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?

Answer 1

[Sean de Wolski]

doc double

double(sym_expr)

and

doc subs

Answer 2

[Walter Roberson]

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]

So if one of my variables was a, would this be the right general idea?

a = 2

y = subs(a)*4;

Thank you so much!

[Walter Roberson]

If your symbolic expression was S then

double(subs(S))

It _is_ possible to substitute for individual variables, but as you have a number of variables you will find the above easiest.

[mkeehan]

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))

[Sean de Wolski]

You could always (It pains me to say it) just use eval on the string.

[mkeehan]

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]

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