How to add columns equally in each page of a multidimensional array within a for loop?

Question

EB 2017-11-23

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/368829-how-to-add-columns-equally-in-each-page-of-a-multidimensional-array-within-a-for-loop

评论： EB 2017-11-27

I have a problem inside a for loop where I am defining a multidimensional array by adding more columns at every loop. The multidimensional array I have problems with is array R. The starting dimension for k =1 of R is 4425x2x11. At the end of the for loop I should get R of dimension 4425x23x11. Hence, for each k within the loop the array R gets more columns but equally for every page. The for loop for R calculation is following: Omega_rs has size 4425x11.

kmax = 21;
A = 11;
for k = 1:kmax
    for i = 1:A
        Vkm1 = 2*sum(Omega_rs(:,i).*R(:,k+1,i).*R(:,k,i).*q(:,i));
        Sk = Omega_rs(:,i).*R(:,k+1,i)-Vkm1*R(:,k,i);
        Dk = sqrt(2*sum((Sk.^2).*q(:,i)));
        R(:,:,i) = [R(:,:,i),Sk/Dk,i];
        coeff(:,k+2,i) = -Vkm1*coeff(:,k,i);
        coeff(2:k+1,k+2,i) = coeff(2:k+1,k+2,i)+coeff(1:k,k+1,i);
        coeff(:,k+2,i) = coeff(:,k+2,i)/Dk;
    end
end

I have checked the for loop line by line, and it is ok for all variables inside except the R array.. After executing this for loop I get a note Dimensions of matrices being concatenated are not consistent. Any advice how to solve this problem would be greatly appreciated.

Thank you in advance!

2 个评论
显示无隐藏无

Rik 2017-11-23

Sk = Omega_rs(:,i).*R(:,k+1)-Vkm1(:,i)*R(:,k,i); is that correct? Or should it be R(:,k+1 ,i)

Also, pre-allocating your matrices (especially large ones) is generally a good idea.

If that doesn't solve your issue, check all your * and / if they should indeed be non-element-wise operation.

EB 2017-11-23

编辑：EB 2017-11-23

在 MATLAB Online 中打开

Thank you Rik Wisselink for your answer. YOu were right, the expression for Sk had an error, so R should be R(:,k+1 ,i). I did pre-allocation and it didn't help. * and / are definitely correct because the expressions were checked for 2d arrays. Still I have the problem in the row where I have to calculate R. The same message appears Error using horzcat Dimensions of matrices being concatenated are not consistent.

I think the problem is in the way how I concatenate the matrix R and the product Sk/Dk. It shoul go like this: So for k=1 and for all 11 pages the starting matrix R that was 4425x1x11 will change to 4425x2x11, for k=2 and all 11 pages it will change from 4425x2x11 to 4425x3x11 and it will continue to 4425x23x11. The error is in

R(:,:,i) = [R(:,:,i),Sk/Dk,i];

I don't know is there another (correct) way of doing this?

Many thanks!

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Rik 2017-11-24

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/368829-how-to-add-columns-equally-in-each-page-of-a-multidimensional-array-within-a-for-loop#answer_292943

在 MATLAB Online 中打开

You need to think about what your syntax does.

R(:,:,i) = [R(:,:,i),Sk/Dk,i];

What that line does is evaluating the right hand side and saving to de left hand side. When it evaluates the right hand, this goes wrong. You are trying to concatenate R(:,:,i) (a 2D matrix) with something that is either a matrix or a scalar (you can easily check with the debugging tools) and with a scalar. I don't think there is any implicit expanding for this syntax. What do you want to achieve? Because you can use a syntax like R(:,k,:)=Some_2D_matrix;. You can also look into the permute and cat functions.

Also, I expect you will get an error for i=2, because it looks like Vkm1 is a scalar, so Vkm1(:,i) will result in an error.

3 个评论
显示 1更早的评论隐藏 1更早的评论

Rik 2017-11-24

在 MATLAB Online 中打开

What you want to do is saving a 4425x1x1 to R. Just do that.

R(:,k+1,i)=Sk/Dk;
%for this syntax it is best to pre-allocate R:
%R=zeros(4425,22,11);

At the end of your loop R will be 4425x22x11

I'm getting the idea you don't understand how concatenation works. The syntax you are using tells Matlab to replace R(:,:,1) with the matrix that is the result of [R(:,:,i),Sk/Dk,i]. That last is the concatenation, but it doesn't affect R, until you save it with the R(:,:,i)=

EB 2017-11-27