CODEWITHSUNDEEP
matlabsymbolic-computation
Petter T
Petter T

Reputation: 3627

Matlab Symbolic expression creates overflow

I am using Matlab symbolic toolbox to create a function of high complexity. This function is then written to a .m-file (using matlabFunction). For some reason, after simplifying the function, the function is returned on a form that looks like fun = (A*1.329834759483753e310 + B*5.873798798237459e305 + ...)*7.577619127319697e-320, where A and B are functions of my variables (too complex to repeat here). That is, all the terms within the parenthesis are in the order of about 1e280 to 1e300. The problems arises when the exponents become larger than about 1.79e308, as this causes an overflow for doubles (when calling the generated .m-function). The real size of the function is nowhere close to create an overflow, but this way of expressing the function does. This would be solved if the simplify function multiplied the 1e-320 into the parenthesis, but for some reason it doesn't.

Any idea why the symbolic toolbox chooses to represent my function this way?

I have found that I can call call expand(fun) to multiply 1e-320 into the parenthesis. The resulting expression then has exponents with the expected sizes (in the range -1 to -30) but I would prefer to know the reason why the expression looks like this in the first place, and if there are better options than calling expand to avoid the problem. Besides, calling expand seems to create a more complex function than the one I have, and I am trying to obtain a function that evaluates very fast here.

Upvotes: 1

Views: 138

Answers (1)

marsei
marsei

Reputation: 7751

Large exponent multipliers are probably due to some floats in the formula. Try to avoid them and use rationals (1/2 instead of 0.5).

The best reason I've read is this one:

The floating point numbers tend to get converted to rational numbers, which usually involves multiplying by 2^53 and then factoring out the gcd from the top and bottom of the ratio. Square such a value and you are working with numbers on the order of 2^100... and so on.

Upvotes: 1

Related Questions