Major cell

In the APL array model and leading axis theory, a major cell is a cell of an array which has rank one smaller than the rank of the array, or equal to it if the array is a scalar. The number of major cells in an array is its Tally, and a function can be called on the major cells of an array individually by applying it with rank ¯1 using the Rank operator. Functions designed to follow leading axis theory often manipulate the major cells of an array. For example, Reverse First (⊖) is considered the primary form of Reverse in leading-axis languages because it can be interpreted as reversing the major cells of its argument; J removes last-axis Reverse entirely.

Examples

A is an array with shape 3 4. Using Tally we see that the number of major cells in A is the first element of the shape, 3:

      ⊢A ← 5 3 1 ∘.∧ 2 3 4 5
10 15 20  5
 6  3 12 15
 2  3  4  5
      ≢A
3

We can separate A's major cells using Enclose with rank ¯1:

      ⊂⍤¯1 ⊢A
┌──────────┬─────────┬───────┐
│10 15 20 5│6 3 12 15│2 3 4 5│
└──────────┴─────────┴───────┘

Given another array B we can search for cells of B which match major cells of B. High-rank Index-of always searches for right argument cells whose rank matches the rank of a left argument major cell: if the right argument is a vector and not a matrix then it searches for the entire vector rather than its major cells (which are scalars).

      ⊢B ← ↑ 4,/⍳6
1 2 3 4
2 3 4 5
3 4 5 6
      A ⍳ B
4 3 4
      A ⍳ 2 3 4 5
3