Re: How to manipulate globals in a function?
- To: mathgroup at smc.vnet.net
- Subject: [mg73710] Re: How to manipulate globals in a function?
- From: "oshaughn" <oshaughn at northwestern.edu>
- Date: Sun, 25 Feb 2007 04:36:55 -0500 (EST)
- References: <erop34$94v$1@smc.vnet.net>
This is an order-of-evaluation issue : when mathematica evaluates a
function, the arguments are evaluated *first*.
So for example if I have
f[a_,b_] := a=b
then
a=3
Trace[ f[a,2]]
returns an error, because the first step in evaluation is to replace
'a' by 3, before the function is called.
{{a, 3}, f[3, 2], 3 = 2, {Message[Set::
setraw, 3], {Set::setraw, Cannot assign to raw object `1`.},
However, the mathematica attribute HoldFirst fixes this problem,
preventing the first argument from being evaluated "too soon"
SetAttributes[f, HoldFirst]
Trace[f[a,2]]
--> {f[a, 2], a = 2, 2}
On Feb 24, 1:22 am, davi... at gmail.com wrote:
> I am trying to write code to manipulate a global array in a function.
> How do something like this:
>
> Moo [A_List, i_Integer] :=
>
> A[[i]] = 7;
>
> Moo[A, 1]
>
> This fails. But, if I do this without a funciton it works.
>
> i=1
> A[[i]] = 7
>
> JD