On Tue, 28 Apr 2009, Don Watson wrote:
> Hi everyone,
>
> I can't believe I have been as thick as I have been. I was blinded by
> the whole structure of tacit J. Two things have drastically made me see the
> light - Ric talking about converting tacit back to explicit and actually
> reading the definition of tacit J. Tell me what's wrong with the following:
>
> Here is an explicit J expression to find a standard deviation:
> %:(+/*:y -(+/y)%#y)%<:#y
Don, I would suggest that if a student derives tacit form using `13 :'
or your purposed S, it shows his inability in writing tacit form. I
have no problem in accept this explicit definition. However if I was
asked to find the tacit I will proceed like factorisation in algebra.
- explicit
%: (+/ *: y - (+/y) % #y ) % <:#y
- fork atop
%: (+/ *: y - (+/ % #) y) % <:@# y
- hook (also as a fork, as exercise)
%: (+/ *: (- (+/ % #)) y) % <:@# y
- atop atop
%: (+/ @: *: @ (- (+/ % #)) y) % <:@# y
- fork
%: ( (+/ @: *: @ (- (+/ % #)) ) % <:@# ) y
- atop
%: @ ( (+/ @: *: @ (- (+/ % #)) ) % <:@# ) y
(untested)
Just like idioms in APL, whenever I see <:# I know it is atop <:@#, and
similar for fork and hook. With practice it should be able to write
fork or hook naturally. To compel oneself to strive for tacit form
beyond his current capacity can only hinder the train of thought.
--
regards,
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