[Date Index]
[Thread Index]
[Author Index]
RE: Do Modules Produce Side Effects?
*To*: mathgroup at smc.vnet.net
*Subject*: [mg47992] RE: [mg47964] Do Modules Produce Side Effects?
*From*: "David Park" <djmp at earthlink.net>
*Date*: Wed, 5 May 2004 08:10:59 -0400 (EDT)
*Sender*: owner-wri-mathgroup at wolfram.com
Harold,
But you didn't include m in the list of local Module variables. Therefore it
treated it as the already defined Global variable.
m = i^2;
Module[{m = 4}, 2m]
m
8
i^2
For a case like this, the more efficient statement is probably
With[{m = 4}, 2m]
8
With, as the first step, replaces every occurrence of m in the body of the
expression by 4. Module creates a new internal variable and initializes it
to the value 4. It replaces every occurrence of m in the body by the new
internal variable. If we don't give the new internal variable a value, then
it will return the new internal variable.
Module[{m}, 2m]
m
2 m$29
i^2
Block creates a new value (but not variable) for m, which is only used
internally.
Block[{m = 4}, 2m]
m
8
i^2
If we don't give m a value, which is really a misuse of Block, then we see
that the original value of the original variable is used.
Block[{m}, 2m]
m
2 i^2
i^2
David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/
From: Harold Noffke [mailto:Harold.Noffke at wpafb.af.mil]
To: mathgroup at smc.vnet.net
$Version "5.0 for Microsoft Windows [2000] (November 18, 2003)"
MathGroup:
The MathBook definition of Module tells me, "Module creates new
symbols to represent each of its local variables every time it is
called." I am led by this, and other Module descriptions, to conclude
Modules do not produce side effects, like Blocks do. However, we have
...
In[1]:= m=i^2
Out[1]= i^2
In[2]:= Module[ {}, m=4; 2*m ]
Out[2]= 8
In[3]:= m
Out[3]= 4
I expected m to remain unchanged from its original i^2. But Module
changed m to 4, just as I would expect a Block to do.
Am I misunderstanding something about the "side effect safety" of
Modules?
Regards,
Harold
Prev by Date:
**RE: Re: Re: bug in IntegerPart ?**
Next by Date:
**RE: Re: bug in IntegerPart ?**
Previous by thread:
**RE: Do Modules Produce Side Effects?**
Next by thread:
**Re: Three graph into one**
| |