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