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Re: majority voting [message #65202 is a reply to message #65079]

Mon, 16 February 2009 04:11

Allan Whiteford
Messages: 117
Registered: June 2006

Senior Member

David Fanning wrote:
> Allan Whiteford writes:
>
>> I remember the awful day when the result of "help,({a:[1]}).a" changed
>> and all my widget based code collapsed in a heap - I live in fear of
>> that day coming again so share your worries. It's possible that someone
>> will look at ++ and think they can split it across multiple cores - that
>> will be a bad day.
>
> I'm still trying to wrap my head around this whole
> issue, but I confess I don't understand the first thing
> about this paragraph. Do you think you could elaborate
> a bit, Allan?
>
> Thanks,
>
> David
>

David,

Previously (I dunno, sometime before IDL 5.something) if you extracted a
one element array from a structure then it would return as a scalar
rather than a array. This behaviour was clearly a bug but I had stacks
of code which relied upon it. Namely stuff like:

widget_control,textwidget,get_value=myval ; myval 1-element strarr
myval = float(myval) ; myval 1-element fltarr
results = {myval:myval}

inside an event handler then much later...

myval=results.myval ; myval now magically a scalar

answer=myval+[1.,2.,3.]

"answer" would end up being a 3-element fltarr because myval had turned
into a scalar. With the improvement answer now ends up as a 1 element
fltarr. Plotting this isn't so useful!

I never actually knew that get_value from a text widget returned a
1-element array in the case of only one line of text because I never
looked at the value explicitly and all subsequent testing meant that I
never noticed the problem in my code.

"help,({a:[1]}).a" demonstrates the problem since it would return either:

<Expression> INT = Array[1]

or

<Expression> INT = 1

depending on your version of IDL.

I have a concern that someone at ITTVIS will one day decide that the
behaviour that I was relying on in the ++ operator can be changed
because they want to make is consistent with += or because they decide
that if they don't treat repeated indices as they are just now they can
parallelise it across multiple CPU cores.

I don't like it when an IDL expression can change in result just due to
a version change of IDL.

Thanks,

Allan

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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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