⍀) is a dyadic function or monadic operator that copies each element of the right argument a given number of times and inserts prototype elements, ordering the copies along a specified axis. Typically
\ is called Expand while
⍀ is called "Expand First" or an equivalent.
Expand is usually associated with Replicate (
/), and the two functions are related to Mesh and Mask. It shares a glyph with Scan even though Expand is naturally a function and Scan must be an operator. This incongruity is sometimes resolved by making Expand an operator itself, and sometimes by function-operator overloading allowing both syntactic elements to coexist.
If the right argument is a vector, the number of positive values in the left argument must match the length of the right argument. Each positive value produces copies of the corresponding item on the right argument; a zero or negative value produces one or more prototype elements.
1 0 3 ¯2 2\'abc' a bbb cc
1 0 0 1 0 1\'abc' a b c 1 0 0 1 0 1/'a b c' abc
⎕←A←3 4⍴⎕A ABCD EFGH IJKL 1 0 3 2 ¯2 1\A A BBBCC D E FFFGG H I JJJKK L 3 1 ¯1 2⍀A ABCD ABCD ABCD EFGH IJKL IJKL
Some implementations allow the right argument to have length 1 along the expansion axis even if other axes have lengths not equal to 1.
1 ¯2 3 \ ⍪'abc' a aaa b bbb c ccc