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Re: Finding the closest value in an array... [message #38774 is a reply to message #38771]

Tue, 30 March 2004 08:54

Wayne Landsman
Messages: 117
Registered: January 1997

Senior Member

Tim Robishaw wrote:

> Seems like every few minutes I'm taking a scalar and trying to locate
> which value in an array it's closest to. VALUE_LOCATE() finds the
> interval of a monotonic vector that the value lives in, so it's not
> quite what I'm looking for, but it's awfully close! I end up just
> doing this:
>
> IDL> useless = min(abs(vector-value),minindx)
> IDL> closest = vector[minindx]

Presuming your vector is not monotonic, then your two lines perform the
minimum necessary number of calculations (and are completely vectorized)
-- you have to check every value in vector. Though you don't need to
save the "useless" scalar value returned by MIN, this value did need to
be calculated, and the extra overhead is very MINimal ;-)

If you have a monotonic vector then -- like VALUE_LOCATE() -- you can
use bisection to minimize the number of values in vector that you need
to check.

> I'd bet
> VALUE_LOCATE() is doing a lot more stuff behind the scenes than the
> simple two lines above (judging from the old Goddard library routine).

You probably saw Craig Markwardt's implementation of the VALUE_LOCATE
algorithm for users of IDL V5.2 and earlier, with its bizarre call
SPL_INTERP. The program is not really performing cubic
interpolation, but rather taking advantage of the bisection algorithm
within the intrinsic SPL_INTERP function. Even with the unnecessary
spline calculations this method is still faster than coding the
bisection algorithm within IDL. Since V5.3 the bisection algorithm
is available directly in the VALUE_LOCATE function.

--Wayne Landsman

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[Message index]

		Re: Finding the closest value in an array... By: JD Smith on Tue, 30 March 2004 10:04
		Re: Finding the closest value in an array... By: Wayne Landsman on Tue, 30 March 2004 08:54
		Re: Finding the closest value in an array... By: Mark Hadfield on Tue, 30 March 2004 02:13
		Re: Finding the closest value in an array... By: timrobishaw on Wed, 31 March 2004 00:41

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