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Re: extracting values from an array [message #44278 is a reply to message #44277]

Wed, 01 June 2005 12:49

Michael Wallace
Messages: 409
Registered: December 2003

Senior Member

> Actually I thought maybe there is a shortcut like:
> B = A gt -1
>
> But if
> A = [2,-1, -1, 3,-1,-1, 4, -1]
> the command
> B = A gt -1
> leads to
> B = [1,0,0,1,0,0,1,0]
> which this doesn't help me as I want B = [2,3,4]

In this case, the "a gt -1" returns a boolean value based on whether the
expression is true or not. That's why you see the array of only 1
(true) and 0 (false). Also note that whenever you do array operations,
your resultant array will be the same size as the input array. You want
your output array to be potentially smaller and therefore you need at
least one call to a function such as "where" to handle the manipulation.
By the way, the problem you describe is the very reason why where was
invented.

If you want something a little cleaner, I'd suggest using the count
parameter set by the where command rather than checking if w is -1 or
not. I just find this a little easier to read.

a = [2,-1, -1, 3,-1,-1, 4, -1]
w = where(a gt -1, count)
if count gt -1 then b = a[w]

> The array A is never bigger than 50 elements.
> So I will keep my three lines of code.

All of us like to tinker and find some slick way of doing things, but if
the code works as it should and there's no real gain to be achieved from
optimizing, don't optimize! And never optimize just for the sake of
optimization. Profile your code, find the major bottlenecks and
optimize those first. Rinse, wash, repeat.

Of course, if you're just tinkering with something, playing around with
it to see how it works, optimize the crap out of it. It's a good
learning experience. I still have fond memories of taking one of my
co-worker's programs and bringing its running time from about 20 minutes
down to under a minute and even under 30 seconds in some cases. And I
could have taken it down further had I written some DLMs. :-)

-Mike

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[Message index]

		Re: extracting values from an array By: Kenneth P. Bowman on Thu, 02 June 2005 17:32
		Re: extracting values from an array By: caitouer on Thu, 02 June 2005 14:01
		Re: extracting values from an array By: Michael Wallace on Wed, 01 June 2005 13:14
		Re: extracting values from an array By: Benjamin Hornberger on Wed, 01 June 2005 13:06
		Re: extracting values from an array By: Michael Wallace on Wed, 01 June 2005 12:49
		Re: extracting values from an array By: Francois L. on Wed, 01 June 2005 12:19
		Re: extracting values from an array By: R.G. Stockwell on Wed, 01 June 2005 11:58

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