Re: Passing zero as a Parameter/ NOT KEYWORD_SET

["J.D. Smith" <jdsmith(at)astrosun.tn.cornell.edu>]

Andy Sowter wrote:
> 
> Hmmm!  These solutions are all very elegant, aren't they?  Gives me a
> headache, though!  There's something in my IDL manual about avoiding IF
> statements.......
> 
> For the less accomplished among us, and I'm included, if the parameter
> you're trying to pass is an unsigned integer, add 1 to it before you pass it
> to the subroutine and then subtract 1 when you get in.  Easy.  And you
> didn't have to spend ages with the manual either (actually, there's 8
> manuals with my IDL and it's a nightmare to find things - it was better when
> there were only 2!) :)
> 

That's a bit dangerous.  IDL may have strange semantics and constructs,
but that's no reason not to learn them.  I recommend peeking at a bit of
IDL source code available all over the web.  You'll find common
constructs such as:

; give foo a default value if it is undefined.
if n_elements(foo) eq 0 then foo=5  
; perform some action if flag is defined and non-zero
if keyword_set(flag) then compute_something 
; return something to the caller if var is available to them (by
reference)
if arg_present(var) then var=compute_something_else() 


The best way to proceed is pretend keyword_set() was really named
is_defined_and_non_zero().  Forget that it's called keyword_set().  It
really has nothing to do with keywords necessarily, and RSI has managed
to confound many new users with this unfortunate appellation.  I use it
in many other places.  It's good if the caller may want to explicitly
turn *off* an option by passing a value, as with a flag.  Here's a
simple guide to the use of the keyword_set vs. n_elements methods.

1.  IDL> foo, /FLAG  \ same results for "n_elements(flag) ne 0"
2.  IDL> foo, FLAG=0 /  \
3.  IDL> foo            / same results for "keyword_set(flag)"

And using an optional argument instead of keyword:

1.  IDL> foo, 1 \ same results for "n_elements(arg) ne 0"
2.  IDL> foo, 0 /  \
3.  IDL> foo       / same results for "keyword_set(arg)"

Once you look past the naming and see what these functions do, you can
easily set up any kind of argument/keyword processing you might want.

Good Luck,

JD

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