Method One: replace with NaN (best solution):
a = [1;2;3];
a(2) = NaN;
Method Two: store a mask:
a = [1;2;3]
mask = true(size(a));
mask(2) = false;
Method Three: use a cell array:
a = {1;2;3};
a{2} = [];
Personally I would use either one or two: they do not require moving the data in memory. Any method that actually changes the size of the array will require the array to be copied when it is changed, which is slow and unnecessary.
