Processing a 2d matrix into a 3d and can't avoid a subscripted assignment dimension mismatch.

Hello, I currently have a matrix of the following form

(x)(y)(z)
 1  1  1
 2  2  1
 3  3  1
 4  4  1
 1  1  2
 2  2  2
 3  3  2...

The important part being that the third column has several entries of the same value. I was attempting to process it so that I could display all of the entries with the same value at once. My intuition was to convert the original 2-D matrix into a 3-D matrix where the third dimension was determined by the (z) value. So in my code I have the following line:

NewPC(j,:,z) = OldPC(i,:);

At this point I receive my subscripted assignment dimension mismatch. How would one recommend proceeding in order to correct this error?

[Star Strider]

One ( at least I ) would request that you post the entire loop or nested loops encompassing that statement.