# Converting symbolic expression into double.

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mkeehan on 19 Jul 2011
When I perform a series of functions, the result is:
1.009168*rS + 0.25*rS*dcellCurrent(1) + 0.157*rS*dcellCurrent(2) + 0.057*rS*dcellCurrent(3) - 0.002*rS*dcellCurrent(4) - 0.093*rS*dcellCurrent(5) - 0.145*rS*dcellCurrent(6) - 0.197*rS*dcellCurrent(7) - 0.232*rS*dcellCurrent(8) - 0.313*rS*dcellCurrent(9) ... *exp((120843*iSat*rSh)/(3125*n)))/(3125*n*exp((120843*rSh*dcellCurrent(13))/(3125*n))))
The expression is extremely long, I cut out a good bit of its innards so it would be viewable here. When I copy and paste the expression that it shows into the command window, the result is this number: 13.076812258420913
I know that I can just copy and paste it, but I would like to use logical expressions in an if statement, and I need to convert the expression in code because I would not like to do it by hand. How should I go about converting?

Sean de Wolski on 19 Jul 2011
doc double
double(sym_expr)
and
doc subs

Walter Roberson on 19 Jul 2011
You should use subs() to substitute the current values of the variables in to the expression, and you should use double() on the result to convert it from a symbolic number to a double precision value.
mkeehan on 19 Jul 2011
While the pain part of your answer makes me a bit nervous, it does work! Thank you both of you! How do I go about giving reputation?
Walter Roberson on 19 Jul 2011
You do not appear to have defined dcellCurrent.
Anyhow:
syms x dcC
S = simplify(solve(iP - iSat * (exp(38.66976*(x+dcC*rS)/n)-1) -(dcC*rS + x)/rSh - dcC));
subtotal = 0;
for K = 1:length(dcC)
subtotal = subtotal + (double(subs(S,dcC,dcellCurrent(K))-dcellVoltage(K)).^2;
end