Re: Keyword precedence

[Craig Markwardt <craigmnet(at)cow.physics.wisc.edu>]

"Mark Hadfield" <m.hadfield@niwa.cri.nz> writes:

> It's interesting that David Fanning and Martin Shultz have both recommended
> the following idiom for establishing overridable defaults
> 
> pro my_plot, COLOR=color, _EXTRA=extra
>     if n_elements(color) eq 0 then color = 12
>     plot, COLOR=12, _EXTRA=extra
****       Ooops  ^^     ****
> end
> 
> This has the effect, unintended and normally irrelevant, that if the
> following call is made with the COLOR keyword set to an undefined variable
> 
> my_plot, COLOR=color
> 
> then this variable is set to 12 on output. It isn't too hard to imagine a
> situation (successive calls to different routines with different default
> colours) where this will bite an unwary programmer, though in several years
> of using this idiom I have seldom thought about this side-effect and have
> very seldom been bitten.

I have had a difficult time keeping up with this thread.  Whew!  I
often do my keyword passing with the following draconian but safe
technique.

pro my_plot, COLOR=color0, _EXTRA=extra
    if n_elements(color0) eq 0 then color = 12 $
    else color = floor(color0(0))
    plot, COLOR=color, _EXTRA=extra
end

COLOR0 is the value passed in, which is distinct from the value of the
local variable COLOR.  I agree. It's ugly.

Craig

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Craig B. Markwardt, Ph.D.         EMAIL:    craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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