The dyad { (from) is used to select items from an array. This is sometimes called indexing an array.
a =. 5 7 9 4 8 3 { a NB. select item at index 3 4 0 { a NB. indexes start at 0 5 2 4 { a NB. select 2 items 9 8 2 0 4 { a NB. select 3 items 9 5 8
From selects items from its argument, and therefore selects rows from tables.
m =. i. 3 4 m 0 1 2 3 4 5 6 7 8 9 10 11 0 { m 0 1 2 3 2 1 { m 8 9 10 11 4 5 6 7
From selects tables from rank 3 arrays.
1 { i. 2 3 4 12 13 14 15 16 17 18 19 20 21 22 23