You want 128!:5 :
128!:5 y
Is NaN. If z=: 128!:5 y , then z has the same
shape as y and an atom of z is 1 if and only
if the corresponding atom of y is _. (NaN) or contains _. .
http://www.jsoftware.com/help/dictionary/dx128.htm#5
Specifically, to scrub _.s from y you want (#~ 1-128!:5)y . To replace _.s
with a number, you want the monad 128!:5`(,:&99)}y
where 99 is the replacement number you want.
-Dan
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