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Fun with Int_tabulated.pro [message #68497]

Fri, 30 October 2009 10:20

wlandsman
Messages: 743
Registered: June 2000

Senior Member

I ran into a couple of gotchas with int_tabulated.pro this
morning, It performs numerical integration for the simple case:

IDL> x = findgen(5)
IDL> y = x^2
IDL> print,int_tabulated(x,y)
21.3333

But if one has flipped both vectors (say while converting wavelength
to frequency), then -- even though a plot of the two vectors looks
exactly the same -- int_tabulated gives a different answer.

IDL> x = reverse(x) & y = reverse(y)
IDL> print,int_tabulated(x,y)
42.3619

A closer look at the int_tabulated documentation tells us that if X if
not monotonic increasing, then one needs to set the /SORT keyword.
So now we follow the documentation:

IDL> print,int_tabulated(x,y,/sort)
21.3333

and get the right answer again. But as the documentation also "sort"
of warns
you, setting the /SORT keyword reorders the input X and Y variables.
This is
probably not a problem unless one has modified the Y vector on the fly
by
some factor f

f = findgen(5) + 0.5
print,int_tabulated(x, y*f,/sort)

Upon output, the X vector has been sorted, but the Y vector has not,
since it
was passed as part of a temporary variable y*f. So one has lost
the one to
one correspondence between X and Y, screwing up all subsequent
processing.

--Wayne

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[Message index]

		Fun with Int_tabulated.pro By: wlandsman on Fri, 30 October 2009 10:20
		Re: Fun with Int_tabulated.pro By: Chris[6] on Wed, 04 November 2009 09:50
		Re: Fun with Int_tabulated.pro By: edward.s.meinel@aero. on Wed, 04 November 2009 08:47
		Re: Fun with Int_tabulated.pro By: Paul Van Delst[1] on Tue, 03 November 2009 16:18

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