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Re: Histogram [message #72826 is a reply to message #44556]

Mon, 11 October 2010 07:16

David Grier
Messages: 35
Registered: July 2010

Member

On 10/11/10 9:39 AM, silje wrote:
> Hey!
> I'm trying to get a grip of the Histogram function in IDL, but have
> run into a problem that I don't understand. As you can see from the
> code below I have created an array with six differend elements,but
> when I call HISTOGRAM with a binsize equal to 0.1 then it seems like
> 1.2 and 1.3 is put in the same bin. Why is this?
>
>
> IDL> arr = [1.1,1.2,1.3,1.4,1.5,1.6]
> IDL> print, HISTOGRAM(FLOAT(arr), binsize=0.1)
> 1 2 1 1 1
>
>
> Thanks!
> Best regards Silje

This question hinges on (1) where HISTOGRAM places the origin of
its bins and (2) the representation of floating point numbers.
If you explicitly specify where you want the bins to start, then
HISTOGRAM does what you were expecting:

IDL> print, HISTOGRAM(arr, binsize=0.1, min=1.05)
1 1 1 1 1 1

The command you tried automatically set the origin of the first bin
to 1.1. Each data point then falls right on the dividing line between
two bins. Which way they should fall is clear in decimal notation.
The answer can be different when the same numbers are represented at
finite numerical precision. Your example exercises this distinction.
Here's why:

IDL> print, 1.1 + 2.*0.1 - 1.3
1.19209e-7

which is not zero. Therefore, your third data point falls into the
second bin because of round-off error.

Doing the same calculation in double precision "solves" the problem
IDL> print, 1.1d + 2.d*0.1d - 1.3d
0.0000000

TTFN,

David

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[Message index]

		Histogram By: James[1] on Tue, 28 June 2005 09:24
		Re: Histogram By: R.G. Stockwell on Fri, 01 July 2005 11:37
		Re: histogram By: rinceboy on Wed, 12 August 2009 07:12
		Re: histogram By: jameskuyper on Wed, 12 August 2009 06:15
		Re: histogram By: rinceboy on Wed, 12 August 2009 04:00
		Re: histogram By: David Fanning on Tue, 11 August 2009 10:17
		Re: histogram By: Maxwell Peck on Mon, 29 March 2010 03:08
		Re: Histogram By: David Grier on Mon, 11 October 2010 07:16
		Re: Histogram By: David Fanning on Mon, 11 October 2010 06:51
		Re: HISTOGRAM By: d.poreh on Wed, 23 January 2013 05:06
		Re: HISTOGRAM By: Helder Marchetto on Wed, 23 January 2013 03:44
		Re: HISTOGRAM By: Mats Löfdahl on Wed, 23 January 2013 03:26

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