fill

From The K Language Wiki
Fill
a^y

The fill primitive is used to fill in "gaps" present in an array using a single atom a.

You can use any null value to indicate a gap in the array.

 1^0 0N 2 3 0N
0 1 2 3 1

 "b"^," "
,"b"

 "A"^(1;0N;" ")
(1
 "A"
 "A")