Imagine I've defined the following name in J:
m =: >: i. 2 4 5
This looks like the following:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
I want to create a monadic verb of rank 1 that applies to each list in this list of lists. It will double (+:) or add 1 (>:) to each alternate item in the list. If we were to apply this verb to the first row, we'd get 2 3 6 5 10.
It's fairly easy to get a list of booleans which alternate with each item, e.g., 0 1 $~{:$ m gives us 0 1 0 1 0. I thought, aha! I'll use something like +:`>: @. followed by some expression, but I could never quite get it to work.
Any suggestions?
UPDATE
The following appears to work, but perhaps it can be refactored into something more elegant by a J pro.
poop =: monad define
(($ y) $ 0 1 $~{:$ y) ((]+:)`(]>:) @. [)"0 y
)
I would use the oblique verb, with rank 1 (/."1)- so it applies to successive elements of each list in turn.
You can pass a gerund into /. and it applies them in order, extending cyclically.
+:`>: /."1 m
2
3
6
5
10
12
8
16
10
20
22
13
26
15
30
32
18
36
20
40
42
23
46
25
50
52
28
56
30
60
62
33
66
35
70
72
38
76
40
80