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 shapeA 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 4Boxed 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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