清空筛选条件
清空筛选条件

filling up an m X n X o size matrix without loops

1 次查看（过去 30 天）
Pal
Pal 2012-10-13
Hi,
I am looking for an efficient way to 'fill up' a three-dimensional (m by n by o size) matrix M with elements based on a simple rule.
I have a number of matrices of size m X n X 1. I want the matrix M to contain weighted sums of these m X n X 1 size matrices. The weighted sums are obtained by the general formula
M(:,:,i) = f1(A, theta(i)) + f2(B, theta(i)) + f3(C, theta(i)) + ...
where f1, f2, f3, etc... are arbitrary functions and A, B, C, etc... are the m X n X 1 size matrices. theta is a 1 X o size vector containing angles (the functions f1, f2... are trigonometrical functions). The functions f1, f2... apply elementwise operations on A, B,... using a single value of theta, thus their outputs are also m X n X 1.
I can fill the matrix M up by looping through i=1:o and inserting the appropriate level M(:,:,i). However I feel that there must be a vectorized solution that works much faster.
I'd appreciate if you could give me ideas where to look for this functionality.
Thanks a lot,
Paul
  2 个评论
Pal
Pal 2012-10-13
Azzi,
There is no difference between nXmX1 and nXm.
theta is 1 by 'o', not 1 by zero.

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 2012-10-13
编辑：Matt J 2012-10-13
I doubt that the loop over i is what's slowing you down. My bet is that it's the evaluation of f1(A,theta), f2(B,theta), etc.... However, one possibility to avoid the loop over i is as follows;
theta=reshape(theta,1,1,[]);
M = bsxfun(f1,A, theta) + bsxfun(f2,B, theta) + bsxfun(f2,C, theta)...
It still seems to me that you need a loop over f1,f2,f3, etc... but a vectorized solution to that will require knowledge of what those functions look like.
  3 个评论
显示 1更早的评论
Matt J
Matt J 2012-10-13
编辑：Matt J 2012-10-14
That will also depend greatly on the number of terms f1...fj that you have. In any case, BSXFUN is not my preferred solution. Now that we know the form of your f_i(), the more appropriate thing to do is,
theta=theta(:).'; %make sure it's a row vector
M=[A(:),B(:),C(:),...]*[trig1(theta); trig2(theta); trig3(theta);...];
M=reshape(M,m,n,[]);
Pal
Pal 2012-10-14
That is about five times faster than the for loop. Brilliant!
Thank you very much.

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