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Box - monad <
So far you have dealt with atoms that are either numeric or character. The monad < (box) introduces a new type of atom called boxed. The monad < applies to any noun and returns an atom that is a box which contains the argument.
An array is either a numeric array that contains numbers, or a literal array that contains characters, or a boxed array that contains boxes. Arrays of numbers and characters are referred to as open to distinguish them from boxed arrays.
b =. < 2 3 4
$b NB. an atom has empty shape
A boxed array is displayed in a box.
b
+-----+
|2 3 4|
+-----+
c =. < 4 7 9
d =. b , c NB. append
d
+-----+-----+
|2 3 4|4 7 9|
+-----+-----+
$d NB. list with shape 2
2
(< 2 3 4) , < 4 7 9
+-----+-----+
|2 3 4|4 7 9|
+-----+-----+
Arrays of different types (numeric, character, and boxed) cannot be appended to one another.
'a' , 3
|domain error
| 'a' ,3
3 , < 2 3 4
|domain error
| 3 ,<2 3 4
Boxed arrays are of the same type and can be appended no matter what they contain.
(< 'abc') , < 4 5 6
+---+-----+
|abc|4 5 6|
+---+-----+
(< 2 3 $ 'abcdef') , (< i. 3 4) , < 23
+---+---------+--+
|abc|0 1 2 3|23|
|def|4 5 6 7| |
| |8 9 10 11| |
+---+---------+--+
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