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Re: Bug in N_PARAMS? [message #63974]

Wed, 26 November 2008 07:49

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

Carsten Lechte writes:

> Kenneth P. Bowman wrote:
>> to provide default values for optional parameters.
>
> I wrote this little procedure for setting default values
> that I use a zillion times a day:
>
>
> PRO defaultarg, arg, defval, VALUEGIVEN=valuegiven
>
> IF N_ELEMENTS( arg) EQ 0 THEN BEGIN
> arg = defval
> valuegiven = 0
> ENDIF ELSE valuegiven = 1
>
> END

This is a pretty good idea. I can see how it would
save some typing! :-)

Here is a slightly modified version. I'll add this,
with documentation, to the Coyote Library later today.

PRO SetDefaultValue, argument, defaultValue, BOOLEAN=boolean

; If BOOLEAN, we need just a 0 or a 1.
IF Keyword_Set(boolean) THEN BEGIN

argument = Keyword_Set(argument)

ENDIF ELSE BEGIN

; We only need assignment if the argument is undefined.
IF N_Elements(argument) EQ 0 THEN BEGIN

; If the default value is undefined, treat as BOOLEAN.
; Otherwise, assign default value to the argument.
IF N_Elements(defaultValue) EQ 0 THEN BEGIN
argument = Keyword_Set(argument)
ENDIF ELSE BEGIN
argument = defaultValue
ENDELSE

ENDIF

ENDELSE

END

Cheers,

David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")

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Re: Bug in N_PARAMS? [message #63983 is a reply to message #63974]

Tue, 25 November 2008 23:53

Carsten Lechte
Messages: 124
Registered: August 2006

Senior Member

Kenneth P. Bowman wrote:
> to provide default values for optional parameters.

I wrote this little procedure for setting default values
that I use a zillion times a day:

PRO defaultarg, arg, defval, VALUEGIVEN=valuegiven

IF N_ELEMENTS( arg) EQ 0 THEN BEGIN
arg = defval
valuegiven = 0
ENDIF ELSE valuegiven = 1

END

then use it like this:

PRO somepro, arg1, arg2, KEY1=key1

defaultarg, arg1, 0
defaultarg, arg2, 'test'
defaultarg, key1, 2343d8

...

END

chl

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Re: Bug in N_PARAMS? [message #63986 is a reply to message #63983]

Tue, 25 November 2008 06:29

Kenneth P. Bowman
Messages: 585
Registered: May 2000

Senior Member

In article <MPG.2394d6a934f41609989775@news.giganews.com>,
David Fanning <news@dfanning.com> wrote:

> Kenneth P. Bowman writes:
>
>> I have been having some very odd problems with the N_PARAMS function lately.
>> ...
>> Am I missing something obvious here?
>
> N_PARAMS only tells you *how many* parameters the
> command is called with. It tells you nothing about
> the *type* of the parameter. Since "undefined" is
> a valid variable type in IDL, it is possible to
> pass an "undefined" variable and have it counted
> as a passed *number* of variables by N_PARAMS.
>
> If you are concerned with if a variable is undefined
> or not, you can't use N_PARAMS to determine this. The
> only way to tell is to use N_ELEMENTS.

Ah. That significantly restricts the utility of

SWITCH N_PARAMS() OF
0 : ...

ENDSWITCH

to provide default values for optional parameters. I suppose
that is the only way it could work with positional parameters,
as you might have an undefined parameter in the middle of the
list.

Ken

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Re: Bug in N_PARAMS? [message #64010 is a reply to message #63986]

Mon, 24 November 2008 14:20

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

Kenneth P. Bowman writes:

> I have been having some very odd problems with the N_PARAMS function lately.
> ...
> Am I missing something obvious here?

N_PARAMS only tells you *how many* parameters the
command is called with. It tells you nothing about
the *type* of the parameter. Since "undefined" is
a valid variable type in IDL, it is possible to
pass an "undefined" variable and have it counted
as a passed *number* of variables by N_PARAMS.

If you are concerned with if a variable is undefined
or not, you can't use N_PARAMS to determine this. The
only way to tell is to use N_ELEMENTS.

Cheers,

David
--
David Fanning, Ph.D.
Coyote's Guide to IDL Programming (www.dfanning.com)
Sepore ma de ni thui. ("Perhaps thou speakest truth.")

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Re: Bug in N_PARAMS? [message #64033 is a reply to message #63974]

Tue, 02 December 2008 07:03

Brian Larsen
Messages: 270
Registered: June 2006

Senior Member

David, (and others)

this is certainly one of the best things about this newsgroup
community. A question gets asked about a bug and there is some
helpful discussion then one of our resident experts (who is now a java
programmer?) puts up a bit of code that is so obvious that it is
incredibly useful. So many of my codes have

if n_elements(var) eq 0 then $
var = 245 ; or some other value

that repeat over and over and over, thanks again for providing the
clean solution.

Cheers,

Brian

------------------------------------------------------------ --------------
Brian Larsen
Boston University
Center for Space Physics
http://people.bu.edu/balarsen/Home/IDL

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Re: Bug in N_PARAMS? [message #64071 is a reply to message #63986]

Thu, 27 November 2008 13:53

Mark[1]
Messages: 66
Registered: February 2008

Member

On Nov 26, 3:29 am, "Kenneth P. Bowman" <k-bow...@null.edu> wrote:
> Ah. That significantly restricts the utility of
>
> SWITCH N_PARAMS() OF
> 0 : ...
>
> ENDSWITCH
>
> to provide default values for optional parameters.

Indeed. N_ELEMENTS is what you need for that. The only use I have
found for N_PARAMS is for writing wrapper routines

pro wrapper, P1, P2, P3, P4, _REF_EXTRA=_extra
;; Do some other stuff
case n_params() of
0: wrappee, _STRICT_EXTRA=_extra
1: wrappee, P1, _STRICT_EXTRA=_extra
2: wrappee, P1, P2, _STRICT_EXTRA=_extra
3: wrappee, P1, P2, P3, _STRICT_EXTRA=_extra
4: wrappee, P1, P2, P3, P4, _STRICT_EXTRA=_extra
endcase
end

Ugly, but sometimes necessary.

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Re: Bug in N_PARAMS? [message #64142 is a reply to message #63974]

Wed, 03 December 2008 09:06

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

David Fanning writes:

> Carsten Lechte writes:
>
>> Kenneth P. Bowman wrote:
>>> to provide default values for optional parameters.
>>
>> I wrote this little procedure for setting default values
>> that I use a zillion times a day:
>>
>>
>> PRO defaultarg, arg, defval, VALUEGIVEN=valuegiven
>>
>> IF N_ELEMENTS( arg) EQ 0 THEN BEGIN
>> arg = defval
>> valuegiven = 0
>> ENDIF ELSE valuegiven = 1
>>
>> END
>
> This is a pretty good idea. I can see how it would
> save some typing! :-)
>
> Here is a slightly modified version. I'll add this,
> with documentation, to the Coyote Library later today.

I have a doppelganger at work who takes all my good ideas
as soon as I publish them and points out all their deficiencies
and the obvious ways in which I could have made them better. :-(

To that end, I've updated the SetDefaultValue program I published
earlier to something a good deal simpler that also works in a more
intuitive way. You can find the update, if you are interested in
this sort of thing, here:

http://www.dfanning.com/programs/setdefaultvalue.pro

Cheers,

David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")

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