Mathematica 9 is now available
Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2000
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2000

[Date Index] [Thread Index] [Author Index]

Search the Archive

RE: Using Block instead of Module?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg24268] RE: [mg24251] Using Block instead of Module?
  • From: Wolf Hartmut <hwolf at debis.com>
  • Date: Wed, 5 Jul 2000 23:10:50 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com



> -----Original Message-----
> From:	d8442803 at student.nsysu.edu.tw [SMTP:d8442803 at student.nsysu.edu.tw]
To: mathgroup at smc.vnet.net
> Sent:	Tuesday, July 04, 2000 9:22 PM
> To:	mathgroup at smc.vnet.net
> Subject:	[mg24251] Using Block instead of Module?
> 
> Dear listers,
> 
> I try to output results to a file whose name is variable, and find Module 
> statement cannot fulfill this purpose, only Block can do. For example,
> 
> outputTest[lst_List, id_Integer] := Module[{fname, tmp = lst},
>       fname = ToString[id] <> ".txt";
>       ToExpression["tmp >>" <> fname];
>       ];
> 
> outputTest[{1, 2, 3}, 3]
> 
> The output file is 3.txt, but the content is "tmp". But if I change 
> Module statement into Block as below:
> 
> outputTest[lst_List, id_Integer] := Block[{fname, tmp = lst},
>       fname = ToString[id] <> ".txt";
>       ToExpression["tmp >>" <> fname];
>       ];
> 
> outputTest[{1, 2, 3}, 3]
> 
> The output file is 3.txt, and the content is correct, {1, 2, 3}. How if 
> one used to use Module, like me, should do for the same purpose?
> 
[Wolf Hartmut]  

Hallo Wen-Feng,

the problem with Module in your case, was with the name of the temporary
variable. Within Module the variable tmp (and also fname for that matter)
are renamed to tmp$nnn and fname$nnn, however not when occurring in a text
string. So after ToExpression you effectively refer to the global variable
tmp (which presumable was undefined in your case).

Observe:

 
In[43]:= $ModuleNumber
Out[43]= 16
In[44]:=
outputTest[lst_List, id_Integer] := 
    Module[{fname, tmp = lst}, fname = ToString[id] <> ".txt";
      ToExpression["tmp$16 >>" <> fname];];

In[45]:= outputTest[{1, 2, 3}, 3]

In[48]:= !! 3.txt
>From In[48]:= {1, 2, 3}

In[49]:= outputTest[{4, 5, 6}, 3]

In[50]:= !! 3.txt
>From In[50]:= tmp$16

You see, here we carefully arrange the name, such that it worked for the
first call. It did no so for the second, because then $ModuleNumber was
incremented. Block works because there the variable name is not renamed.

Your problems however presumably arose with the special form for Put

   expr >> filename

Therein filename is taken literally and automatically enclosed in
parentheses (I suppose by the frontend ???). All you have to do, is to avoid
that, e.g. (1) use Put:

 
In[54]:=
outputTest[lst_List, id_Integer] := 
    Module[{fname, tmp = lst}, 
      fname = "C:\\Temp\\" <> ToString[id] <> ".txt";
      Put[tmp, fname];];

In[55]:= outputTest[{1, 5, 5}, 3]

In[56]:= !! "C:\\Temp\\3.txt"
>From In[56]:= {1, 5, 5}


or (2) use an expression which cannot be misunderstood as a literal
filename:
 
In[65]:= 
outputTest[lst_List, id_Integer] := Module[{tmp = lst},
      tmp >> "C:\\Temp\\" <> ToString[id] <> ".txt";];

In[66]:= outputTest[{7, 8, 9}, 5]

In[67]:= !! "C:\\Temp\\5.txt"
>From In[67]:= {7, 8, 9}


Kind regards,  Hartmut


  • Prev by Date: Re: Piecewise functions definition and usage
  • Next by Date: Re: Drawing polytopes
  • Previous by thread: Re: Using Block instead of Module?
  • Next by thread: Re: Using Block instead of Module?