Re: Error message need explaination in english
- To: mathgroup at smc.vnet.net
- Subject: [mg78664] Re: Error message need explaination in english
- From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
- Date: Fri, 6 Jul 2007 03:29:31 -0400 (EDT)
- Organization: The Open University, Milton Keynes, UK
- References: <f6if0b$428$1@smc.vnet.net>
JGBoone at gmail.com wrote:
> So i put in these commands
>
> Q=Range[10];
> size[group_]=Length[group];
> metareplace[group_] := (group[[Random[Integer, {1, size[group]}]]] =
> hubl[metacommunity]);
> (*takes random element of community size and replaces it with random
> element of metacommunity by way of the function hubl[group_]*)
> metareplace[Q]
>
>
> and get this error message
>
> Set::setps: {1, 2, 3, 4, 5, 6, 7, 8, 9, 10} in assignment of part is
> not a symbol
>
> looking at the help menu only confuses me further. can anyone tell me
> in layman's terms what the messages means?
> thanks
Arguments are passed by value. That is, when you ask Mathematica to
evaluate metareplace[Q], Mathematica first evaluates the symbol Q and
replace it by whatever value is returned, in your case by the /list/ {1,
2, 3, ... }. Only then the body of the function is evaluated and the
assignment looks like {1, 2, 3, ... }[[...]] = hubl[...], that is a list
(the actual values) without a reference to a variable/symbol. You can
see what is going on with *Trace* as in metareplace[Q] // Trace
To solve this issue, you could add a *Module* construct in your function
definition and create a local symbol as in the following example:
Q = Range[10];
size[group_] = Length[group];
metareplace[group_] :=
Module[{grp = group},
(grp[[Random[Integer, {1, size[grp]}]]] = hubl[metacommunity]);]
metareplace[Q]
Regards,
Jean-Marc