Vocabulary/aco

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a: Ace (Boxed Empty) Noun

An atom consisting of the boxed empty list (<0$0).

   # a:   NB. Tally (#) shows a: consists of 1 (list) item
1
   $ a:   NB. But the Shape Of ($) a: is the empty list

   $$ a:
0

Common uses

1. For convenience inside a tacit definition

   listnameswithprefix=: 0 1 2 3&$: :(] ((] -: ({.~ #))S:0 _ # [)~ a: , [ 4!:1~ [: {. ])

   NB. a: appears here ____________________________________________^

2. Remove unwanted empty strings from a list of boxed strings

   ] z=: 'alpha' ; '' ; 'bravo' ; 'charlie' ; '' ; 'delta'
+-----++-----+-------++-----+
|alpha||bravo|charlie||delta|
+-----++-----+-------++-----+
   z -. a:
+-----+-----+-------+-----+
|alpha|bravo|charlie|delta|
+-----+-----+-------+-----+

3. Denote an empty axis for  x { y or  x u;.3 y

   i. 3 4
0 1  2  3
4 5  6  7
8 9 10 11
   (<a:;2) { i. 3 4  NB. Fetch column 2
2 6 10

4. To select all of y, whatever its rank, using: {

   a: { ''

   a: { 0
0
   a: { i. 3
0 1 2

More Information

1. (a:) is the value used for framing fill for boxed results.

2. Strictly a: is an empty Boolean list, boxed (i. e. <0$0). But, by and large, J treats all empty lists as equivalent, as tested by (-:)

It matters if you unbox a: and overtake the contents,

or join it to another empty list, or explicitly check the type with 3!:0, or use it as y in x ": y

   lace=: <''
   assert lace -: a:
   NB. But...
   <  3{. > lace
+---+
|   |
+---+
   <  3{. > a:
+-----+
|0 0 0|
+-----+

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