Y may be any array. X must be a simple integer scalar or vector.
If Y is a scalar, it is treated as a one-element array of shape (⍴,X)⍴1. The length of X must be the same as or less than the rank of Y. If the length of X is less than the rank of Y, the missing elements of X default to the length of the corresponding axis of Y.
R is an array of the same rank as Y (after possible extension), and of shape |X. If X[I] (an element of X) is positive, then X[I] sub-arrays are taken from the beginning of the Ith axis of Y. If X[I] is negative, then X[I] sub-arrays are taken from the end of the Ith axis of Y.
If more elements are taken than exist on axis I, the extra positions in R are filled with the fill element of Y (⊂∊⊃Y with ⎕ml←0).
Examples
5↑'ABCDEF' ABCDE 5↑1 2 3 1 2 3 0 0 ¯5↑1 2 3 0 0 1 2 3 5↑(⍳3) (⍳4) (⍳5) 1 2 3 1 2 3 4 1 2 3 4 5 0 0 0 0 0 0
M 1 2 3 4 5 6 7 8 2 3↑M 1 2 3 5 6 7 ¯1 ¯2↑M 7 8 M3←2 3 4⍴⎕A 1↑M3 ABCD EFGH IJKL ¯1↑M3 MNOP QRST UVWX