MATLAB: How to store a matrix, A, in variable X, where X is also carrying a counter “i”

arraycounterindices don't matchindices errorindices not compatibleMATLABmatricesmatrixmatrix arraymatrix manipulationvariablevariables

I have a matrix A, which I want to store in a varaible X.

Now I can do,

X=A;

But the varaible is X is having a counter i, beacuse the value of X variable in one iteration will be used in the next iteration and so on.

So, it put a counter like this;

X(i)=A;

But, in the above line, the indices error – Unable to perform assignment because the indices on the left side are not compatible with the size of the right side – is shown.

What should be the correct syntax, plaease tell me.

Best Answer

Hi Neeraj,

simply create a structure.

X = struct();

X(i).A = A;

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First, let's fix your starting point. As I said, for what you're doing tables are the wrong storage. They're more complicated to use and use more memory. Ideally, you'd go back to whichever code created your a tables so that a) they're not numbered and b) they're matrices. So instead of tables, we're going to use matrices. Since your matrices are 2D and have all the same size, the simplest is to store them all in a single 3D matrices:

allmatrices = cat(3, table2array(a1), table2array(a2), table2array(a3), table2array(a4), table2array(a5), table2array(a6));

Numbered variables forces you to repeat the same code many times, as you can see above. Always a bad idea...

I'm still unclear of your goal.

If you want to keep only the rows that are all ones, it's simply:

selectedrows = allmatrices(all(allmatrices == 1, [2 3]), :, :);  %keep rows that are all 1 across columns and matrices. This syntax requires R2018b or later

which will output a ?x100x6 matrix with only the rows containing all ones.

If you want to keep only the rows that sum to 100, it's simply:

selectedrows = allmatrices(sum(allmatrices, [2 3]) = 100, :, :);  %keep only the rows that sum to 100 across the columns and matrices. Syntax requires R2018b or later

MATLAB: Update values in while loop and store values from a while loop in arrays

% Exercise 2.1 Order of Convergence
clear
close
clc
% Define Variables
F = 12;
x_old = 0;
itr = 0;
epsilon_new = 1e30;
epsilon_target = 1e-6;
while epsilon_new > epsilon_target
    itr = itr+1;
   
    x_new = springcalc(F, x_old);
    epsilon_new = abs(x_new - x_old);
    epsilon(itr)=epsilon_new;
    x_old = x_new;  
end
semilogy(1:length(epsilon), epsilon);
function [x_new] = springcalc(F, x_old)
k_0 = 30;
x_new = F/(k_0*(1+x_old^(0.1)));
end

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