Populating a matrix with variables

Question

Steven Taggart 2016-9-29

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https://ww2.mathworks.cn/matlabcentral/answers/305007-populating-a-matrix-with-variables

评论： Massimo Zanetti 2016-9-29

Hello,

I am currently working on some finite volume method problems and as such I need to populate a matrix diagonal, I have used the 'diag' function successfully for populating the matrix with numbers but I would like to insert variables in the matrix diagonal. Would someone be able to advise me on the best way to do this. I am looking for something like the matrix included below, I know I could easily type in a matrix of that size but something of the 100*100 size just isn't practical.

Thanks for your help.

a_p1 a_e1   0   0    0
a_w2 a_p2 a_e2  0    0 
 0   a_w3 a_p3 a_e3  0
 0    0   a_w4 a_p4 a_e4 
 0    0    0   a_w5 a_p5

P.S. I am fairly new to Matlab and I am using this finite volume method problem as a way to learning the software.

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Answer 1

Massimo Zanetti 2016-9-29

3
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https://ww2.mathworks.cn/matlabcentral/answers/305007-populating-a-matrix-with-variables#answer_236612

在 MATLAB Online 中打开

Best way to populate diagonal matrix is to use spdiags (see help page).

For example, to obtain

A=[1,2,0,0,0,0;
   0,2,3,0,0,0;
   0,0,3,4,0,0;
   0,0,0,4,5,0;
   0,0,0,0,5,6];

Do this:

%define a matrix where columns are the diagonals
D = [1,2;
     2,3;
     3,4;
     4,5;
     5,6];
%put the columns in diagonal positions 0 (main diag) and 1 (upper diag)
%and specify matrix size 5x6
A = full(spdiags(D,[0,1],5,6));