| Re: Arg_Present, XSurface, and Other Assorted Blunders [message #9708] |
Fri, 08 August 1997 00:00 |
davidf
Messages: 2866
Registered: September 1996
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Senior Member |
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Kirt Schaper writes in response to my longish article:
> I might be missing something here, but it seems to me that 'arg_present'
> is doing precisely what it is supposed to do, report whether the
> variable passed to it is (1) an argument to the current routine AND
> (2) a variable into which a value will be copied when the current
> routine exits. If the user called the routine w/o specifying the a
> value for "data", then there is NO way that any value assigned to a
> variable "data" w/in the routine. (This is assuming that they did not
> pass in an unassigned variable as a keyword parameter.)
>
> 'Arg_present' is intended to allow avoiding computation of output
> variables if the calling routine has no way of retrieving those values.
Yes, I am pretty clear now about its rather limited usefulness.
My point is exactly this: its *name* implies things about it that
are not true. Hence, it is likely to be misused by unwary programmers
like me.
Cheers,
David
------------------------------------------------------------ ---
David Fanning, Ph.D.
Fanning Software Consulting
Customizable IDL Programming Courses
Phone: 970-221-0438 E-Mail: davidf@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com
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| Re: Arg_Present, XSurface, and Other Assorted Blunders [message #9711 is a reply to message #9708] |
Fri, 08 August 1997 00:00  |
Kirt Schaper
Messages: 6
Registered: February 1997
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Junior Member |
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David Fanning wrote:
>
> ... alot of stuff and then ...
>
> I looked at the code near the LoadData call and here is
> what I found:
>
> Catch, error
> IF error NE 0 THEN BEGIN ; Can't find LoadData.
> data = DIST(41)
> x = Findgen(41)
> y = Findgen(41)
> ENDIF
>
> IF Arg_Present(data) EQ 0 THEN BEGIN
> data = LoadData(2)
> ENDIF
>
> Here is what this piece of code is *meant* to do:
> I want to supply some default data if the user doesn't
> pass data into the program in the argument "data".
> If I need to create some data I want to use my LoadData
> program to get it. But I also know that some people won't
> have LoadData, so I want to "Catch" the error that happens
> when you try to call a program that is not on your path.
> If I catch the error, I create the "data" variable and
> continue program execution.
>
> Because this is a program using IDL 5 specific functionality,
> I also wanted to use the new Arg_Present routine to prove
> that I am an up-to-date and with-it IDL programmer.
> Apparently, I forgot to read the documentation. In any case,
> here is what happens.
>
> When the call is first made, "data" is not present and
> Arg_Present reports this correctly. The LoadData error occurs
> and I bounce up to my error handler. I define "data" and
> continue. But Arg_Present *STILL* reports data as missing
> in action. This is so even when it is defined AND a variable
> that is passed by reference. As a result, my code goes into
> an infinite loop.
>
> So now I am confused about exactly what Arg_Present is suppose
> to do, but I do know this: it is a grievous mistake to treat
> Arg_Present as an function that tells you if an argument
> is present! To fix the problem I swallowed my pride and
> went back to the terribly misnamed N_Elements to solve
> my problem.
I might be missing something here, but it seems to me that 'arg_present'
is doing precisely what it is supposed to do, report whether the
variable passed to it is (1) an argument to the current routine AND
(2) a variable into which a value will be copied when the current
routine exits. If the user called the routine w/o specifying the a
value for "data", then there is NO way that any value assigned to a
variable "data" w/in the routine. (This is assuming that they did not
pass in an unassigned variable as a keyword parameter.)
'Arg_present' is intended to allow avoiding computation of output
variables if the calling routine has no way of retrieving those values.
To illustrate, consider the following program/output:
<<<<<<<<<<<<<<<<<<<<< start program >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
pro idl_test, arg1, ARG2=arg2
print,'n_params=',n_params()
if (arg_present(arg1)) then print,'arg1 present' else print,'arg1
absent'
if (keyword_set(arg1)) then print,'arg1 set' else print,'arg1 not
set'
print,'n_elements(arg1)=',n_elements(arg1)
if (arg_present(arg2)) then print,'arg2 present' else print,'arg2
absent'
if (keyword_set(arg2)) then print,'arg2 set' else print,'arg2 not
set'
print,'n_elements(arg2)=',n_elements(arg2)
end
<<<<<<<<<<<<<<<<<<<<< end program >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
<<<<<<<<<<<<<<<<<<<<< start output >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
IDL> idl_test
n_params= 0
arg1 absent
arg1 not set
n_elements(arg1)= 0
arg2 absent
arg2 not set
n_elements(arg2)= 0
IDL> help,xxx
XXX UNDEFINED = <Undefined>
IDL> idl_test,xxx
n_params= 1
arg1 present
arg1 not set
n_elements(arg1)= 0
arg2 absent
arg2 not set
n_elements(arg2)= 0
IDL> help,xxx,yyy
XXX UNDEFINED = <Undefined>
YYY UNDEFINED = <Undefined>
IDL> idl_test,xxx,arg2=yyy
n_params= 1
arg1 present
arg1 not set
n_elements(arg1)= 0
arg2 present
arg2 not set
n_elements(arg2)= 0
<<<<<<<<<<<<<<<<<<<<< end output >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
So, 'arg_present' (as opposed to 'n_elements') can detect whether there
is any reason to supply a OUTPUT variable. It isn't as useful for
detecting
whether or not an INPUT variable needs to have a default value supplied
for
it, and 'n_elements' should be used for this.
-------------------------------------------------------
Don't just do something, sit there!
-------------------------------------------------------
Kirt Schaper | Bell: (612) 725-2000 x4791
PET Center (11P) | net: kirt@pet.med.va.gov
VA Medical Center | FAX: (612) 725-2068
MPLS, MN 55417 | URL: http://pet.med.va.gov:8080
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