Re: NCL bug

Dave,

Sorry for the delay in my response. I appreciate your attention to this
matter.

When I ran the sample code you provided, I got the same floating point
number for y and x(90). This made me a bit suspicious, so I ran my
nclbug.ncl code (Attached below) to make sure I was still having the
same problem. Sure enough, x(90) = 0.9 this time around, so the
"problem" is hiding for a bit or somehow got fixed. The only difference
is that I restarted my system. Can the floating point roundoff change
just by restarting the system?

Gus

On 9/12/11 1:02 PM, Dave Allured wrote:
> Gus,
>
> This is a classic example of roundoff error. The number 0.9 does not
> have an exact representation in standard floating point binary. Your
> code has computed 0.9 as an NCL "float" by two different methods:
>
> (1) x = ispan(0,100,1)*.01, then take x(90)
> (2) Entered directly as an NCL constant "0.9"
>
> Now, the NCL print statement does *not* display all significant digits
> by default. If you show all significant digits with the sprintf
> function, the issues will become clearer:
>
> mac56:~ 3> ncl
> NCAR Command Language Version 5.2.1
> ncl 1> x = ispan(0,100,1)*.01
> ncl 2> y = 0.9
> ncl 3> print (sprintf ("%15.9f", x(90)))
> (0) 0.899999976
> ncl 4> print (sprintf ("%15.9f", y))
> (0) 0.899999976
>
> On my system, the two methods get exactly the same floating point
> number. But floating point roundoff can vary from system to system.
> Do you get two different numbers by the above method?
>
> --Dave
>
> On 9/12/2011 10:49 AM, Ghassan J. Alaka, Jr. wrote:
>> Hi,
>>
>> I have found a bug in NCL version 5.1.1, although a colleague of
>> mine confirmed that the same error in NCL version 6.0.0.
>>
>> If I create an array (called "x") that contains increments from 0 to
>> 1 by 0.01, I run into problems when x should equal 0.9. The print()
>> function says that x(90) = 0.9, yet x(90) > 0.9 after some
>> investigation. This only seems to be occurring at x = 0.9. Why is
>> this the case?
>>
>> I have attached a simple script to view this error.
>>
>> Thank you,
>> Gus Alaka
>>
>>
>> nclbug.ncl
>>
>>
>>
>>
>> ;================================================================
>> ; Load scripts
>> ;================================================================
>>
>> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
>> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
>> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
>> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/shea_util.ncl"
>>
>>
>> begin
>>
>> ;x = fspan(0.,1.0,101)
>> x = ispan(0,100,1)*.01
>>
>> val1 = ind(x.eq.0.1)
>> val2 = ind(x.eq.0.5)
>> val3 = ind(x.eq.0.9)
>>
>> print(""+val1)
>> print(""+val2)
>> print(""+val3)
>> print(""+x(90))
>>
>> if(x(90).eq.0.9)then
>> print("!Equal!")
>> end if
>>
>> if(x(90).lt.0.9)then
>> print("!LessThan!")
>> end if
>>
>> if(x(90).gt.0.9)then
>> print("!GreaterThan!")
>> end if
>>
>> ; if x(90) = 0.9 (as seen from print()), why doesn't x(90) = 0.9?
>> ; This problem does not seem to exist at 0.8, 0.7, 0.6, etc.
>>
>> end
>>
>>
>>
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-- 
Ghassan J. Alaka, Jr., Ph.D. Candidate
Department of Atmospheric Science
Colorado State University
1371 Campus Delivery
Fort Collins, CO 80523-1371
E: gusalaka@atmos.colostate.edu
P: (970) 491-1487
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