Very new to J. I love it but still far from fluent in it. I have managed to hit an issue and I don't know why it is happening. If somebody could please explain why this is occuring I could learn more about this straight-to-the-point language.
Basically I'm doing difference (-) then squared (*:) then sum (+/) then want to take the square root (%:). Now the sum square error part ( (+/@:*:@:-) ) works fine:
1 2 3 4 5 (+/@:*:@:-) 2 2 2 2 2
15
Then I can get the square root this way (just add %: to left):
%: 1 2 3 4 5 (+/@:*:@:-) 2 2 2 2 2
3.87298
But when I add square root to left of the atco (tacit?) part in the middle it doesn't give me the right result:
1 2 3 4 5 (%:@:+/@:*:@:-) 2 2 2 2 2
1.57001
It instead returns 1.57001 instead of 3.87298.
My understanding is all I have done is add a 'then do square root' but obviously I am wrong. But I don't understand why.
When I dissect:
require 'debug/dissect'
dissect '1 2 3 4 5 (%:@:+/@:*:@:-) 2 2 2 2 2'
I see diff (-) and square (*:) are in their own dissect boxes. But then the last box combines sum (+/) and square root (%:) together, showing %:@:+/ . Not a separate box for +/ and then %: .
As I said I am new to J and am struggling to understand why this is occuring here.
I've tried changing to &, &: and @ instead but as expected that didn't fix this issue. It doesn't appear to be due to some composition limit either since this works fine (has many more atco's combined):
(>:@:>:@:>:@:>:@:>:@:>:@:>:@:>:@:>:) 2 2 2
11 11 11
Thank you
The issue that you are encountering is that
(%:@:+/@:*:@:-)
is being evaluated as
1 2 3 4 5 ((%:@:+)/@:*:@:-) 2 2 2 2 2
1.57001
and not
1 2 3 4 5 %:@:(+/@:*:@:-) 2 2 2 2 2
3.87298
because of the way that conjunctions take as much of the left operand as possible. This means that your partial result from *:@:- is being processed by %:@:+/ when you want %: to apply to the result of +/@:*:@:-
Since you are just starting your journey with J, this may be a bit advanced, but at some point you may want to read the tacit programming section of J for C programmers by Henry Rich. It has a very good explanation about how J sentences are parsed. Parenthesis are your friend when you want to change the order of execution. https://www.jsoftware.com/help/jforc/contents.htm#_Toc191734581
Edit: As @Eelvex points out in the comment below the break actually occurs at (+/), but the result is the same because of the way that conjunctions are evaluated left to right.