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Re: acting on a matrix

  • To: mathgroup at
  • Subject: Re: acting on a matrix
  • Date: 17 Sep 1992 00:00:01 -0700 (MST)

Riccardo Rigon (rigon at asks:
>  I would like to define functions acting on a  list ( really a matrix )
>  defined outside them. I found it is possible to do something like this:
>  ( it modify the list "list" in some suitable specified location
>  assigning to it the value {1,1} )
>  list=Table[{i,j},{i,10},{j,10}];
>  MyFunction[position_list]:=Module[{vaule={1,1}},
>  list[[position[[1]],position[[2]] ]=vaule;
>  ]
>  The example works but I would like to specify in the arguments of
>  the function the name of the matrix.
>  Something like:
>  MyFunction[position_list,nameofthelist_]:=Module[{value={1,1}},
>  nameofthelist[[position[[1]],position[[2]] ]]=value
>  ]
>  This last code indeed does not works. What is wrong and/or lacking?
>  The final scope is to have some matrix (rappresenting a physical system)
>  on which different operations can act changing its internal values
>  but without reallocating each time the whole matrix or part of it except
>  for the values changed.
(First, I want to note that there must be a capital L in position_list. I
assume this is just a typo, since both functions behave really screwy without
The problem here is that by default, functions evaluate all of their arguments
before passing them into the body of the function. Look at what happens with
your second version:
nameofthelist[[position[[1]],position[[2]] ]]=value
when you evaluate
thelist = {{a,b},{c,d}};
MyFunction[{2,2}, thelist]
You want MyFunction to execute 
thelist[[2,2]] = {1,1}
but what happens is that thelist is evaluated to {{a,b},{c,d}} and then passed
into the function. Therefore, it tries to execute
{{a,b},{c,d}}[[2,2]] = {1,1}
You get the error message "Part::setps: {{a, b}, {c, d}} in assignment of part
is not a symbol.", because Mma can't assign back into a list this way. (And
even if it could, the value of the variable thelist would not be affected).
You need to give MyFunction the Attribute HoldRest (or HoldAll) to prevent it
from evaluating thelist:
SetAttributes[MyFunction, HoldRest]
Now it will work as expected. Another way to get the desired behavior is to
leave MyFunction as it is, but call it with Unevaluated wrapped around the list
MyFunction[{2,2}, Unevaluated[thelist] ]
This also overrides the default evaluation.
Mma has a built-in function, ReplacePart, that makes it easy to modify pieces
of lists. I don't know if this method would be any less efficient, but it has
the advantage that it will work without modification for any dimension of list:
SetAttributes[MyFunction2, HoldRest];
     nameofthelist = ReplacePart[nameofthelist, value, position];
Todd Gayley
Department of Ecology and Evolutionary Biology
University of Arizona
tgayley at       -or-        tgayley at

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