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MathGroup Archive 1995

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Re: Replacing a part of a list/matrix?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg2221] Re: Replacing a part of a list/matrix?
  • From: withoff (David Withoff)
  • Date: Tue, 17 Oct 1995 02:33:42 -0400
  • Organization: Wolfram Research, Inc.

In article <45fl35$2t8 at ralph.vnet.net> ianc at wri.com (Ian Collier) writes:
>In article <45d22j$63m at ralph.vnet.net>, olee at ripco.com (O. Lee) wrote:
>
>> I am trying to assign a value to a matrix position, such as:
>> 
>>         pop[[ x,y ]] = value
>> 
>> in a module function. When I try manually assigning a value
>> to the matrix position outside of the module, it works. When
>> I run the function with the statement, Mathematica gives me
>> the following error message:
>> 
>> Part::setps:
>> 
>>         {{1, 1, 0, 0}, <<1>>}
>>           in assignment of part is not a symbol.
>> 
>> Can anyone tell me how to get the assignment statement to
>> work! I isolated the problem in the function to this line
>> and I am sure that what I just described was the problem.
>> I have tried using the command:
>> 
>>         pop=ReplacePart[pop,value,{x,y}]
>> 
>> but it gives me even more error messages! Help me please!
>> I appreciate any help. Thanks.
>> 
>> O. Lee
>
>ReaplacePart is the way to do this. It workes in this example:
>
>In[9]:=
>    mat = {{a,b},{c,d}}
>Out[9]=
>    {{a, b}, {c, d}}
>In[11]:=
>    mat = ReplacePart[ mat, 4, {1,2}]
>Out[11]=
>    {{a, 4}, {c, d}}
>In[12]:=
>    mat
>Out[12]=
>    {{a, 4}, {c, d}}

Part assignments (assignments of the form mat[[1,2]] = value) will
also work fine.  For example:

In[1]:= pop = Array[e, {3, 3}]

Out[1]= {{e[1, 1], e[1, 2], e[1, 3]}, {e[2, 1], e[2, 2], e[2, 3]}, 
 
>    {e[3, 1], e[3, 2], e[3, 3]}}

In[2]:= pop[[2,2]] = value

Out[2]= value

In[3]:= pop

Out[3]= {{e[1, 1], e[1, 2], e[1, 3]}, {e[2, 1], value, e[2, 3]}, 
 
>    {e[3, 1], e[3, 2], e[3, 3]}}

The fact that you got the Part::setps message indicates that you must
have used a part assignment to assign parts to the value of something
that wasn't a symbol.  Part assignments only work with symbols.

In[4]:= mat[3] = Array[e, {3, 3}]

Out[4]= {{e[1, 1], e[1, 2], e[1, 3]}, {e[2, 1], e[2, 2], e[2, 3]}, 
 
>    {e[3, 1], e[3, 2], e[3, 3]}}

In[5]:= mat[3][[2,2]] = value

Part::setps: mat[3] in assignment of part is not a symbol.

Out[5]= value

Also, the content of the message

    Part::setps:
         {{1, 1, 0, 0}, <<1>>}
           in assignment of part is not a symbol.

indicates that the left-hand side of the assignment was not only not
a part of a symbol, it was part of a fully evaluated matrix.  There
are several ways that this could happen, as in

    In[9]:= {{1, 1, 0, 0}, {0, 0, 1, 1}}[[1, 1]] = value

    Part::setps: {{1, 1, 0, 0}, {0, 0, 1, 1}}
         in assignment of part is not a symbol.

    Out[9]= value

but in order to know exactly what to do about it we'd have to see
the rest of your example.  There are easy ways to do all of this,
and the stuff that you described should work fine, so something
else significant must be going on.

Dave Withoff
Research and Development
Wolfram Research


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