Re: Replacing a part of a list/matrix?
- Subject: [mg2215] Re: [mg2180] Replacing a part of a list/matrix?
- From: hay at haystack.demon.co.uk (Allan Hayes)
- Date: Mon, 16 Oct 1995 15:54:39 GMT
- Approved: usenet@wri.com
- Distribution: local
- Newsgroups: wri.mathgroup
- Organization: Wolfram Research, Inc.
- Sender: daemon at wri.com ( )
In [mg2180] Replacing a part of a list/matrix?
"O. Lee" <olee at ripco.com> writes
> 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.
>
> ............
> I appreciate any help. Thanks.
I hope that the following examples may help.
First define pop
In[1]:=
pop = {1,2};
Example 1:
In[2]:=
foo[p_] := p[[2]] = 4 (**)
In[3]:=
{foo[pop],pop}
Part::setps: {1, 2} in assignment of part is not a symbol.
Out[3]=
{4, {1, 2}}
The crucial parts of the evaluation of foo[pop] are
0. (**) is used with p_ replaced by pop;
1. pop is evaluated on the *left* to give ,{1,2};
2. then the symbol p on the right is replaced by this value
to give
{1,2}[[2]] = 4
3. the evaluation of {1,2}[[2]] = 4 causes the message.
Since the symbol pop never got to the right side its value is unchanged
Example 2:
To make a symbol with value {1,4} on the right , we can do this:
In[4]:=
foo[p_] := {p2 = p; p2[[2]] = 4, p2}
In[5]:=
{foo[pop],pop}
Out[5]=
{{4, {1, 4}}, {1, 2}}
Again, the value of pop is not changed.
Example 3:
To actually change the value of the symbol pop, we must prevent the
evaluation of anything put in place of p_ on the left.
This can be done by giving foo the attribute HoldAll (or HoldFirst)
In[6]:=
SetAttributes[foo, HoldAll]
In[7]:=
foo[p_] := {p[[2]] = 4, p} (****)
In[8]:=
{foo[pop], pop}
Out[8]=
{{4, {1, 4}}, {1, 4}}
Here the symbols p on the right of (****) have been replaced by the
*symbol* pop, not its value; so that we evaluate pop[[2]] = 4; and
after this pop has value {1,4}
Allan Hayes
hay at haystack.demon.co.uk