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Re: majority voting [message #65141 is a reply to message #65079]

Thu, 12 February 2009 14:45

JD Smith
Messages: 850
Registered: December 1999

Senior Member

On Feb 12, 3:11 pm, David Fanning <n...@dfanning.com> wrote:
> Paolo writes:
>> And in case any other person a bit more slow on the intake
>> (like me) is wondering what the heck ++ and -- do in IDL,
>> they can be found in the documentation under "Mathematical Operators"
>> (searching for "++" did not return anything useful for me).
>
> Thank you. I have been searching for it off and on all
> morning. :-)
>
> There is nothing in the documentation, though, to imply this
> is a vectorized operation. In fact, just the opposite for the
> discussion on postfix operations. And insights, JD, on
> how that can be so?

The surprise isn't that it's vectorized: all of IDL's mathematical,
relational, and bit operators are vectorized. The surprise is in how
it treats repeated indices. I can only guess that ITT regards the old
"no repeats" behavior as undesirable but inalterable due to legacy
code, whereas when ++ and -- were introduced recently, no such legacy
baggage existed, so they were free to improve the behavior (at the
cost of consistency). That said, I know IDL has accumulated lots of
cruft over the years, but I can imagine getting different answers for a
[i]++ and a[i]+=1 might turn some people off.

I also noticed that a[i]+=1, in addition to being plagued by the "no
repeats" issue, is at least 4x slower than a[i]++. As for ++a vs. a++
in expressions, when you don't care about the order of evaluation,
using the former is somewhat faster, since it saves making a temporary
copy of a.

JD

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[Message index]

		majority voting By: Mort Canty on Wed, 11 February 2009 08:14
		Re: majority voting By: Allan Whiteford on Fri, 13 February 2009 01:31
		Re: majority voting By: David Fanning on Sat, 14 February 2009 08:44
		Re: majority voting By: JD Smith on Thu, 12 February 2009 14:45
		Re: majority voting By: Foldy Lajos on Thu, 12 February 2009 12:47
		Re: majority voting By: David Fanning on Thu, 12 February 2009 12:11
		Re: majority voting By: pgrigis on Thu, 12 February 2009 11:58
		Re: majority voting By: JD Smith on Thu, 12 February 2009 10:49
		Re: majority voting By: Mort Canty on Thu, 12 February 2009 07:45
		Re: majority voting By: JD Smith on Thu, 12 February 2009 07:08
		Re: majority voting By: David Fanning on Thu, 12 February 2009 05:43
		Re: majority voting By: Allan Whiteford on Thu, 12 February 2009 04:17
		Re: majority voting By: Mort Canty on Thu, 12 February 2009 01:15
		Re: majority voting By: JD Smith on Wed, 11 February 2009 15:20
		Re: majority voting By: ben.bighair on Wed, 11 February 2009 14:03
		Re: majority voting By: Allan Whiteford on Mon, 16 February 2009 04:11

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