The crux of the issue is that
gridx(P(:,1))
is a row vector (because P is a row vector).
In some cases, like simple assignment statements, MATLAB will allow some wiggle room and transpose the right-hand side of the assignment for you (if the lengths of the vectors match). For example, the following will work:
A = magic(3)
A(:,2) = [1 2 3]
In other cases, like vector math operations, MATLAB is more stringent, requiring an exact match in dimensions as well as length. So, for example, the following gives the same error as you see in your example:
B = [1 2] - [1 2]'
One simple solution in your case is to define gridx as the transpose of how you did it:
gridx=[0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5]'
Then it should work fine.
