Re: How to change the argument of a function?
- To: mathgroup@smc.vnet.net
- Subject: [mg10970] Re: How to change the argument of a function?
- From: Allan Hayes <hay@haystack.demon.co.uk>
- Date: Sun, 15 Feb 1998 02:10:36 -0500
- References: <199802050558.AAA14348@smc.vnet.net.> <6brkhm$lqs$3@dragonfly.wolfram.com> <6bups6$lgg@smc.vnet.net>
Stonewall Ballard wrote:
>
> In article <6brkhm$lqs$3@dragonfly.wolfram.com>,
> seanross@worldnet.att.net wrote:
>
> > Vilis Nams wrote:
> > >
> > > I want to define a function that in the course of doing something, also
> > > changes the value of its argument, for example: f[x_]:=Module[{},
> > > x=2*x;
> > > x
> > > ]
> > > When I run the function, I get the error: "Set::setraw : Cannot assign
> > > to raw object ...." Can this be done in some way?
> > >
> > > --------------------
> > > Vilis O. Nams
> > > Dept of Biology, NSAC
> > > Box 550, Truro, NS, Canada
> > > vnams @ nsac.ns.ca
> > > -------------------
> >
> >
> > If I have an argument I want to change in the course of a functional
> > evaluation, I make a "working" copy at the beginning of the function:
> >
> > f[x_]:=Module[{y},
> > y=x;
> > > y=2*y;
> > > y
> > > ]
> >
> > I don't think you can change the value of the actual argument inside a
> > function, or if you could that it would be a good idea. If you made
> > global changes inside a local function, it might make for some
> > interesting bugs that would be very hard to track down. --
> > Remove the _nospam_ in the return address to respond.
>
> This is a classic programming problem, solved by using "reference" args
> or "call by name". The example used in most cases is swap[x, y], which
> swaps the values of the arguments.
>
> In Mathematica it can be done like:
>
> swap[x_, y_] := Module[{t = x}, x = y; y = t;]
>
> SetAttributes[swap, HoldAll]
>
> then try:
>
> foo = 1
> 1
> bar = 5
> 5
> swap[foo, bar]
>
> foo
> 5
> bar
> 1
>
> - Stoney
>
> --
> Stonewall Ballard StoneSoft, Inc.
> sb.nospam@stonesoft.com http://www.stonesoft.com/
Or
In[1]:=
$Line=0;
In[1]:=
SetAttributes[swap, HoldAll]
swap[x_, y_] := ({x,y} = {y,x};)
In[3]:=
{foo, bar} = {1,5}
Out[3]=
{1, 5}
In[4]:=
swap[foo,bar]
In[5]:=
{foo,bar}
Out[5]=
{5, 1}
--
Allan Hayes
Training and Consulting
Leicester, UK
hay@haystack.demon.co.uk
http://www.haystack.demon.co.uk
voice: +44 (0)116 271 4198
fax: +44 (0)116 271 8642
- References:
- How to change the argument of a function?
- From: Vilis Nams <vnams@nsac.ns.ca>
- How to change the argument of a function?