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OptimizeExpression

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86170] OptimizeExpression
  • From: Evan Drumwright <edrumwri at hotmail.com>
  • Date: Tue, 4 Mar 2008 02:09:44 -0500 (EST)

Hi everyone,

I'm trying to send a very large (200+MB) expression to a C file using CForm, which, as you might imagine, crashes my compiler.  From other discussions on this list, I have seen that you can use the OptimizeExpression[] function to eliminate common subexpressions, and I think that this will work.  However, I don't understand how to process the resulting compound expression.  For instance:


Experimental`OptimizeExpression[(3+3*a^2 + Sqrt[5 + 6*a + 5*a^2] + a*(4 + Sqrt[5 + 6*a + 5*a^2]))/6]

generates

Out[1]= Experimental`OptimizedExpression[Block[{$$11, $$13, $$14, $$15,

                      2
>      $$16}, $$11 = a ; $$13 = 6 a; $$14 = 5 $$11; $$15 = 5 + $$13 + $$14;

                          3 + 3 $$11 + $$16 + a (4 + $$16)
>      $$16 = Sqrt[$$15]; --------------------------------]]
                                         6

Now, I'd like to get out the individual set of statements:

$$11 = a;
$$13 = 6a;
$$14 = 5 $$11;
$$15 = 5 + $$13 + $$14;
$$16 = Sqrt[$$15];
(3 + 3$$11 + $$16 + a(4+$$16)/6


But, if I were to look at the function in the optimized expression, I get the original function:

In[2]:= %[[1]]

               2                     2                             2
        3 + 3 a  + Sqrt[5 + 6 a + 5 a ] + a (4 + Sqrt[5 + 6 a + 5 a ])
Out[2]= --------------------------------------------------------------
                                      6


I suppose part of the difficulty is that I don't see a way to treat a CompoundExpression as a list of statements to be CForm'd.  Can anyone tell me how I'd do this?


Thanks,
Evan



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