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 I^th axis of Y.  If X[I] is negative, then X[I] sub-arrays are taken from the end of the I^th 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).

      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