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Re: Garbage collection and mutable data structures

  • To: mathgroup at smc.vnet.net
  • Subject: [mg85639] Re: Garbage collection and mutable data structures
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Sun, 17 Feb 2008 07:16:12 -0500 (EST)
  • Organization: University of Bergen
  • References: <fp67h9$5f6$1@smc.vnet.net>

massyah at gmail.com wrote:
> Hi MathGroup,
> 
> This snippet of code is giving me headaches:
> 
> -----
> $HistoryLength = 0;
> NewCell[i_] := Module[{c}, CellVal[c] ^= i; c]
> MemoryInUse[]
> NewCell /@ Range[10^5];
> MemoryInUse[]
> -----
> 
> It occurred while trying to build mutable data structures by using
> upvalues. I can't understand why the temporary variable c is not freed
> since I don't keep any reference to it.
> Have I missed something in the reference counting scheme ?

Hi,

It looks like if you let a temporary escape from the Module[] (i.e. you 
create references to it outside the Module[] by returning it), then it 
won't be cleaned up.

In[1]:= Module[{var}, var; 1]
Out[1]= 1

In[2]:= ?Global`*

Global`var

OK, temporary was cleaned up.  Now let us return it from the Module[]:

In[3]:= Module[{var}, var]
Out[3]= var$79

In[4]:= ?Global`*

Global`
var	var$79

As you see, now it persists.  Now let's try assigning something to it.

In[5]:= Module[{var}, var = 10; Hold[var]]
Out[5]= Hold[var$87]

In[6]:= ?Global`*

Global`
var	var$79	var$87


In[7]:= ?var$87

Global`var$87

Attributes[var$87]={Temporary}

var$87=10

It looks like it keeps the value.


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