When assignment is allowed?

8 次查看（过去 30 天）
Bruno Luong
Bruno Luong 2023-8-6
编辑： Bruno Luong 2023-8-7
The question concerns the requirement of the sizes of the assignment between two arrays.
To my great surprise this is allowed
lhs = zeros(3,4,5);
rhs = ones(4,5);
lhs(1,:,:) = rhs;
I would thought the last statement would throw an error
"Unable to perform assignment because the size of the left side is 1-by-4-by-5 and the size of the right side is 4-by-5."
but obviously it's not.
Can someone points me to the official document that explains such assigment is allowed and how?
  2 个评论
Paul
Paul 2023-8-6
Ran in:
The documentation on how assignment works is nonexistent as far as I can tell.
Will be interested to see if there is a doc-based answer, or if this is a case of "Matlab knows what you want"
Also works for other dimensions, so the leading unity dimension is not some sort of special case
lhs = zeros(3,4,5);
rhs = ones(3,5);
try
lhs(:,2,:) = rhs;
disp('success');
catch
disp('error');
end
success
the cyclist
the cyclist 2023-8-6
This would have taken me by surprise as well. I did not find this behavior in the documentation, but I did find it in this blog post from @Loren Shure. It is in the section "Retaining Array Shape During Assignment".

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

Matt J
Matt J 2023-8-6
编辑：Matt J 2023-8-6
Ran in:
The relevant documentation is at,
https://www.mathworks.com/help/matlab/ref/subsasgn.html?searchHighlight=subsasgn&s_tid=srchtitle_support_results_1_subsasgn#br5hkvm-7
Singleton dimensions are ignored when determining if the right and left hand sides have matching dimensions.
x=1:8
x = 1×8
1 2 3 4 5 6 7 8
x(1:3)=[10;20;30] %assign 3x1 into a 1x3 data region
x = 1×8
10 20 30 4 5 6 7 8
  8 个评论
显示 6更早的评论
Rik
Rik 2023-8-7
It really does help for 3D images. The mri.mat example dataset is [row,col,color,page] (even though it is grayscale), but most other places will use [row,col,page,color]. This sytax allow subindexing and swapping conventions with relative easy, without having to use permute or squeeze every second line
Bruno Luong
Bruno Luong 2023-8-7
编辑：Bruno Luong 2023-8-7
@Rik understood why you have 2 different orders of dimensions. In one case you want to process by color, in other you want to process by chromatic images.

请先登录，再进行评论。

更多回答（0 个）

类别

在 Help Center 和 File Exchange 中查找有关 Matrix Indexing 的更多信息

标签

产品

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by