cut.md

Conjunction ;. with a right argument becomes an adverb:

monadic adverb ;.0 (rank _)

Apply verb to y with all axes reversed.

[    i.2 3 4
[;.0 i.2 3 4        NB. reverses dimensions -> result shape unchanged
|: i.2 3 4          NB. for comparison; reverses shape -> shape changes

dyadic adverb ;.0 (ranks 2 _)

Apply verb to window at position and shape x. x is table with first line giving starting positions per axis, and second line giving number of elements to take. Thus x can be created pos ,: shape.

(1 2 ,: 3 4) ];.0 i.10 10

monadic adverbs ;.1, ;._1, ;.2 and ;._2 (all rank _)

These adverbs apply their verb to each part of y which is delimited by occurrences of a certain element of y. The variants with 1 start each part with the first element of y, while the variants with 2 end each part with the last element of y. The negative variants remove the delimiting element before applying the verb.

< ;.1  '-ab-=cd='
< ;.2  '-ab-=cd='
< ;._1 '-ab-=cd='
< ;._2 '-ab-=cd='

dyadic adverbs ;.1, ;._1, ;.2 and ;._2 (all rank 1 _)

These adverbs are similar to their monadic variants but instead of using every occurrence of the first/last item as delimiter, the positions of the delimiters are given explicitly by boolean list x. To select per axis/dimension supply a boolean list for each as list of boxes; an empty element takes the whole dimension.

0 1 0 1 0 <;.1  'a-b-a' NB. everything before first delimiter is missing
0 1 0 1 0 <;.2  'a-b-a' NB. everything after last delimiter is missing
0 1 0 1 0 <;._1 'a-b-a' NB. without demiliter
0 1 0 1 0 <;._2 'a-b-a' NB. without delimiter

0 1 0 0 0 <;.1  'a-b-a' NB. x is delimiters not this elem's occurrences

NB. per line put delimiters on first and second elements
(''; ''; 1 1 0 0 0) <;.1 i.3 4 5
NB. + per table put delimiters on second and third line
(''; 0 1 1 0; 1 1 0 0 0) <;.1 i.3 4 5
NB. + per brick put delimiters on second table
(0 1 0; 0 1 1 0; 1 1 0 0 0) <;.1 i.3 4 5

dyadic adverbs ;.3 and ;._3 (ranks 2 _)

These create a moving window and apply their verb to its contents. The difference is that the negative variant stops moving the window in a direction if there are no more elements, while the positive variant keeps moving the window in a direction as long as it can see something. Therefore what the window of a negative variant sees always has the same shape.

The first line in x specifies the movement of the window, but can be omitted, in which case the window moves by 1. (The second line in) x specifies how many elements to take per axis/dimension, which can also be thought of as the shape of the window for low dimensional data.

i.5 5
2 3 <;._3 i.5 5             NB. x of rank 1 interpreted as (1,:x)
2 3 <;.3  i.5 5

i.3 4 5
  2 3 <;._3 i.3 4 5         NB. unexpected result as it means: (2 3 _)
1 2 3 <;._3 i.3 4 5         NB. while this window shape was intended

(1 2,:2 3) <;.3  i.5 5      NB. move by 2 on lines and by 1 on tables
(2 1,:2 3) <;.3  i.5 5
(2 2,:2 3) <;.3  i.5 5

NB. non-overlapping views by setting movement to window's shape
(,:~ 2 3)  <;.3  i.5 5
(,:~ 2 3)  <;._3 i.5 5

monadic adverbs ;.3 and ;._3 (rank _)

These are same as the dyadic versions but automatically compute the shape of the window: Each dimension is as long as the shortest dimension of y and the rank is the same as y.

<;._3 i.2 3           NB. 2 (lowest in shape of y) 2x (rank of y): (2 2)
<;._3 i.2 3 4         NB. 3#2 gives a shape of (2 2 2)
$;.3  i.2 3 4         NB. note: *rank* doesnt change; but shape does