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Re: A ToExpression question
*To*: mathgroup at smc.vnet.net
*Subject*: [mg58351] Re: [mg58316] A ToExpression question
*From*: Andrzej Kozlowski <andrzej at akikoz.net>
*Date*: Tue, 28 Jun 2005 21:56:48 -0400 (EDT)
*References*: <200506280913.FAA05098@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
On 28 Jun 2005, at 18:13, 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 ?
>
>
To analyze this we need a more explicit model for the commonly used
construction
f[x_]:=Body
I believe that, with the above definition, when you evaluate f[u]
you get
Unevaluated[Body]/.HoldPattern[x]->u
So using this model let's first consider f2[0]. This is
Unevaluated[ToExpression["Print[x]"]]/.HoldPattern[x]->0
However, there is nothing to match x in the unevaluated ToExpression
["Print[x]"] , ("Print[x]" is a string so the x is not matched) hence
the replacement has no effect and x is printed.
Now consider f4[0]. This is:
Unevaluated[Block[{y = x}, ToExpression["Print[y]"]]]/.HoldPattern[x]->0
This time we get
Block[{y = 0}, ToExpression["Print[y]"]]]
Now the evaluation of ToExpression["Print[y]"]] proceeds with the
value of y set to 0, so first ToExpression["Print[y]"]] is evaluated
to Print[y] and then 0 is printed.
Finally, consider f3[0]. This gives
Unevluated[Module[{y = x}, ToExpression["Print[y]"]]]]/.HoldPattern
[x]->0
This is turned into
Module[{y=0},ToExpression["Print[y]"]]]
but unlike in the case of Block, before evaluation y is renamed as y
$somenumber but of course the y in "Print[y]" is not renamed since it
is still a part of a string. So in effect we get
Block[{y$somenumber=0},ToExpression["Print[y]"]]] and of course this
will print y and not 0.
Andrzej Kozlowski
Chiba, Japan
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