| comparing numbers using WHERE doesn't work [message #25243] |
Sat, 02 June 2001 12:15  |
Miklos Z. Kiss
Messages: 7
Registered: April 2001
|
Junior Member |
|
|
Hi folks:
I have noticed a strange occurence when I try to compare a number with
all the elements of an array. I'll show my situation in an example.
Consider the following array:
a = [2.0000, 2.1000, 2.2000, 2.3000, 2.4000, 2.5000, 2.6000].
Let, x = 2.2000, y = 2.4000
So, I want to find which elements of a match up with x and y.
Naturally, I do the following:
index0 = WHERE(a EQ x)
index0 = index[0] ; convert from 1-D array to single integer.
index1 = WHERE(a EQ y)
index1 = index[0]
So, when I execute, I may get the following:
PRINT, index0
2
PRINT, index1
-1
The output clearly tells me that x occurs at a[2] and that y does not exist
in a at all. By inspection, IDL should return the value 4. Does anyone
have any idea why this happens? Thanks,
Mik Kiss
|
|
|
|
| Re: comparing numbers using WHERE doesn't work [message #25343 is a reply to message #25243] |
Wed, 06 June 2001 04:47  |
R.Bauer
Messages: 1424
Registered: November 1998
|
Senior Member |
|
|
"Miklos Z. Kiss" wrote:
>
> Hi folks:
> I have noticed a strange occurence when I try to compare a number with
> all the elements of an array. I'll show my situation in an example.
> Consider the following array:
> a = [2.0000, 2.1000, 2.2000, 2.3000, 2.4000, 2.5000, 2.6000].
> Let, x = 2.2000, y = 2.4000
> So, I want to find which elements of a match up with x and y.
> Naturally, I do the following:
>
> index0 = WHERE(a EQ x)
> index0 = index[0] ; convert from 1-D array to single integer.
index0 is now redefined by index[0].
> index1 = WHERE(a EQ y)
> index1 = index[0]
index1 is now redefined by index[0].
But where is index redefined normally index0 and
index1 should have the same value.
I believe you missed something in your example.
>
> So, when I execute, I may get the following:
> PRINT, index0
> 2
>
> PRINT, index1
> -1
>
Reimar
--
Reimar Bauer
Institut fuer Stratosphaerische Chemie (ICG-1)
Forschungszentrum Juelich
email: R.Bauer@fz-juelich.de
http://www.fz-juelich.de/icg/icg1/
=============================================
a IDL library at ForschungsZentrum J�lich
http://www.fz-juelich.de/icg/icg1/idl_icglib/idl_lib_intro.h tml
http://www.fz-juelich.de/zb/text/publikation/juel3786.html
|
|
|
|
| Re: comparing numbers using WHERE doesn't work [message #25363 is a reply to message #25243] |
Tue, 05 June 2001 06:18 |
Martin Schultz
Messages: 515
Registered: August 1997
|
Senior Member |
|
|
"Miklos Z. Kiss" <mzkiss@unity.ncsu.edu> writes:
> Hi folks:
> I have noticed a strange occurence when I try to compare a number with
> all the elements of an array. I'll show my situation in an example.
> Consider the following array:
> a = .
> Let, x = 2.2000, y = 2.4000
> So, I want to find which elements of a match up with x and y.
> Naturally, I do the following:
>
>
> index0 = WHERE(a EQ x)
> index0 = index ; convert from 1-D array to single integer.
> index1 = WHERE(a EQ y)
> index1 = index
>
>
> So, when I execute, I may get the following:
> PRINT, index0
> 2
>
> PRINT, index1
> -1
>
> The output clearly tells me that x occurs at a and that y does not exist
> in a at all. By inspection, IDL should return the value 4. Does anyone
> have any idea why this happens? Thanks,
>
> Mik Kiss
You must have a strange computer system ;-). The above should normally
work, if (1) you are positively sure that all number are single
precision or all numbers are double, (2) you didn't get 2.4000 as a
result of some computation but explicitely wrote it into the vector.
But these 2 conditions show you excatly why you shouldn't use an EQ
comparison on floating point numbers.
Cheers,
Martin
--
[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[ [[[[[[[
[[ Dr. Martin Schultz Max-Planck-Institut fuer Meteorologie [[
[[ Bundesstr. 55, 20146 Hamburg [[
[[ phone: +49 40 41173-308 [[
[[ fax: +49 40 41173-298 [[
[[ martin.schultz@dkrz.de [[
[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[ [[[[[[[
|
|
|
|
| Re: comparing numbers using WHERE doesn't work [message #25373 is a reply to message #25243] |
Mon, 04 June 2001 14:24 |
John-David T. Smith
Messages: 384
Registered: January 2000
|
Senior Member |
|
|
"Miklos Z. Kiss" wrote:
>
> Hi folks:
> I have noticed a strange occurence when I try to compare a number with
> all the elements of an array. I'll show my situation in an example.
> Consider the following array:
> a = [2.0000, 2.1000, 2.2000, 2.3000, 2.4000, 2.5000, 2.6000].
> Let, x = 2.2000, y = 2.4000
> So, I want to find which elements of a match up with x and y.
> Naturally, I do the following:
>
> index0 = WHERE(a EQ x)
> index0 = index[0] ; convert from 1-D array to single integer.
> index1 = WHERE(a EQ y)
> index1 = index[0]
What is the vector "index" you reference? I think you must have meant
index0=index0[0]
JD
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
