prior

Syntax: v2 prior x, prior[v2;x]
Syntax: (vv)prior x, prior[vv;x]

Where

applies v2 or vv to each item of x and the item preceding it, and returns a result of the same length.

That is, the projections prior[v2;] and prior[vv;] are uniform functions.

q)(+) prior til 10
0 1 3 5 7 9 11 13 15 17
q){x+y%10}prior til 10
0n 1 2.1 3.2 4.3 5.4 6.5 7.6 8.7 9.8

prior is a wrapper for the Each Prior iterator.

First!

See the iterator for detail of how the first item of the result is determined.

It is good q style to use prior rather than the iterator, except where iterators are composed and brevity helps.