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Re: subset a 2d array [message #61930 is a reply to message #61929]

Thu, 14 August 2008 09:34

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

xiao writes:

> Sorry David, I still got problems here. I printed out all the values
> in the array and i found the center value which is -28.0011 but when I
> tried to do this :
>
> index=where(lat eq -28.0011,count)
> xy = array_indices(lat, index[0])
> subarray = lat[xy[0]-400:xy[0]+400, xy[1]-400:xy[1]+400]
>
>
> It cannot find the index number...Why is that? Thank you?
> PS :THIS ARRAY IS A FLOAT ARRAY.

Yeah, I thought I would let you solve one problem
at a time. :-)

The "problem" is that computers cannot represent numbers
"exactly". At least most numbers they can't. (They do
a reasonable job with integers. :-)

http://www.dfanning.com/math_tips/sky_is_falling.html
http://www.dfanning.com/math_tips/razoredge.html

Typically, if we are looking for floating values, we
have to add/subtract a small "delta" to the number
to be able to find it. How big should the delta be?
Big enough to allow you to find the number you are
looking, but not too big to add ambiguity to your
search. You could try something like this, for example"

delta = 1e-3
index = Where(array GE (2.5-delta) AND array LE (2.5+delta), count)

Or, if you are just looking for the center of the array, you
could always do this:

s = Size(array, /Dimensions)
xcenter = s[0]/2
ycenter = s[1]/2
targetValue = array[xcenter, ycenter]

Cheers,

David

--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming (www.dfanning.com)
Sepore ma de ni thui. ("Perhaps thou speakest truth.")

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[Message index]

		Re: subset a 2d array By: xiao zhang on Thu, 14 August 2008 09:46
		Re: subset a 2d array By: David Fanning on Thu, 14 August 2008 09:34
		Re: subset a 2d array By: xiao zhang on Thu, 14 August 2008 09:24
		Re: subset a 2d array By: xiao zhang on Thu, 14 August 2008 07:54
		Re: subset a 2d array By: David Fanning on Wed, 13 August 2008 20:53
		Re: subset a 2d array By: xiao zhang on Wed, 13 August 2008 20:22

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