Re: A ToExpression question
- To: mathgroup at smc.vnet.net
- Subject: [mg58356] Re: A ToExpression question
- From: David Bailey <dave at Remove_Thisdbailey.co.uk>
- Date: Tue, 28 Jun 2005 21:56:52 -0400 (EDT)
- References: <d9r4h4$53i$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Ray Koopman wrote:
> f1[x_] := Print[x];
> f2[x_] := ToExpression["Print[x]"];
> f3[x_] := Module[{y = x}, ToExpression["Print[y]"]];
> f4[x_] := Block[{y = x}, ToExpression["Print[y]"]];
>
> Scan[#[0]&,{f1,f2,f3,f4}]
>
> 0
> x
> y
> 0
>
> Why don't f2 and f3 print 0 ?
>
Hello,
In each of these x is replaced by its value - 0 - before the RHS gets
evaluated. Thus in f2 the expression Print[x] only comes into being
after this substitution has taken place. In a Module, local variables,
like y, are replaced by unique variables such as y$30 - and again, this
replacement happens before the expression Print[y] is created. f4 works
because the values of local variabples are simply cleared and restored
afterwards - no name change is involved.
David Bailey
http://www.dbaileyconsultancy.co.uk