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MathGroup Archive 1999

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Re: List manipulation question

  • To: mathgroup at
  • Subject: [mg15880] Re: [mg15852] List manipulation question
  • From: David Withoff <withoff>
  • Date: Wed, 17 Feb 1999 23:33:33 -0500
  • Sender: owner-wri-mathgroup at

> Dear all,
> In the following piece of code I define a function Swap23 which is ment
> to swap elements 2 and 3 in a list.
> Executing the function on a simple list I get an error. Why do I get
> this error? Why do I not get this error when I execute the commnad from
> Swap23 "by hand" as is shown in In[4]?
> Thanks for any help
> Maarten van der Burgt
> Icos Vision Systems
> Leuven
> Belgium
> In[1]:=Swap23[L_List]:=Module
>         [
>           {temp},
>            temp = L[[2]];
>           L[[2]]=L[[3]];
>           L[[3]] =temp;
>           L
>         ]
> In[2]:= mylist = {1,2,3};
> In[3]:= Swap23[mylist]
> Set::"setps": "\!\({1, 2, 3}\) in assignment of part is not a symbol."
> Set::"setps": "\!\({1, 2, 3}\) in assignment of part is not a symbol."
> Out[3]= {1,2,3}
> In[4]:=   temp = mylist[[2]];
>           mylist[[2]]=mylist[[3]];
>           mylist[[3]] =temp;
>           mylist
> Out[4]= {1,3,2}

Swap23 is an assignment operation (it assigns a new value to mylist),
and like all assignment operations must hold the argument unevaluated
in order to work.  Also, the argument will be the symbol that is getting
a new value, not the expression that results after that symbol has
been evaluated.  

For example, you could use

In[1]:= Swap23[L_Symbol]:=Module[
                    temp = L[[2]];
                   L[[3]] =temp;

In[2]:= mylist = {1,2,3};

In[3]:= Swap23[Unevaluated[mylist]]

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

or you could set the HoldFirst attribute so that the argument will be held
unevaluated by default, as in:

In[4]:= SetAttributes[Swap23, HoldFirst]

In[5]:= Swap23[mylist]

Out[5]= {1, 2, 3}

The need to hold the argument unevaluated in assignment operations is
a logical necessity in all programming languages, since a function can
only change the value of a variable if it knows what the variable is.
All of the assignment functions in Mathematica (Set, SetDelayed,
Increment, etc.) have attributes set for holding arguments unevaluated.

Dave Withoff
Wolfram Research

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