Converting symbolic expression into double.

Question

mkeehan 2011-7-19

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https://ww2.mathworks.cn/matlabcentral/answers/11941-converting-symbolic-expression-into-double

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?

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Answer 1

Sean de Wolski 2011-7-19

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https://ww2.mathworks.cn/matlabcentral/answers/11941-converting-symbolic-expression-into-double#answer_16340

在 MATLAB Online 中打开

doc double
double(sym_expr)

and

doc subs

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Answer 2

Walter Roberson 2011-7-19

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https://ww2.mathworks.cn/matlabcentral/answers/11941-converting-symbolic-expression-into-double#answer_16339

在 MATLAB Online 中打开

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.

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mkeehan 2011-7-19

Okay, I'm certain that I'm doing something wrong. I miss java, the for loops made more sense. I'm a little embarrassed by my failure to use it here, but I have to post the code because I'm incapable of getting this without it, probably.

iP = 0; % this is a cell in the dark

iSat = 3.917e-007;

rS = .41;

rsRange = .1:.01:1;

rSh = 268.5;

n = 1.699;

subtotal = (solve('iP - iSat * (exp(38.66976*(x+dcellCurrent(1)*rS)/n)-1) -(dcellCurrent(1)*rS + x)/rSh - dcellCurrent(1) = 0',x)-dcellVoltage(1))^2 ;

subtotal = subtotal + (solve('iP - iSat * (exp(38.66976*(x+dcellCurrent(2)*rS)/n)-1) -(dcellCurrent(2)*rS + x)/rSh - dcellCurrent(2) = 0',x)-dcellVoltage(2))^2;

subtotal = subtotal + (solve('iP - iSat * (exp(38.66976*(x+dcellCurrent(3)*rS)/n)-1) -(dcellCurrent(3)*rS + x)/rSh - dcellCurrent(3) = 0',x)-dcellVoltage(3))^2 ;

subtotal = subtotal + (solve('iP - iSat * (exp(38.66976*(x+dcellCurrent(4)*rS)/n)-1) -(dcellCurrent(4)*rS + x)/rSh - dcellCurrent(4) = 0',x)-dcellVoltage(4))^2 ;

subtotal = subtotal + (solve('iP - iSat * (exp(38.66976*(x+dcellCurrent(5)*rS)/n)-1) -(dcellCurrent(5)*rS + x)/rSh - dcellCurrent(5) = 0',x)-dcellVoltage(5))^2;

subtotal = subtotal + (solve('iP - iSat * (exp(38.66976*(x+dcellCurrent(6)*rS)/n)-1) -(dcellCurrent(6)*rS + x)/rSh - dcellCurrent(6) = 0',x)-dcellVoltage(6))^2;

subtotal = subtotal + (solve('iP - iSat * (exp(38.66976*(x+dcellCurrent(7)*rS)/n)-1) -(dcellCurrent(7)*rS + x)/rSh - dcellCurrent(7) = 0',x)-dcellVoltage(7))^2 ;

subtotal = subtotal + (solve('iP - iSat * (exp(38.66976*(x+dcellCurrent(8)*rS)/n)-1) -(dcellCurrent(8)*rS + x)/rSh - dcellCurrent(8) = 0',x)-dcellVoltage(8))^2 ;

subtotal = subtotal + (solve('iP - iSat * (exp(38.66976*(x+dcellCurrent(9)*rS)/n)-1) -(dcellCurrent(9)*rS + x)/rSh - dcellCurrent(9) = 0',x)-dcellVoltage(9))^2 ;

subtotal = subtotal + (solve('iP - iSat * (exp(38.66976*(x+dcellCurrent(10)*rS)/n)-1) -(dcellCurrent(10)*rS + x)/rSh - dcellCurrent(10) = 0',x)-dcellVoltage(10))^2;

subtotal = subtotal + (solve('iP - iSat * (exp(38.66976*(x+dcellCurrent(11)*rS)/n)-1) -(dcellCurrent(11)*rS + x)/rSh - dcellCurrent(11) = 0',x)-dcellVoltage(11))^2;

subtotal = subtotal + (solve('iP - iSat * (exp(38.66976*(x+dcellCurrent(12)*rS)/n)-1) -(dcellCurrent(12)*rS + x)/rSh - dcellCurrent(12) = 0',x)-dcellVoltage(12))^2;

subtotal = subtotal + (solve('iP - iSat * (exp(38.66976*(x+dcellCurrent(13)*rS)/n)-1) -(dcellCurrent(13)*rS + x)/rSh - dcellCurrent(13) = 0',x)-dcellVoltage(13))^2;

double(subs(subtotal))

---------------

The error message:

??? Error using ==> mupadmex

Error in MuPAD command: DOUBLE cannot convert the input expression into a double array.

If the input expression contains a symbolic variable, use the VPA function instead.

Error in ==> sym.sym>sym.double at 936

Xstr = mupadmex('symobj::double', S.s, 0);

Error in ==> transcendentalTesting at 32

double(subs(subtotal))

mkeehan 2011-7-19

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 2011-7-19

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

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Converting symbolic expression into double.

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