Re: List Replace Problems
- To: mathgroup at smc.vnet.net
- Subject: [mg59847] Re: List Replace Problems
- From: David Bailey <dave at Remove_Thisdbailey.co.uk>
- Date: Wed, 24 Aug 2005 06:30:13 -0400 (EDT)
- 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,
First, your code would really become a lot more compact (and efficient!)
if you used functional constructs, but since not everyone is comfortable
with this style of programming, let's look at your code as is.
The basic problem is that operations that alter the contents of a list
(as opposed to, say, creating a new list) can only work if that list is
assigned to a variable - 'a' in your case. However, when you call dlst
you actually pass the value of 'a', which is a list - so it is invalid
to try to destructively alter it.
There are many ways round this. One would be to assign u to a local
variable, v (say) inside dlst, manipulate that, and then return the
whole list as the result. Of course, if your list is really vast, maybe
you don't want to do that because it involves taking a copy.
My general advice would be to read a book on Mathematica programming or
go on a course. It is all too easy to get into very bad programming
habits and struggle!
David Bailey
http://www.dbaileyconsultancy.co.uk