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ambassador at fourthworld.com
Sat Feb 14 18:00:17 EST 2015
Mike Kerner wrote:
> REPEAT FOR is .129 seconds, and REPEAT WITH is TWENTY SEVEN THOUSAND
> TIMES SLOWER (for this operation)??!?!?!?!?!???
> Hey, Pete, "That's a common technique"...WHAT? If it's so common,
> and all of this is common knowledge, then how come it isn't
> documented, anywhere
The Dictionary entry for "repeat" notes that the "for each" form is much
faster than "with".
> and how come this is the first time I remember EVER hearing about this
Good question. This comes up in the forums and/or this list almost
every month or so.
The speed difference will vary according to the size of each line and
the size of the lines, but "order of magnitude" is usually a pretty fair
minimal expectation for the speed boost with this.
It's one of those things we don't think about until we see it in action,
and then it seems almost self-evident:
Chunk expressions are handy, but expensive. We usually don't think
about the expense because the engine's pretty fast, but with large-scale
operations like traversing a long list it adds up enough to be significant.
Many chunk expressions require the engine to examine the data character
by character, keeping track of delimiters as it goes.
repeat with i = 1 to the number of lines of tData
DoSomethingWith line i of tData
...the engine first needs to examine every character in tData to count
the number of CRs, then each time through the loop it needs to do it
again to the next line. First time through it reads from the beginning
until the first CR, second time through it goes from the beginning until
the second CR, and so forth, so by the time you get several thousand
lines into it it's doing the same long character-by-character comparison
each time through, getting successively slower and slower.
repeat for each line tLine in tData
...the engine only counts to the next CR, puts it into tLine, and
remembers where it left off so each time through the loop it's only
reading a single line.
While the former takes logarithmically longer to complete, the latter
> What else don't I know about???
Mode 14. :)
> You would think that Edinburgh would think about tweaking an
> algorithm, since REPEAT WITH seems to be a special case of
> REPEAT FOR, and you can generate the REPEAT WITH behavior
> by wrapping the REPEAT FOR...
But there's one key difference which makes each form worth keeping in
case you need it:
With "repeat with i =..." the data in the variable being traversed can
change. Sometimes you may need that.
But with "repeat for each..." the data being traversed is not allowed to
change, because if it did then the line endings might have been altered
and its attempt to keep track of where it is would fail.
So each form has its own special benefits. I tend to use "repeat for
each" most of the time, but I'm glad "repeat with" is available for the
rare cases where it's useful.
Fourth World Systems
Software Design and Development for the Desktop, Mobile, and the Web
Ambassador at FourthWorld.com http://www.FourthWorld.com
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