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Re: question on passing variable to function by reference

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  • Subject: [mg113591] Re: question on passing variable to function by reference
  • From: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>
  • Date: Thu, 4 Nov 2010 03:57:45 -0500 (EST)

Hi Leonid,

Thanks for the your extended responses. Very interesting. I would love to
learn more about your application of the technique in UI programming.

Cheers -- Sjoerd 

Hi Sjoerd,

FWIW, I can confirm that this is indeed pass-by-reference. I find this quite
useful at times.
Perhaps, the most useful for me was the ability to pass symbols to functions
which then 
define those symbols inside their scope as closures (functions which operate
on internal state of those 
functions outside their scope), so that I can later use them in my original
larger function to operate 
on the symbols created in a second larger function. This is very useful in
UI programming with Dynamic, for 
instance - this way you can split the monolithic UI into more managable
chunks and still 
maintain the dynamic updating of the variables. I will be happy to extract
some minimal 
example out of my code if you are interested.

Sometimes it also offers algorithmic advantages, if the algorithm is more
suited for 
in-place modifications. In this thread

http://groups.google.com/group/comp.soft-sys.math.mathematica/browse_thread/
thread/69d53a2ffa5a640e

I gave a pass-by-reference based implementation of qsort, which is
asymptotically better than 
the alternative one using immutable lists. 

The massive simultaneous assignments using Part, which are very fast, also
use pass-by-reference to gain 
the speed which is hard to achieve by other means,and are more
memory-efficient at the same time.

I am also using pass-by-reference  to dynamically generate data structures
(structs),  for example  
as described in my post in this recent thread:

http://groups.google.com/group/comp.soft-sys.math.mathematica/browse_thread/
thread/b2ddf61da56b0be9

The code there is not exactly transparent, but the idea is simple: if you
have a structure field name
say foo, you can take a symbol foo and give it HoldAll attribute. Then,
expression like 

s = With[{var = Unique[]},foo[var]] 

or 

s = foo[Evaluate[Unique[]]]

can be viewed as the "memory cell", so that you can store some information
in it. In particular,
s[[1]]=smth will assign a new value, and s[[1]] will return the currently
stored value. 

I am sure there are other uses for pass-by-reference mechanism, these are
just some off the top of
my head, in case if you are interested.

Now, regarding the check for list being a list in the OP situation.There
will be a problem with such check 
if the passed argument is stored in a variable (so that what is passed is a
variable rather than literal list), 
and if the check is implemented straightforwardly as _List, due to the
interplay between pattern-matching 
and evaluation. There was a question on the group on a very similar setting
about a year ago:

http://groups.google.com/group/comp.soft-sys.math.mathematica/browse_thread/
thread/c062579f93e093a6

and I also discussed this topic in my book at 

http://www.mathprogramming-intro.org/book/node408.html

where I consider this situation in greater detail. 


Cheers,
Leonid




On Tue, Nov 2, 2010 at 1:01 PM, Sjoerd C. de Vries
<sjoerd.c.devries at gmail.com> wrote:

Actually, I thought you couldn't do a 'pass by reference' in
Mathematica, but this really looks like the real thing.

You might want to add a test to check that list is really a List, but
otherwise I don't see a problem.

Cheers -- Sjoerd

On Nov 1, 11:01 am, "Nasser M. Abbasi" <n... at 12000.org> wrote:
> Suppose I want to pass an array to function to be filled in by some value=

 


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