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Not able to add 2d array to 3d array

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Nagesh A P
Nagesh A P on 27 Jun 2018
Commented: Ameer Hamza on 27 Jun 2018
I'm trying to add the 2d array cycle to a 3d array cycle_mat. i is a variable that changes after each loop iteration. I'm getting the following error:
Subscripted assignment dimension mismatch.
Error in The_Code (line 622) cycle_mat(:,:,i)=cycle;
if true
cycle_mat=zeros(50000,4,100);
loop starts
cycle_mat(:,:,i)=cycle;
loop ends
end

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Ameer Hamza
Ameer Hamza on 27 Jun 2018
But on the left side of the assignment, there is a [50000 x 4] matrix. How do you want to assign a [18000x4] to a [50000x4] matrix?
Nagesh A P
Nagesh A P on 27 Jun 2018
Okay. I thought matlab would take care of that, like it does for 2d arrays when u preallocates a matrix .So here I have to change the dimesnsion of that 3d matrix for each loop iteration??
Ameer Hamza
Ameer Hamza on 27 Jun 2018
MATLAB does not do such thing even for a 2D matrix.
A = [1 2 3; 4 5 6; 7 8 9];
is a 3x3 matrix. Now if I try
A(2, :) = [1 2];
it will again throw an error.
The dimension of left and right side of an assignment must be same. Also In a MATLAB matrix, the length of all rows must be same. Similarly for columns and pages. If your matrix size is continually changing along the third dimension then consider using cell arrays.

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Answers (1)

Ameer Hamza
Ameer Hamza on 27 Jun 2018
The error is probably caused because the dimension of cycle are not [50000 x 4]. Also, if cycle is has a constant value then you don't need to use for loop to assign to 3rd dimension of a matrix. MATLAB support auto array expansion feature. So for R201bb and later, the following will work
cycle_mat(:,:,i_vector) = cycle_mat(:,:,i_vector) + cycle;
i_vector is a vector of all indexes to which you want to add cycle.
For R2016a and earlier
cycle_mat(:,:,i_vector) = bsxfun(@plus, cycle_mat(:,:,i_vector), cycle);

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Nagesh A P
Nagesh A P on 27 Jun 2018
Hi ameer.Cycle is not a constant value. It changes in each loop iteration.

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