Re: Named Indices

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

David Fanning wrote:
> 
> Jason P. Meyers (jpm7934@cis.rit.edu) writes:
> 
> >     I am brand new to IDL programming.  I am using version 5.3.1 on a PC
> > and started going through some examples from my Digital Image Processing
> > (DIP) class as well as some from the online manuals and Dave Fanning's
> > book.  I got to wondering if there exists (either built-in or developed
> > by someone else) a list of named indices. Such a beast would be useful
> > for specifying things line styles using names instead of numbers.
> >
> > For example,
> >
> >     PLOT, time, curve, LineStyle=LongDash
> >
> > makes more sense to me than
> >
> >     PLOT, time, curve, LineStyle=5
> >
> > One idea I had was to create a structured system variable named !I (for
> > indices) which then has the various indices as fields.  For example:
> >
> >     !I.Solid = 0
> >     !I.Dot = 1
> >     !I.Dash = 2
> >     !I.DashDot = 3
> >     !I.Dash3Dot = 4
> >     !I.LongDash = 5
> >
> > I couldn't find a reference to such a system variable.  Furthermore, I
> > don't even know if it is possible to create new system variables like
> > this.  Finally, if something like this can be created, I don't want to
> > reinvent the wheel if it already exists.
> 
> Well, it *is* possible to do something like this, since
> you can make new system variables. In fact, to do what
> you want to do requires an IDL statement like this:
> 
> DefSysV, '!LS', {solid:0, dot:1, dash:2, dashdot:3, dashdotdot:4, longdash:5}, 1
> 
> I used !LS for "line style", but what name you choose is up to you.
> You can put this kind of statement in, for example, an IDL
> start-up file.
> 
> The principle reason this is not done more often
> is that programs that rely on it:
> 
>    Plot, Findgen(11), Linestyle=!LS.Dash
> 
> don't work when they are passed along to someone else.
> You always forget to give them your start-up file, or
> they got it, but they forget to run it, etc. So most of
> us stay with the lowest common denominator code:
> 
>    Plot, Findgen(11), Linestyle=2
> 
> And most of us (present company included) don't have any
> idea what that code does five minutes after we write the
> program.
> 

A slightly more portable solution, in the sense that it requires no modification
of startup code but simply inclusion of a program somewhere on the path
(presumably in the same directory tree as the program which would use it), is a
special purpose function.  E.g.

plot, findgen(11), LINESTYLE=linestyle(/DASH)

You could even make an uber-function that encapsulates all such "named
entity"<->"integer value" constant mappings:

const(/LINESTYLE,/DASH)  ; produces 2

const(/AXIS_STYLE,/FORCE,/SUPPRESS) ; produces 4 or 2 = 6

const(/SIZE,/INTEGER)               ; produces 2

const(/WIDGET_DRAW_TYPE,/BUTTON_RELEASE) ; produces 1

const(/AXIS_NUMBER,/X) ; produces 0

const(/COLOR_TABLE,/PRISM) ; produces 6

etc.

The problem is you have to maintain it with new versions of IDL, and ensure
backwards compatibility too.  Other issues crop up, such as private color table
lists, etc.  It would definitely help readability though.  Sounds like a great
beginner programming project that would readily educate one on the subtle
distinctions among keyword_set(), n_elements() ne 0, and arg_present().  

JD

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