⍪ — Table / Catenate first

Function. Keys: Alt-Minus ,. Hanzi: 接/矩. Ranks: ∞ monadic, ∞ ∞ dyadic

⍪Y makes a matrix: first axis becomes rows, remaining axes become columns.

⍪1 2 3             ⍝ 3 1⍴1 2 3
⍴⍪2 3 4⍴0          ⍝ [2 12]ₓ

X⍪Y joins on the first axis. ⍪⍠K follows catenate.

1 2⍪3 4            ⍝ 1 2 3 4

Inverse

⍪⁻Y gives the vector that a one-column matrix holds, and gives back any other matrix. X⍪⁻Y removes the fixed major cells X from the front of Y. With the right argument fixed, (⍪↢X)⁻Y removes them from the end.

⍪⁻⍪1 2 3             ⍝ 1 2 3
[1 2]⍪⁻[1 2⋄3 4]   ⍝ [3 4⋄]