| Re: code snippet [message #25286] |
Wed, 30 May 2001 09:26 |
Brian Jackel
Messages: 34
Registered: January 1998
|
Member |
|
|
But then there's the following fun:
IDL> key= 1 & print,(['first','second','third','fourth'])[[key]]
second
IDL> key= 3 & print,(['first','second','third','fourth'])[[key]]
fourth
IDL> key= 5 & print,(['first','second','third','fourth'])[[key]]
fourth
IDL> key= -999 & print,(['first','second','third','fourth'])[[key]]
first
Which is a "feature" when you expect it, and a bug when you don't :)
Brian
|
|
|
|
| Re: code snippet [message #25293 is a reply to message #25286] |
Tue, 29 May 2001 17:38  |
m.hadfield
Messages: 36
Registered: April 2001
|
Member |
|
|
From: "Craig Markwardt" <craigmnet@cow.physics.wisc.edu>
> choice = (['first','second','third','fourth'])[key]
> choice = (['first','second','third','fourth'])(key)
> choice = (['first','second','third','fourth'])([key])
> choice = (['first','second','third','fourth'])[(key)]
> One of these four expressions has a slightly different meaning than
> the other. As a trivia test to the newer users of IDL on the
> newsgroup (or the older ones!), can you say which one is different?
I think I qualify as one of the older ones.
IMHO you could (and should) eliminate some of the confusion by declaring
"compile_opt STRICTARR" or "compile_opt IDL2" in your startup file so that
numbers 2 and 3 are illegal.
Now, this
choice = (['first','second','third','fourth'])[key]
is the same as this
choice = (['first','second','third','fourth'])[(key)]
and indeed they are both the same as this
choice = (['first','second','third','fourth'])[((((((key))))))]
because all those parentheses around "key" have no effect.
But this
choice = (['first','second','third','fourth'])[[key]]
is different from all the others because the index is now a 1-D array, so
the result is also a 1-D array.
But what about these
choice = (['first','second','third','fourth'])[[[[[[key]]]]]]
choice = (['first','second','third','fourth'])[([([(key)])])]
choice = ([['first','second'],['third','fourth']])[[key]]
choice = ([['first','second','third'],'fourth'])[[key]]
I'll leave them as an exercise (I think they're all valid).
---
Mark Hadfield
m.hadfield@niwa.cri.nz http://katipo.niwa.cri.nz/~hadfield
National Institute for Water and Atmospheric Research
--
Posted from clam.niwa.cri.nz [202.36.29.1]
via Mailgate.ORG Server - http://www.Mailgate.ORG
|
|
|
|
| Re: code snippet [message #25294 is a reply to message #25293] |
Tue, 29 May 2001 15:03 |
Craig Markwardt
Messages: 1869
Registered: November 1996
|
Senior Member |
|
|
"Dick Jackson" <dick@d-jackson.com> writes:
> choice = (['first','second','third','fourth'])[key]
Heh heh, and just to be archane, why not try these:
choice = (['first','second','third','fourth'])(key)
choice = (['first','second','third','fourth'])([key])
choice = (['first','second','third','fourth'])[(key)]
One of these four expressions has a slightly different meaning than
the other. As a trivia test to the newer users of IDL on the
newsgroup (or the older ones!), can you say which one is different?
[ And why ? ]
Craig
--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
------------------------------------------------------------ --------------
|
|
|
|
| Re: code snippet [message #25295 is a reply to message #25294] |
Tue, 29 May 2001 14:21 |
Dick Jackson
Messages: 347
Registered: August 1998
|
Senior Member |
|
|
Hi,
Ken Mankoff wrote...
> Hi everyone,
>
> i just ran across this in an IDL (its inside 'map_continents.pro')
> procedure, and thought its behaviour was pretty neat. [...]
>
> IDL> key = 2
> IDL> ;;; set 'key' to 0 or 1 or 2 or ...
> IDL> possibilities = ['first','second','third','fourth']
> IDL> choice = (possibilities)[key]
> IDL> help, choice
> CHOICE STRING = 'third'
Perhaps less neat is the (equivalent) simple array access:
choice = (possibilities)[key]
But neater still is subscripting an array built on-the-spot
(again equivalent to the original):
choice = (['first','second','third','fourth'])[key]
Note that all parentheses and brackets are required here.
Cheers,
--
-Dick
Dick Jackson / dick@d-jackson.com
D-Jackson Software Consulting / http://www.d-jackson.com
Calgary, Alberta, Canada / +1-403-242-7398 / Fax: 241-7392
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
