Assigning to variables with autowrap

[Nathaniel Virgo]

This would probably be a better solution to the problem in my previous mail, if it can be done. I've posted the same text as a question on Stack Overflow at https://stackoverflow.com/questions/52638892/using-assignment-with-autowrap-in-sympy.

I am writing some code that uses sympy to construct a complicated mathematical expression, then uses the autowrap function to compile it and evaluate it repeatedly.

Unfortunately, my expressions are becoming exponentially large in some cases. To prevent this, I want to generate expressions that contain assignment statements. That is, I want to end up with something like (pseudocode)

x1 = f(a,a);
x2 = f(x1,x1);
x3 = f(x2,x2);
x4 = f(x3,x3);

instead of

f(f(f(a,a),f(a,a)),f(f(a,a),f(a,a))),f(f(a,a),f(a,a)),f(f(a,a),f(a,a))))

in which every subexpression has been substituted twice.

There seems to be a mechanism for this, in the form of sympy.codegen.ast.CodeBlock. However, I can't seem to work out how to get this to work with autowrap, if indeed it can.

If I attempt to do

>>> f = autowrap(ast.CodeBlock(ast.Assignment(y,sym.sin(x))),backend='cython')

then I get a long stack trace ending with

wrapped_code_12.c:5:22: error: use of undeclared identifier 'y'
   autofunc_result = y = sin(x);
                     ^
1 error generated.
error: command '/usr/bin/clang' failed with exit status 1

which suggests to me that using a CodeBlock directly inside autowrap isn't the right way to do it. (I get a similar error using the Fortran backend.)

The question is, is there a right way to do it? Can I use CodeBlock in autowrap, or is there some other way that I can generate intermediate variables with autowrap rather than always having my sympy expressions fully expanded out?

Best regards

Nathaniel

[Aaron Meurer]

autowrap doesn't understand CodeBlock yet.

Ideally your last element in the CodeBlock would be an expression, and
autowrap would assign that (and it would give a better error message
if it isn't). I opened https://github.com/sympy/sympy/issues/15336 for
this.

I'm not sure if is a more proper way to do what you want. autowrap is
fairly complicated, so I can't figure out how to do it, but I suspect
it is possible.

Aaron Meurer

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