| Re: IDL versus MATLAB : could you help me ????? [message #28351 is a reply to message #28135] |
Mon, 03 December 2001 13:56   |
Mark Hadfield
Messages: 783
Registered: May 1995
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Senior Member |
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From: "Nabeel" <nabeel@mathworks.com>
>> Well, in MatLab you must either keep track of the
>> order of input parameters, or you have to parse the
>> input manually (of type which is done in the plotting routines:
>
> Not anymore - there's a relatively common programming pattern in MATLAB
> that makes use of structures to address this functionality. ...
Thanks for that info, Nabeel. I am a long-time IDL user in a discipline
where Matlab is the de facto standard so every so often I try Matlab out.
Invariably I find myself stuck trying to do things in an IDL-ish way. The
issue of optional arguments is a particularly sticky area. When I explain to
colleagues what I am trying to do they invariably say, "Huh? Why would you
want to do that?" The next time I dabble with Matlab I will try out your
suggestion.
But what I would like to know is, does Matlab support anything like IDL's
keyword inheritance? It is extremely useful for wrapper functions. I don't
know if you know that means so I will try to illustrate with an example. The
IDL plot command (like Matlab's plot, i think) accepts a huge list of
optional arguments. In IDL they are implemented as keywords, eg you might
write
plot, x, y, xrange=[10,20], yrange=[0,3], color=10B
Let's say I want a routine that, by default, plots data in red. (Let's also
assume that through a deep and mystical knowledge of IDL
colour-specification I have associated red with value 10B. Hmmm maybe this
isn't such a good example but I will persist.) Then I could write plot_red
like this
pro plot_red, p1, p2, _EXTRA=extra
plot, p1, p2, COLOR=10B, _EXTRA=extra
end
When plot_red is called all its keywords are bundled up (into a structure in
this case) and passed to plot. If there is no COLOR keyword amongst them
then the "COLOR=10B" keyword is passed to plot, but if there is a COLOR
keyword then it takes precedence.
This is extremely useful once you get the hang of it. They key thing is that
in defining plot_red I don't have to know or care what keywords are accepted
by plot, other than COLOR.
> I'm not sure about what other programming languages
> everyone here is familiar with, but if you aren't familiar
> with structures you can think of them as hash tables, with
> the key being specified after the "."
IDL users will find that Matlab structures are a lot like IDL ones, but
there is a significant difference. IDL structures are immutable, i.e. once
you've made one the only way to add a tag is to destroy the old structure
and create a new one. This is what the following code does:
s = {param1: 42}
s = create_struct(s, 'param2', "Nabeel'}
In Matlab, you can extend an existing structure any time you like, and the
syntax is simpler, eg:
S.param1 = 42;
S.param2 = 'Nabeel';
So Matlab wins on flexibility here. Of course, there may be efficiency
advantages to IDL's approach
---
Mark Hadfield
m.hadfield@niwa.cri.nz http://katipo.niwa.cri.nz/~hadfield
National Institute for Water and Atmospheric Research
--
Posted from clam.niwa.cri.nz [202.36.29.1]
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