Repeating matrix multiplication linear system

2014 7 16
3 个回答
回答已采纳
更新时间：2014 7 16
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0 个投票

Hello, I am trying to do a matrix multiplication with a loop, I have a matrix A
A =
0 0 45
1/25 0 0
0 39/100 3/10
and a vector
x =
2000
100
25
Well, I do the multiplication
A*x
ans =
1125
80
93/2
But then I want to do A*ans
A*ans
ans =
4185/2
45
903/20
And I want to repeat this 50 times. My problem that I am running into is when I write a for loop, then it will only select the element. I don't know how to set an instance of a vector, here is what I have tried so far.
e = 2000;
l = 100;
a = 25;
x = [e; l; a];
for t = 0:50
if t == 0
PopulationInfo = A*x
else
PopulationInfo = A*PopulationInfo;
end
But this can't go anywhere because if I do A*x(t) it will just be an element of x, not the entire vector. Also, I want to do a plot starting at t=0 (the initial e, l, and a are populations at t=0), but with this loop there is no 0th element of an array.

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Sara
Sara 2014-7-16
what do you mean "then it will only select the element"?
Rick
Rick 2014-7-16
编辑：Rick 2014-7-16
e stands number for eggs, l stands for larva, and a is for adults. I want a plot of the number of eggs vs. t for t = 1:50, as well as number of adults and number of larva.
The formula is that e(t+1) = A*e(t), l(t+1) = A*l(t), and a(t+1) = A*a(t). The values for e,l, and a are at t=0.

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 采纳的回答

Azzi Abdelmalek
Azzi Abdelmalek 2014-7-16
编辑：Azzi Abdelmalek 2014-7-16

0 个投票

A =[ 0 0 45
1/25 0 0
0 39/100 3/10]
e = 2000;
l = 100;
a = 25;
x = [e; l; a];
for t = 1:50
x(:,t+1) = A*x(:,t);
end
t = 0:1:50;
plot(t,x)

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Rick
Rick 2014-7-16
Thanks, but how do I plot the elements of populatonInfo vs. t?
Sara
Sara 2014-7-16
Using Joseph answer (below), you can do
t = 0:1:50;
plot(t,x(1,:),'r',t,x(2,:),'b',t,x(3,:),'g')

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更多回答（2 个）

Joseph Cheng
Joseph Cheng 2014-7-16

0 个投票

to store each iteration of t.
e = 2000;
l = 100;
a = 25;
x = [e; l; a];
for t = 0:50
x(:,t+2) = A*x(:,t+1);
end
Rick
Rick 2014-7-16

0 个投票

Here is the code
e = 2000;
l = 100;
a = 25;
x = [e; l; a];
PopulationInfo = A*x;
for t = 1:50
PopulationInfo = A*PopulationInfo;
end
for t = 0:50
x(:,t+2) = A*x(:,t+1);
end
t = 0:1:50;
plot(t,x(1,:),'r',t,x(2,:),'b',t,x(3,:),'g');
But I get the error
Error using plot
Vectors must be the same lengths.

4 个评论
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Rick
Rick 2014-7-16
Thanks sara. Let's say I was trying to do it the way Azzi first mentioned (which was closer to what I came up with myself), how would I do it then?
Sara
Sara 2014-7-16
Azzi has modified his answer to include the plotting. What you don't like?
Rick
Rick 2014-7-16
didn't see the edit, thank u to Azzi and Sara and everyone here!

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