Re: Replacing a part of a list/matrix?

*To*: mathgroup at smc.vnet.net*Subject*: [mg2215] Re: [mg2180] Replacing a part of a list/matrix?*From*: Allan Hayes <hay at haystack.demon.co.uk>*Date*: Mon, 16 Oct 1995 11:54:39 -0400

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= 4, p} (****) In[8]:= {foo[pop], pop} Out[8]= {{4, {1, 4}}, {1, 4}} Here the symbols p on the right of (****) have been replaced by the *symbol* pop, not its value; so that we evaluate pop[[2]] = 4; and after this pop has value {1,4} Allan Hayes hay at haystack.demon.co.uk