Re: Shadow error when trying to use ParallelTable
- To: mathgroup at smc.vnet.net
- Subject: [mg107501] Re: Shadow error when trying to use ParallelTable
- From: "Norbert P." <bertapozar at gmail.com>
- Date: Mon, 15 Feb 2010 05:45:54 -0500 (EST)
- References: <hl8l2g$pe8$1@smc.vnet.net>
Hey Todd,
it's not really an error message, it's just a warning. And it appears
only the first time you use Parallel functions. I don't think it will
lead to any problems in your code. It's most likely caused by some
omission in the built-in code, somebody forgot to put variable list
into a Private section of the Parallel`Preferences` package. I just
ignore the message. If you are really worried, don't use the variable
named "list" in your code.
Your message can be easily reproduced, this is what happens on a fresh
kernel:
In[1]:= list = 1;
In[2]:= ParallelTable[1, {3}]
During evaluation of In[2]:= list::shdw: Symbol list appears in
multiple contexts {Parallel`Preferences`,Global`}; definitions in
context Parallel`Preferences` may shadow or be shadowed by other
definitions. >>
Out[2]= {1, 1, 1}
Actually, someone screwed up big time, these are the names in
Parallel`Preferences` that will produce this shadowing warning:
In[3]:= Names["Parallel`Preferences`*"]
Out[3]=
{Parallel`Preferences`add,Parallel`Preferences`addPreference,Parallel`Preferences`clear,Parallel`Preferences`debugPreference,Parallel`Preferences`debugQ,Parallel`Preferences`exists,Parallel`Preferences`get,Parallel`Preferences`InitializePreferences,Parallel`Preferences`list,Parallel`Preferences`load,Parallel`Preferences`Preferences,Parallel`Preferences`prefs,Parallel`Preferences`Scope,Parallel`Preferences`set,Parallel`Preferences`tr}
As you can see, many of these are quite common names that people might
use in their programs.
Best,
Norbert
On Feb 14, 2:59 am, Todd Allen <genesplice... at yahoo.com> wrote:
> Hi Everyone,
>
> I am trying to accelerate some code using ParallelTable, but when I do , I get the following error message:
>
> list::shdw: Symbol list appears in multiple contexts {Parallel`Preferences`,Global`}; definitions in context Parallel`Preferences` may shadow or be shadowed by other definitions. >>
>
> ----------------------
>
> The basic outline of my code, which itself is INSIDE a module is:
>
> DistributeDefinitions[degenes, subraw, addonpos];
>
> finaldegenes =
> Partition[
> Flatten[ParallelTable[
> Append[degenes[[i]],
> Flatten[Select[subraw, #1[[1]] == degenes[[i, 1]] &]][[
> addonpos]]], {i, 1, Length[degenes]},
> Method -> "CoarsestGrained"]], (Length[degenes[[1]]] +
> Length[addonpos])];
>
> -------------------
>
> All the variables are themselves in the Global context and I can say that the code does work without error, if I comment out the DistributeDefinitions statement, and revert to a Table command rather than ParallelTable.
>
> I have tried to fix this problem by referring to the variables by their full context name, such as: Parallel`Preferences`degenes without success. I have also tried localizing all variables inside the module, and I still get the shadow error regardless.
>
> Do you see anything obviously wrong?
>
> What strategies do you use to figure out shadow problems in your own code? The reason I ask, is because I have tried to further isolate this problem to see if the variable contexts are changing....but it is not obvious to me they are.
>
> This is a frustrating one. Thank you for any thoughts you might have!
>
> Todd