| IDL's handling of LOGICAL quantities (WHERE) [message #17411] |
Tue, 12 October 1999 00:00  |
James Tappin
Messages: 54
Registered: December 1995
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Member |
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\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.
\end{rant}
--
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| James Tappin | School of Physics & Astronomy | O__ |
| sjt@star.sr.bham.ac.uk | University of Birmingham | -- \/` |
| Ph: 0121-414-6462. Fax: 0121-414-3722 | |
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