Question

I declared a 3-dimension matrix like this , and later in the iteration, I need to dynamically expand the length of the third dimension.

Notice that currently, I only declare two dimensions but not the third dimension, so I will get an error like this:

Attempted to access bins(1,2,2); index out of bounds because
size(bins)=[2,22,1].

But the problem is, the Xk dimension is variable and I really cannot decide how it is needed. Is there anyway to declare a changeable matrix?

bins=zeros(2,size(Xtrain,2));
%Some for loop
    bins(Y,k,Xk)=bins(Y,k,Xk)+1;
%end loop

Answer 1

You can use the end + k notation to dynamically expand the matrix as follows:

A = zeros(10,10,10);
size(A) %[10 10 10]
Xk = 5;
A(:,:,end+Xk) = 4;
size(A) %[10 10 15]

The size of A will have increased automatically and Matlab will automatically fill in-between values with zeros. However, resizing an array inside a loop is not a good way to program. You might want to rethink your method such that you aren’t doing this.