How to extract vertices from a matrix.

13 次查看（过去 30 天）
Meghan
Meghan 2016-9-29
回答： David Goodmanson 2016-9-30
Hi everyone
I wonder if you can help me with a problem I'm having.
Right now I have a matrix which is 40x3 and contains nodal points related to triangles making each row a triangle. I'm trying to make a different matrix which would just contain two columns, with the nodal points associated with a vertex. For example:
B(1,1)=A(1,1);
B(1,2)=A(1,2);
B(2,1)=A(1,2);
B(2,2)=A(1,3);
B(3,1)=A(1,3);
B(3,2)=A(1,1);
Now I could continue doing it that way to make the matrix I want, but this code will be used many times and the dimensions of the matrices will change so I've been trying to find a way to do it in a for loop, or something of the sort.
Any help would be much appreciated :)
  11 个评论
显示 9更早的评论
Meghan
Meghan 2016-9-30
编辑：Meghan 2016-9-30
That worked! Thank you so much :) You're an absolute star! It seems really quite simple, I think I was making it a lot more complex in my head than it needed to be!
p.s. If you put write it as an answer I'll accept it :)
David Goodmanson
David Goodmanson 2016-9-30
I'm glad you liked this solution and I will post it as an answer. Thank you for mentioning that idea.
The great thing about Matlab is that its syntax simplifies things compared to 'for' loops and such, and you can look at matrix calculations in an almost pictorial way.

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

David Goodmanson
David Goodmanson 2016-9-30
Suppose you create an index vector
ind = [1 2 2 3 3 1]
which is the same as your column index for A in your original posting. Then for the sample matrix
A =
4 5 6
10 11 12
22 23 24
36 37 38
the command
C = A(:,ind)
concatenates the columns of A in the correct order to make the pairs you want:
C =
4 5 5 6 6 4
10 11 11 12 12 10
22 23 23 24 24 22
36 37 37 38 38 36
but for matrix B the pairs need to be stacked on top of each other. The 'reshape' command will do this but it reads elements out columnwise, so it's necessary to do some transposing back and forth using the single quote operator:
B = reshape(C',2,120)'
B =
4 5
5 6
6 4
10 11
11 12
12 10
22 23
23 24
24 22
36 37
37 38
38 36
The comments section for this question shows some intermediate matrices in this process.

更多回答（1 个）

Andrei Bobrov
Andrei Bobrov 2016-9-30
编辑：Andrei Bobrov 2016-9-30
A = [...
4 5 6
10 11 12
22 23 24
36 37 38]
B = reshape([A.',circshift(A,[0,-1]).'],[],2);

类别

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

标签

Community Treasure Hunt

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

Start Hunting!

Translated by