MATLAB: I need help with a for loop that gives me an assignment error.

assignment errorfor loop

Hi all, I'm a beginner to Matlab but I have this code:

clear

clc

X_nf=randi([10 50],1,10);

sigma_x=rand(1,10);

N=numel(X_nf);

V=zeros(length(X_nf),10);

for i=1:10

V(i)=X_nf+sigma_x.*randn(1,N)

end

and it is giving me this error: In an assignment A(:) = B, the number of elements in A and B must be the same. I would like it to create the variable V that has a 10×10 matrix of numbers that are varied a little bit from the variable X_nf with the random noise I am adding.

Any help would be great. Thanks!

Best Answer

When you get this kind of error you want to look at the line (11 in this case) and check the sizes of the different variables. So let's look at

size(V(i))
size(X_nf)
size(sigma_x.*randn(1,N))

Now we see that V(i) is just a 1x1, while the others are 1x10. So instead we should probably write:

V(i, :) = X_nf+sigma_x.*randn(1,N)

This indexes the i-th row instead of just the i-th element.

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Vidhan - what are the values for your predefined variables? If I randomly assign some numbers (to them) then I see two lines. Once the loop has finished, what does the data in the arrays m and q look like?

Note that each time that you call plot, the code is creating a graphics object on the parent figure. Since you are collecting all of your data into two arrays, then you may as well just plot the data once the arrays have been populated. For example,

m = zeros(40,2);
q = zeros(40,2);
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for k = 1:1:40    
  n = n+0.1;
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  T4 = T3/n^l;
  efficiency = 1-1/n;
  m(k,1) = n;
  q(k,1) = n;
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figure;
hold on;
plot(m(:,1),m(:,2),'b.');
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There are several problems here, but I think the biggest problems you need to overcome is that the result of a PARFOR loop needs to be expressed as a sliced or a reduction variable, and that the iterations of the loop need to be provably order-independent. Simply put, a sliced output is one where MATLAB can see from the text of your program that each loop iteration assigns into a slice of an array, and the slice depends on the loop variable. So, the following is a sliced output:

x = zeros(10);
parfor idx = 1:10
  x(:, idx) = rand(10, 1);
end

In this case, each iteration of the loop assigns into a column slice of x. In your case, the indexing expression into t1.v2 is problematic firstly because it is not a simple 'sliced' form, and secondly because the indexing expression depends on previous values in t1.v2.

You might be able to run the first expression in the loop as a PARFOR, and then perform the assignment in an ordinary FOR loop later - but this will probably not get you any improvement in performance.

parfor i = 1:size(t2,1)-1
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end
for i = 1:size(t2,1)-1
  x = xc{i};
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end

Best Answer

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