matrix multiplication after a for loop

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hani daniel
hani daniel 2016-3-16
编辑： per isakson 2016-3-18
Dear Help, I have an element of a 2x2 matrix that needs to ne updated in a for loop, i=0, 100. After the iterations, I need to multiply all the sub matrix elements to calculate the resultant matrix. I am not sure how to complete the following code. Thank you. HD.
% series matrix calculation in a for loop
clear all
b=1;
c=5.12;
d=3.04
dz=0.1
G=1.2
L=10
% evlaute matrix elements at increments idz
for i=0,1,100;
idz=i*dz;
Fi11=1+g*cos(idz/L);
Fi12=b;
Fi21=c;
Fi22=d;
Fi=[Fi11,Fi12,Fi21,Fi22];
end
% not sure how to multiply all 100 matrix idz elements to calculate
% final matriz F
% F=F1*F2*......F100
plot (idz,F);

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回答（3 个）

Christoph
Christoph 2016-3-16
This should do it:
b=1;
c=5.12;
d=3.04
dz=0.1
G=1.2
L=10
F0=[0,b;c,d];
F =[0,b;c,d];
for i=1:100
Fi = F0;
idz=i*dz;
Fi(1,1)=1+G*cos(idz/L);
F = F * Fi;
end
hani daniel
hani daniel 2016-3-17
编辑：per isakson 2016-3-18
Christoph, Thank you. Being new to Matlab, I am still struggling with this problem. Essentially, I simplified the code for clarity and tried to follow your suggestions. The modified program (shown below) can now update the matrix element (A11) in increments of 1. However, I still do not know how to multiply correctly the resulting sequential three matrices. thank you for your help.
clear all
b=2;
c=3;
d=4;
a0=1;
for i=0:1:2;
A=[a0,b;c,d];
a(i+1)=a0+i;
Ai=[a(i+1),b;c,d]
end
A=prod(Ai
  1 个评论
Christoph
Christoph 2016-3-18
You multiply the matrices sequentially within the loop, not after it. Have you tried my code, what didn't work? I suggest you run my code line-by-line in debug mode (just click on the line number left to the first line), it should help you understand how loops work in general and adjust the code to do what you want.
But maybe it's best if you look into some programming fundamentals first. You should really understand that code in general (not just Matlab) works differently than mathematical formulas. a(i+1)=a0+i; inside a loop will actually grow a vector which you didn't even create before. I think Matlab should be able to handle it, but it's extremely poor practice and actually won't even run in most languages.

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hani daniel
hani daniel 2016-3-17
Christoph, Thank you. Being new to Matlab, I am still struggling with this problem. Essentially, I simplified the code for clarity and tried to follow your suggestions. The modified program (shown below) can now update the matrix element (A11) in increments of 1. However, I still do not know how to multiply correctly the resulting sequential three matrices. thank you for your help.
clear all
b=2;
c=3;
d=4;
a0=1;
for i=0:1:2;
A=[a0,b;c,d];
a(i+1)=a0+i;
Ai=[a(i+1),b;c,d]
end
A=prod(Ai)

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