Thanks - This is what I had concluded and Jasmin and Linda have also
emphasized the [: rather than @: Both, in this case do what I wanted.
Don
On 16/11/2013 9:43 PM, km wrote:
Focus on what you want to DO. You want to do "the square root of the mean of the
square".
A novice's tip for using [: is to read [: f g as "the f of the g" . What you
want is
[: %: ( [: (+/%#) *: )
which you read as "the square root of ( the mean of the square )" . (Omitted one
"the".)
Because of J's right-to-left processing you can omit the outer parentheses and
still read
[: %: [: (+/%#) *:
as "the square root of the mean of the square".
Thus you can use
rms =: [: %: [: (+/%#) *:
Another tip is to read @: as "after". You want to do square root after doing
mean after doing square. Thus
rms =: %: @: (+/%#) @: *:
also works. Pick what you are comfortable with and remember focus on what is
to be DONE. J's verbs are DOERS.
-- Kip Murray
Sent from my iPad
On Nov 16, 2013, at 7:20 PM, Don Kelly <[email protected]> wrote:
This is a novice problem (which arose from reading "Easy J" by Linda Alvort
I think that I have it worked out but this is something (IMHO) that possibly
should be in a primer.
I have occasion to use what is called rms or root mean square (usually applied
to a wave form).
I can define root, mean and square and can do something like:
root mean square 1 2 1 2
or
%:(+/%#)*: 1 2 1 2 NB. in immediate
execution
1.58114 OK
A natural tendency equivalent to saying mean=: +/%#
is to try
rms=: %:(+/%#)*:
But rms 1 2 1 2 fails.
However rms=: 3: ' %:(+/%#)*:y' or rms=: 13: ' %:(+/%#)*:y'
is OK
and the latter is equivalent to
rms=:[:%:[:(+/%#)*: which works
Alternatively rms=:%:@:(+/%#)@:*: also workS
It is obvious that either the capped fork (from use of 13: or use of @: work
and I see
the reasoning behind this- forcing a sequential " root after mean after square".
Am I right in assuming that immediate execution of
%:(+/%#)*: y where y is explicitly entered
is treated as 3: or 13: ' %:(+/%#)*::y ' (explicitly in that the *:y is
the argument for mean(+/%#) whose value is the argument for %: i.e. as in APL
for which 'tacit' doesn't exist)
and rms=: %:(+/%#)*: as a tacit verb with no argument is parsed as a fork
rather than sequentially as desired and runs into never never land?
The classic example of 'mean=:+/%# as a verb is misleading in that the example
is a single fork, but trying to extend it
does need a flag as to the need to emphasize the parsing.
(+/%#) *: 1 2 1 2 works but msg =:(+/%#)*: doesn't work but msg=:(+/%#)@:*:
or msg=:[:(+/%#)*: do work
Don Kelly
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