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Re: List Replace Problems

  • To: mathgroup at smc.vnet.net
  • Subject: [mg59892] Re: List Replace Problems
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Wed, 24 Aug 2005 06:33:00 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, U.K.
  • References: <deeom3$3rl$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

wesh wrote:
> I want to perform some element by element operations on a class of
> lists. For a given list The operation in a much simplified form can be
> characterized by
> 
> a = {1, 3, 5, 7, 9}; 
> Do[a[[i]] /= i, {i, 1, 5}]; 
> a
> 
> Out[38]=
> {1, 3/2, 5/3, 7/4, 9/5}
> 
> Now, however, if I try to define a function to do this for a general
> list, namely,
> 
> dlst[u_List] =
>    Do[u[[i]] /= i,  {i, 1, 5}]; 
> a = {1, 3, 5, 7, 9}; 
> dlst[a]; 
> a
> 
> Set::setps: 
> ({1, 2, 3, 4, 5}) in assignment of part is not a symbol. ! More ...
> 
> Out[39]=
> {1, 3, 5, 7, 9}
> 
> I get the above error. It says I'm trying to assign one number to
> another number. Why does Mathematica perform in the first case but 
> refuse to in the second.
> 
> I tried
> 
> b = Do[ReplacePart[a,  a[[i]]/i, i], {i, 1, 5}]
> 
> but it doesn't even bother to return an error message.
> 
> 
> How, can I get the desired function?
> 
> Thanks,
> 
> Wesh
> 
Hi Wesh,

You could try a more efficient way to code your function by using a 
functional approach. For instance, use *Range* to generate a list of 
number form 1 to 5 and divide the list "a" by it.


In[1]:=
a = {1, 3, 5, 7, 9}
b = Range[5]
a/b

Out[1]=
{1, 3, 5, 7, 9}

Out[2]=
{1, 2, 3, 4, 5}

Out[3]=
{1, 3/2, 5/3, 7/4, 9/5}

Now you can write it as a function that is independent of the length of 
the list to be divided as follows:

In[4]:=
dlst[u_List] := u/Range[Length[u]];

In[5]:=
a = dlst[a]

Out[5]=
{1, 3/2, 5/3, 7/4, 9/5}

The following link may be of interest:

http://documents.wolfram.com/mathematica/book/section-1.2.3

Hope this helps,
/J.M.


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