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Re: How to change the argument of a function?

• To: mathgroup@smc.vnet.net
• Subject: [mg10827] Re: How to change the argument of a function?
• From: Allan Hayes <hay@haystack.demon.co.uk>
• Date: Tue, 10 Feb 1998 21:01:40 -0500
• References: <6baeau$bno@smc.vnet.net>

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:

Could you explain more fully what you wish to do please? I'm not sure
what  "changes the value of its argument" means. The trouble with

In[1]:=
f[x_]:=Module[{},
x=2*x;
x
]

In[2]:=
f[2]

>From In[2]:=
Set::setraw: Cannot assign to raw object 2.

Out[2]=
2

is that  every x on the right is replaced by 2,  so that Mathematica is
asked to evaluate Module[{}, 2 = 2  2, 2], and can't assign to 2.

But,

f[x_] := 2 x

does what you seem to have been aiming.

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