Distinguishing between scalar rvars and arrays-of-size-1

オープン
  • 何か問題がありますか？
#187 コメント 1 件 リアクション 1 件 担当者 0 名 GitHub で見る

まだ誰も着手していません。

評価

難易度
5/5
見積もり時間
1週間以上
初心者へのやさしさ
25/100
issue の種類
バグ
明瞭さ
説明が足りない
活発さ
停滞
技術スタック
r
領域
data

調査の方向性

まず、rvar の変換関数、dim() と draws_of() の処理、および issue で説明されている配列インデックスの flattening/unflattening ヘルパーを確認します。最初に、スカラーとサイズ 1 の配列の同一性を保持すべきかどうかを明確にします。選択した挙動が文書化され、変換のラウンドトリップでこれらのケースが一貫して区別されるか、意図的に統一されれば完了です。

索引モデルが issue の本文から書いたものです。

説明

feature

Copying @martinmodrak's post from discourse here since it should probably be resolved in some way:

Hi,
it appears that the rvar interface in posterior cannot distinguish between a scalar and array of size 1, i.e.

scalar <- draws_matrix("y" = 1:10)
array1 <-  draws_matrix("y[1]" = 1:10)
all.equal(scalar, array1) # Correctly unequal
# [1] "Attributes: < Component “dimnames”: Component “variable”: 1 string mismatch >"
all.equal(as_draws_rvars(scalar), as_draws_rvars(array1)) # Incorrectly equal
# [1] TRUE
all.equal(as_draws_matrix(as_draws_rvars(array1)), array1) # Incorrectly unequal
# [1] "Attributes: < Component “dimnames”: Component “variable”: 1 string mismatch >"

Is this the desired/expected behaviour? Or is this a bug?

Also copying my thoughts from later in the thread:

Hmm. One behavior that base R vectors do that I did not mimic in rvars because it creates many corner cases in code is that vectors do not require a dim attribute (i.e. dim(x) can be NULL), whereas rvars will always return a numeric vector for dim (sometimes it's just the length of the vector). I did this because otherwise a bunch of code has to check for NULL dims and use length instead, which at the time did not seem useful.

That said, one way to distinguish between a "scalar" in base R and an array of size 1 is if dim(x) == NULL or dim(x) == 1. So one option would be to have the conversion functions recognize this convention. It will be slightly annoying to fix since we'd have to check all uses of dim() on rvars in the code that assume dim() is guaranteed non-null, and other functions that assume the internal array always has at least two dimensions (since currently this is also guaranteed). This would also mean the value of draws_of() returned to a user would not be guaranteed to have at least two dimensions. I'm also not sure if this would make some other operations on rvars kind of annoying to use if they are returning values that the user would expect to be treated as scalar when converted to other formats but which are actually 1-element arrays.

The other option would be a more focused solution that just uses an attribute, as you suggest. It might be an attribute for array-ness rather than scalar-ness; then 1-element arrays that get converted to rvars from another format could retain their array-ness through this attribute. Would be curious what folks think. (I'm not sure either solution sounds great to me at the moment, at least from the perspective of implementing it :) )

Another idea that occurred to me if we don't want to change how rvars treat scalars versus arrays-of-1 would be to instead expose the helper functions for flattening/unflattening array indices, since it sounds (maybe) like that's what you were intending to use the rvar stuff for @martinmodrak?

主要言語
R
スター
172
フォーク
27
平均マージ
5日 12時間
マージ済み PR（30日）
5

環境構築

はじめの一歩

  1. issue を最後まで読み、次にプロジェクトのコントリビューションガイドを読みます。
  2. 着手することを issue にコメントします — 二人が同じ作業をするのを防げます。
  3. リポジトリをフォークし、ブランチを切って変更します。
  4. issue 番号を参照したプルリクエストを送ります。

stan-dev/posterior のほかの issue

stan-dev/posterior の issue をすべて見る

似ている issue

R の issue をもっと見る

新しい issue をメールで受け取る

初心者向けの GitHub issue を短くまとめたダイジェスト。