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| Re: About keywords and positional parameters [message #32790 is a reply to message #32720] |
Fri, 08 November 2002 12:32  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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JD Smith (jdsmith@as.arizona.edu) writes:
> Just to be clear, I've never actually used this method (using
> arg_present() essentially as an input test) in real-life code... it's
> not really all that helpful. In fact, I wrote it just to get under
> your skin, what with your braggadocious claim that you could never
> know if a keyword is *used* ;). From my perspective, there are only
> three real cases worth keeping track of:
>
> 1. An input keyword: Use n_elements() to see if it's present, or
> keyword_set() if it's intended to be a boolean.
> 2. An output keyword: Use arg_present() if computing the output is
> actually optional in your program, and you want to save the
> trouble of calculating it if the caller didn't ask for it.
> 3. An input/output keyword: Use a combination of the above two,
> reserving n_elements/keyword_set for input tests, and arg_present
> for output tests.
JD and I don't really need to be having this
conversation, because both of us know perfectly
well how to handle keywords correctly (and neither of
us would probably ever have a need to know if a keyword
was "used" or not). But for the sake of completeness, and
for the edification of the 63.58% of IDL programmers
(according to an informal survey I've taken) that use
KEYWORD_SET incorrectly in their programs, I reiterate:
KEYWORD_SET [capitals deliberate] should be used ONLY
for keywords that have Boolean values. :-)
Cheers,
David
P.S. Let's just say that if everyone ripped KEYWORD_SET
out of their code and substituted N_ELEMENTS the world
would probably be a better place. (Or, at the very least,
more functional.)
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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| Re: About keywords and positional parameters [message #32791 is a reply to message #32720] |
Fri, 08 November 2002 11:45 |
JD Smith
Messages: 850
Registered: December 1999
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Senior Member |
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On Thu, 07 Nov 2002 14:15:18 -0700, David Fanning wrote:
> JD Smith (jdsmith@as.arizona.edu) writes:
>
>> I agree that you can't tell if a parameter is *meant to be* an input
>> parameter, an output parameter, or both without reading the
>> documentation, but you certainly can tell whether it is *capable* of
>> passing output:
>>
>> if arg_present(parameter) then parameter=myungodlyfunction(foo)
>>
>> ARG_PRESENT() may or many not reveal anything useful, depending on the
>> context. Its primary legitimate use is in avoiding calculations of
>> optional keyword/argument outputs if the caller didn't ask for them.
>
> I have a feeling this piece of helpful advice is more likely to confuse
> Mr. Zhang than enlighten him, but please don't get me started on the
> havoc caused by mis-named functions. Remember, if you want to know if a
> keyword is *used* or not in your program, you have to follow JD's
> helpful advice:
>
> pro testme, KEY=k
> if n_elements(k) ne 0 OR arg_present(k) then $
> print,'You used KEY!' else $
> print,'You neglected KEY!'
> end
>
Just to be clear, I've never actually used this method (using
arg_present() essentially as an input test) in real-life code... it's
not really all that helpful. In fact, I wrote it just to get under
your skin, what with your braggadocious claim that you could never
know if a keyword is *used* ;). From my perspective, there are only
three real cases worth keeping track of:
1. An input keyword: Use n_elements() to see if it's present, or
keyword_set() if it's intended to be a boolean.
2. An output keyword: Use arg_present() if computing the output is
actually optional in your program, and you want to save the
trouble of calculating it if the caller didn't ask for it.
3. An input/output keyword: Use a combination of the above two,
reserving n_elements/keyword_set for input tests, and arg_present
for output tests.
JD
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| Re: About keywords and positional parameters [message #32811 is a reply to message #32720] |
Thu, 07 November 2002 13:15 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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JD Smith (jdsmith@as.arizona.edu) writes:
> I agree that you can't tell if a parameter is *meant to be* an input
> parameter, an output parameter, or both without reading the documentation,
> but you certainly can tell whether it is *capable* of passing output:
>
> if arg_present(parameter) then parameter=myungodlyfunction(foo)
>
> ARG_PRESENT() may or many not reveal anything useful, depending on the
> context. Its primary legitimate use is in avoiding calculations of
> optional keyword/argument outputs if the caller didn't ask for them.
I have a feeling this piece of helpful advice is more
likely to confuse Mr. Zhang than enlighten him, but please
don't get me started on the havoc caused by mis-named
functions. Remember, if you want to know if a keyword
is *used* or not in your program, you have to follow
JD's helpful advice:
pro testme, KEY=k
if n_elements(k) ne 0 OR arg_present(k) then $
print,'You used KEY!' else $
print,'You neglected KEY!'
end
You can find all the nitty-gritty details here:
http://www.dfanning.com/tips/keyword_check.html
Cheers,
David
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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| Re: About keywords and positional parameters [message #32812 is a reply to message #32720] |
Thu, 07 November 2002 12:37 |
JD Smith
Messages: 850
Registered: December 1999
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Senior Member |
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On Thu, 07 Nov 2002 09:57:54 -0700, David Fanning wrote:
> lily_zhang (fengliza@sina.com) writes:
>
>> Would you tell me how to distinguish the keywords and positional
>> parameters in IDL procedures and functions?
>
> Positional parameters are those variables that come after the command
> name. They have a position after the name (1, 2, 3, etc.). Keyword
> parameters also come after the command name, but they have a
> KEYWORD=variable (or sometimes a /KEYWORD, which means KEYWORD=1)
> appearance. Keyword parameters can come in any order and don't affect
> the count of positional parameters.
>
> You can't tell from looking whether a positional or keyword parameter is
> an input parameter or an output parameter (or both). You have to read
> the documentation to discover this.
>
>
>
I agree that you can't tell if a parameter is *meant to be* an input
parameter, an output parameter, or both without reading the documentation,
but you certainly can tell whether it is *capable* of passing output:
if arg_present(parameter) then parameter=myungodlyfunction(foo)
ARG_PRESENT() may or many not reveal anything useful, depending on the
context. Its primary legitimate use is in avoiding calculations of
optional keyword/argument outputs if the caller didn't ask for them.
JD
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| Re: About keywords and positional parameters [message #32816 is a reply to message #32720] |
Thu, 07 November 2002 10:26 |
R.G. Stockwell
Messages: 363
Registered: July 1999
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Senior Member |
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lily_zhang wrote:
> Would you tell me how to distinguish the keywords and positional
> parameters in IDL procedures and functions?
> Thanks in advance!
"=" is the difference.
for instance:
function positionparam,positionparam,keyword=keyword
-bob stockwell
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| Re: About keywords and positional parameters [message #32818 is a reply to message #32720] |
Thu, 07 November 2002 08:57 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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lily_zhang (fengliza@sina.com) writes:
> Would you tell me how to distinguish the keywords and positional
> parameters in IDL procedures and functions?
Positional parameters are those variables that come
after the command name. They have a position after
the name (1, 2, 3, etc.). Keyword parameters also come
after the command name, but they have a KEYWORD=variable
(or sometimes a /KEYWORD, which means KEYWORD=1)
appearance. Keyword parameters can come in any order
and don't affect the count of positional parameters.
You can't tell from looking whether a positional or
keyword parameter is an input parameter or an output
parameter (or both). You have to read the documentation
to discover this.
Cheers,
David
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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