problem in symbolic matrix multiplication

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Andrew
Andrew 2013-11-10
评论： Andrew 2013-11-11
Hi, I am performing simple multiplication on two matrices. However getting error while doing so. I am not able to understand what is error and where I am going wrong. Therefore I am not able to correct error. Can someone provide any ideas or insights? here is my code:
clear all;
clc;
syms xlc
%sr=sym([]);
sr=zeros(4,1);
B=[ 0.64*xlc - 1.2, 1.6 - 1.28*xlc, 0.64*xlc - 0.4; 0.64*xlc - 2.8, 4.8 - 1.28*xlc, 0.64*xlc - 2.0; 0.64*xlc - 4.4, 8.0 - 1.28*xlc, 0.64*xlc - 3.6; 0.64*xlc - 6.0, 11.2 - 1.28*xlc, 0.64*xlc - 5.2];
d=1.0e-03 *[0.000000000095833;0.058593750095833; 0.109375000095833; 0.152343750095834; 0.187500000095834; 0.214843750095834; 0.234375000095834; 0.246093750095834; 0.250000000095834];
e_n=[1,2, 3; 3, 4, 5; 5, 6, 7; 7, 8, 9];
for i=1:1:4
for j=1:1:3
I = e_n(i,j);
sr(i)=sr(i)+(B(i,j)*d(j));
end
end
What I am doing: I want to multiply
sr(1)=B(1,1)*d(1)+B(1,2)*d(2)+B(1,3)*d(3)
sr(2)=B(2,1)*d(3)+B(2,2)*d(4)+B(2,3)*d(5)
and so on. But matlab is giving errors. Any help or ideas are welcome. Thanks.

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采纳的回答

Walter Roberson
Walter Roberson 2013-11-11
You initialize sr as empty, so sr(i) does not exist yet to be indexed by "i" for each different "i" value.
sr = sym(zeros(4,1));

更多回答（2 个）

Andrei Bobrov
Andrei Bobrov 2013-11-11
syms xlc
B=[ 0.64*xlc - 1.2, 1.6 - 1.28*xlc, 0.64*xlc - 0.4; 0.64*xlc - 2.8, 4.8 - 1.28*xlc, 0.64*xlc - 2.0; 0.64*xlc - 4.4, 8.0 - 1.28*xlc, 0.64*xlc - 3.6; 0.64*xlc - 6.0, 11.2 - 1.28*xlc, 0.64*xlc - 5.2];
d=1.0e-03 *[0.000000000095833;0.058593750095833; 0.109375000095833; 0.152343750095834; 0.187500000095834; 0.214843750095834; 0.234375000095834; 0.246093750095834; 0.250000000095834];
ii = bsxfun(@plus,1:size(B,2),2*(0:size(B,1)-1)');
sr = sum(B.*d(ii),2);
  1 个评论
Andrew
Andrew 2013-11-11
Thank you very much for your help. However to be honest, I did not understand your code. I am a beginner of matlab and your code, at first glance, seems complicated. This may be the way that professionals or experts use, however I am not one among them. But, I am trying to find out what you have done. Again, thanks!

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Roger Stafford
Roger Stafford 2013-11-10
It isn't clear whether you are receiving a matlab error message or are getting erroneous results. If it is the latter the line in the inner for-loop should be changed to:
sr(i)=sr(i)+(B(i,j)*d(I));
  3 个评论
显示 1更早的评论
Andrew
Andrew 2013-11-11
Making sr variable symbolic or double does not seem to make difference. Any ideas or suggestions?
Walter Roberson
Walter Roberson 2013-11-11
What error message is it that you are getting for the first situation?

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