| Re: IDL's handling of LOGICAL quantities (WHERE) [message #17408 is a reply to message #17403] |
Tue, 12 October 1999 00:00   |
Liam Gumley
Messages: 473
Registered: November 1994
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Senior Member |
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James Tappin wrote:
>
> \begin{rant}
> I've finally decided to have a public moan about one of the weaknesses of IDL's
> handling of logical operations: to boot -- that the WHERE function follows
> a C-like interpretation while most other things are Fortran-like.
>
> for example suppose we have an array (m) some of whose values are NaN then the
> (inefficient) loop:
> for j=0, n_elements(m) do if not finite(m(j)) then m(j)=0
> will set all non-finite elements of m to 0.
> However:
> m(where(not finite(m))) = 0
> will zero out the whole array since where sees (not 1) as a Yes.
> [The correct solution is of course:
> m(where(finite(m) ne 1)) = 0
> ]
>
> Or a simpler example:
> IDL> a = [0, 1, 0, 1]
> IDL> print, where(a eq 0)
> 0 2
> IDL> print, where(not (a ne 0))
> 0 1 2 3
>
> I guess the proper answer isto have aproper logical or boolean type and
> functions like FINITE and logical operations should return it, and of course
> WHERE should accept it.
I think it's useful to look at the output of NOT to understand what it's
doing. For example,
IDL> print, not 0
-1
IDL> print, not 1
-2
This shows that NOT is a bitwise operator for integer operands, which
sets each bit in the operand to it's complement. Funnily enough, if you
use a float operand, the results are what you'd expect of a logical
(rather than bitwise) operator, e.g.
IDL> print, not 0.0
1.00000
IDL> print, not 1.0
0.00000
To filter out non-finite values in an array, I'd use a function:
FUNCTION CHECK_FINITE, DATA, VALUE=VALUE
;- Check arguments
if n_params() ne 1 then message, 'Usage: RESULT = CHECK_FINITE(DATA)'
if n_elements(data) eq 0 then message, 'DATA is undefined'
if n_elements(value) eq 0 then value = 0.0
;- Set any non-finite elements to VALUE
index = where(finite(data) eq 0, count)
if count gt 0 then data[index] = value
;- Return the result
return, data
END
Example:
IDL> a = findgen(5)
IDL> a[0:1] = 1.0/0.0
% Program caused arithmetic error: Floating divide by 0
IDL> print, a
Inf Inf 2.00000 3.00000 4.00000
IDL> a = check_finite(a)
IDL> print, a
0.00000 0.00000 2.00000 3.00000 4.00000
Cheers,
Liam.
--
Liam E. Gumley
Space Science and Engineering Center, UW-Madison
http://cimss.ssec.wisc.edu/~gumley
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