Changing values in 2D array using logical operators

I'm sure I've solved this before, but it's befuddled me today.

I have a 2D array and I want to subselect some values and set them to nan:

aa=rand(10,10);
ab(aa<0.5)=nan;

ab is now a 1x100 row vector - how can I perform the operation but maintain the 2D array?

Answer 1

[Sebastian Holmqvist]

aa = rand(10, 10);
aa(aa < 0.5) = NaN;

[Bob]

:facepalm:

Thanks!