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Re: Strange problem [message #28220 is a reply to message #28141]

Mon, 26 November 2001 10:03

James Kuyper Jr.
Messages: 10
Registered: November 2001

Junior Member

Andre Kyme wrote:

> Hi everyone,
>
> type: for i=0., 0.8., 0.1 do print,i
> That's bad isn't it?

You can get a better idea of what's going wrong by modifying that as
follows:

for i=0., 0.8., 0.1 do print,i,format='(f20.10)'

There's a warning about this in the User's Guide, in the part describing
the FOR statement.

> Or what about:
>
> for i=0., 9.6, 0.1 do print,i
> for i=0., 9.7, 0.1 do print,i
> Same last element?
>
> If you stick a "d" after the first 0 - ie. make it double precision:
>
> for i=0.d, 0.8, 0.1 do print,i
>
> - then it seems to be OK, kind of
>
> Or try this:
> a=fltarr(100)
> for i=0., 9.7., 0.1 do a[i*10]=i
> Oh darn, 2.3 is not there!
> Neither is 9.3.
> Using the "d" trick seems to fix it, but why the need when we're only
> using steps of 0.1?
>
> Anybody know what's going on?

Yes. You should never use floating point values as your loop variable;
there's only a finite set of numbers that can be represented exactly in
floating point notation; all other numbers are stored as the nearest
approximation from that set. Most decimal fractions can't be represented
exactly. None of the numbers you used except 0.0 can be represented
exactly. As a float, 0.1 is actually represented by approximately
0.100000001490, and 0.8 is represented by approximately 0.800000011921.
Therefore, you can't guarantee exactly how many times your loop will
execute. Also you get roundoff errors which accumulate every time you
increment the counter.

My preferrred solution is as follows:

for i=0, 8 do print,i*0.1

for i=0, 97 do a[i] = i*0.1

The integer increments are exact, so the number of values is guaranteed
to be correct. You only get 1 floating point roundoff error, from the
multiplication, so the numbers printed are more accurate.

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[Message index]

		Strange problem By: Andre Kyme on Sun, 25 November 2001 16:46
		Re: Strange problem By: Paul van Delst on Mon, 26 November 2001 14:16
		Re: Strange problem By: James Kuyper Jr. on Mon, 26 November 2001 10:03
		[Offtopic] Re: Strange problem By: Bhautik Joshi on Wed, 05 December 2001 16:24
		Re: [Offtopic] Re: Strange problem By: Jeff Hester on Fri, 07 December 2001 08:29
		Re: [Offtopic] Re: Strange problem By: Bhautik Jitendra Josh on Fri, 07 December 2001 06:28
		Re: [Offtopic] Re: Strange problem By: Jeff Hester on Thu, 06 December 2001 23:57

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