One note of caution regarding the use of the "repeat for each" loop, whilst you
will get a loop iteration for every value in the collection (fldhexa3 in your
example), you are not guaranteed the order in which they will occur. This
doesn't matter in a lot of cases but does matter when the sequence is
significant. In the case of your example I believe sequence is critical,
otherwise the pixels might appear to be scrambled!
The following adjusted loop guarantees the sequence at the expense of speed:
put 1 into i
repeat for each word theWord in fldhexa3
put word i of fldhexa3 into theWord
put 00 & theword & theword & theword after tVar2
add 1 to i
end repeat
The original "improved" loop reduces the run-time to 25%. However, the
"modified improved" loop only manages to reduce the original run-time to 50%.
The suggested loop above takes advantage of the "for each" mechanism to produce
a set of iterations very rapidly but gets slowed by the need to guarantee
sequence. I wonder whether the LC engine could impose strict sequence more
effectively with a variant of the "for each" loop such as
repeat for each sequenced word x in theCollection
...
end repeat
My own tests, comparing the speed of the 4 common repeat loops, imply that the
current "for each" form is hugely faster than the others. I tested "repeat for
each...", "repeat while...", "repeat until...", "repeat with..." and a
simulated "repeat for each sequenced..." forms using a simple loop body that
added lines of text one after another, e.g.
put empty into tData
repeat with i = 1 to tMaxI
put line i of tList & return after tData
end repeat
I ran this test for 250,000 iterations for each type of loop, which produced
the following timings:
Starting test for 250,000 iterations...
repeat for each... 0 mins 0 secs 111 millisecs
repeat while... 0 mins 30 secs 569 millisecs
repeat until... 0 mins 30 secs 379 millisecs
repeat with... 0 mins 30 secs 341 millisecs
repeat for each seq... 0 mins 30 secs 524 millisecs
As you can see, in this test the "repeat for each..." form was approx. 275
times faster than the other forms. Also the simulated "repeat for each
sequenced..." form was no faster than the other forms. This shows how variable
the speed will be with the simulated "repeat for each sequenced...", depending
on the details of the loop body.
If there was a "repeat for each sequenced..." form of loop in LC, any speed-up
could be very beneficial even if the amount of speed-up was only 10 times
faster!
Cheers
Peter
--
Peter Reid
Loughborough, UK
> On 9 Oct 2017, at 10:18am, [email protected] wrote:
>
> Message: 12
> Date: Sat, 7 Oct 2017 15:53:44 +0200
> From: Malte Pfaff-Brill <[email protected]>
> To: [email protected]
> Subject: Re: Atkinson dither algorithm
> Message-ID: <[email protected]>
> Content-Type: text/plain; charset=us-ascii
>
> Hi Al,
>
> I already posted on the forums, but for completeness also here:
>
> a lot can be done by replacing repeat with with repeat for each where you can.
>
> -- repeat with i = 1 to the number of words of fldhexa3
> -- put 00 & word i of fldhexa3 & word i of fldhexa3 & word i of
> fldhexa3 after tVar2
> -- end repeat
>
> repeat for each word theWord in fldhexa3
> put 00 & theword & theword & theword after tVar2
> end repeat
>
>
> A sidenode:
>
> I always use strict compile mode, therefore I added the needed variable
> declarations and noticed you use startTime as a variablename, which is a
> reserved keyword. That is not a good idea. (I noticed, because I managed to
> freeze liveCode where I fixed only half of the use of startTime. Booom.)
>
> Cheers,
>
> malte
_______________________________________________
use-livecode mailing list
[email protected]
Please visit this url to subscribe, unsubscribe and manage your subscription
preferences:
http://lists.runrev.com/mailman/listinfo/use-livecode
