Define what a rect function is, since I find it difficult to read your mind. I'll guess it might be a unit rectangular pulse of some sort. Perhaps this:
In that case, a rect function is simple enough to create, as the difference of two heaviside functions.
help heaviside
For example:
rect = @(x) heaviside(x + 0.5) - heaviside(x - 0.5);
fplot(rect,[-2,2])
I've defined it there to be consistent with what mathworld shows.
