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List manipulation question


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}





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