Empty Array

An array is empty if it contains no atoms. An empty array has a 0 in its shape.

   a =. 0 $ 0	NB. empty list
   $a
0

   $ ''	NB. empty list
0

   b =. 2 0 3 $ 'a'	NB. empty rank 3 array
   $b
2 0 3

Empty arrays have no atoms to display and their display can be confusing if you don't know what to expect. An empty list displays as a blank line. A table with 0 rows displays as 0 lines; a table with 3 rows, but 0 columns, displays as 3 blank lines.

   2 $ 5
5 5

   1 $ 5
5

   0 $ 5	NB. empty list displays as blank line

 
   2 2 $ 5
5 5
5 5

   1 2 $ 5
5 5

   0 2 $ 5	NB. 0 rows displays 0 lines
   2 0 $ 5	NB. 2 rows displays 2 lines

The display of lists and tables with 1 row can look the same, and you have to look at their shape to distinguish them.

   a =. 2 $ 5
   b =. 1 2 $ 5
   a
5 5
   b		NB. b displays the same as a
5 5
   $ a
2
   $ b		NB. but b has a different shape
1 2