Mathematica 9 is now available
Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
1999
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 1999

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: List manipulation question

  • To: mathgroup at smc.vnet.net
  • 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 wolfram.com

> 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},
                    temp = L[[2]];
                   L[[2]]=L[[3]];
                   L[[3]] =temp;
                   L
                 ]

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


  • Prev by Date: Re: QM,physics, mathematica
  • Next by Date: Help - Single Variable Calc
  • Previous by thread: RE: List manipulation question
  • Next by thread: Re: List manipulation question