and grades

I didn't understand this sentence.

Anyway, i just want to give another explanation why it is possible to assume that there is only one empty matrix.  With this assumption the function of #transpose will become correct, as [] == [[]] == [[],[],[]] == ... (as matrices).

One of the ways to define a matrix is as an  m x n  rectangular table of numbers, more precisely a function from {0, ..., m-1} x {0, ..., n-1} to the numbers.  If m or n is equal to 0, then the domain is the empty set, and so is the only function on this domain: {} x {} = {} x {0,1,2} = {0,1,2} x {} = {} = ���, and the only function define on ��� is ���.  I used here set theoretic conventions.

Of course matrices can also be viewed as functions from R^m to R^n, then R^0 = {���}, and "empty matrices" of different dimension will not be the same.  But i think this view point is less standard.

Alexey.

----------------------------------------
Feature #6852: [].transpose should behave specially
https://bugs.ruby-lang.org/issues/6852#change-28840

Author: boris_stitnicky (Boris Stitnicky)
Status: Open
Priority: Normal
Assignee: 
Category: core
Target version: 2.0.0


p = [1, 2, 3]
q = [4, 5, 6]
[p, q].transpose
# => [[1, 4], [2, 5], [3, 6]]
As expected, 2 x 3 vector was converted into 3 x 2.

[p].transpose
# => [[1], [2], [3]]
As expected, 1 x 3 => 3 x 1.

[].transpose
# => []
Unexpected, 0 x 3 did not become 3 x 0: [[], [], []]

In other words, when [] is the receiver, transpose has no way to know
what kind of ** 2 dimensional ** object is it - whether 0 x 3, 0 x 4, 0 x 1
or perhaps 0 x 0. #transpose should not assume it is 0 x 0. It should raise,
or warn, or complain, or require argument for this case, in short, it should
behave differently than today.


-- 
http://bugs.ruby-lang.org/