To me, that the minimum is greater than the maximum does not make sense.
That aside, try this:
a = max(0, min(a,5))
Testing it:
a = -2:7
a = max(0, min(a,5))
produces:
a =
-2 -1 0 1 2 3 4 5 6 7
a =
0 0 0 1 2 3 4 5 5 5
Experiment to get the result you want.
.
