xprev (previous verb)
The uniform verb xprev returns the nth previous element to each item in the right argument list.
q)r:n xprev list
The result has the same length as the right argument. The first n elements of the result are null, empty or blank as appropriate, and thereafter each element is the nth previous element.
One can note that there is no xnext function. Fortunately xprev with a negative number on the left can achieve this.
q)2 xprev 2 7 5 3 11 0N 0N 2 7 5 q)-2 xprev 2 7 5 3 11 5 3 11 0N 0N q)1 xprev "abcde" " abcd"
For a complete list of functions, see the kdb+ Function Reference.