Not able to add 2d array to 3d array

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

[Ameer Hamza]

What is the dimension of cycle. The error will be caused if the dimension of cycle is not equal to [50000 x 4].

[Nagesh A P]

Each loop iteration will produce a different cycle. The dimension will also be different each time, ranging between [18000x4] and [19000x4]

[Ameer Hamza]

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]

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]

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.

Answer 1

[Ameer Hamza]

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);

[Nagesh A P]

Hi ameer.Cycle is not a constant value. It changes in each loop iteration.