MATLAB: Problems with quad

quad

I'm trying to get a numerical value from a function using quad. In a separate .m-file I got a function looking like this:

function Wi = Wifun(y,T,TM,TMO,G2,A,b,x,Epsi,ny)
syms G2;syms T;syms TM;syms TMO;syms ny;syms A;syms Epsi;syms b;syms x;syms y;
G = G2.*(1+((T-300)./TM).*TMO);
Ei = -(1/2.*pi).*((1+ny)./(1-ny)).*G.*A.*Epsi;
Wi = -(2/3).*b.*x.*Ei.*(1./(x.^2+y.^2));

This function will produce this formula:

-(pi*A*Epsi*G2*b*x*((TMO*(T - 300))/TM + 1)*(ny + 1))/(3*(x^2 + y^2)*(ny - 1))

If I write like this:

syms y
T = 1273;
TM = 2163;
TMO = -0.5;
G2 = 126000;
A = 1.2E-29;
b = 0.000000000258;
x = 2/3*b;
Epsi = 0.00487903650119246;
ny = 0.3;
fun = @(y)-(pi.*A.*Epsi.*G2.*b.*x.*((TMO.*(T - 300))./TM + 1).*(ny + 1))./(3.*(x.^2 + y.^2).*(ny - 1));
z2 = quad(fun,-2,2)

It works and z2 gets the value of 4.0641e-027. But if instead of the 2 last lines try to write like this:

z = quad(Wifun,-2,2)

There will be this error:

??? Error using ==> fcnchk at 108
If FUN is a MATLAB object, it must have an feval method.
Error in ==> quad at 66
f = fcnchk(funfcn);
Error in ==> test at 19
z = quad(Wifun,-2,2)

Someone told me that you should write like this:

Wifun2 = str2func('Wifun');
z = quad(Wifun2,-2,2)

But then the error looks like this instead:

??? Undefined function or method 'isfinite' for input arguments of type 'sym'.
Error in ==> quad at 81
if ~isfinite(y(1))
Error in ==> test at 19
z = quad(Wifun2,-2,2)

Is there someone here that knows how I can do this?

Best Answer

Use matlabFunction() to convert the symbolic expression in to an executable routine.

That executable routine will want to take all those names as parameters but the routine you pass to quad() should only accept one parameter. To take care of this, follow the documentation about "Passing additional parameters" to quad and so on. Which, in short, would look like

wifun_handle = matlabFunction(wifun, {'y' 
T' 'TM' 'TMO' 'G2' 'A' 'b' 'x' 'Epsi' 'ny'});
fun = @(y) wifun_handle(y,T,TM,TMO,G2,A,b,x,Epsi,ny);

and then pass fun to quad.

By the way, you can change your line

function Wi = Wifun(y,T,TM,TMO,G2,A,b,x,Epsi,ny)

function Wi = Wifun

as you never actually pass any parameters to Wifun itself.

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MATLAB: Sending parameters to a function

Of course, "But it don't work" is very vague.

http://www.mathworks.com/matlabcentral/answers/6200-tutorial-how-to-ask-a-question-on-answers-and-get-a-fast-asnwer

Now, even though you don't say what happens, I am guessing that your input variables are immediately overwritten in the function by your declarations. When a value is passed in, you don't then re-declare it inside the function, because then whatever you passed in is lost. For example, if you pass in a value for T of 1273, as you show, the first thing that happens (on the first line!) in your function is that the value you passed in is overwritten and T is declared as a symbolic variable.

Try this simple function as an example. It simply takes a number and doubles it. First try to double any number using the function as I have written it, then try it again with the first line of the function erased. See if you get a different answer!

function B = mydouble(A)
A = rand;  % Erase this line after you try to call, say:  mydouble(3)
B = 2*A;

EDIT In response to your 'answer' below...

The first error is because you have declared your function to take 14 argument with this line:

function ci = cifun(y,T,TM,TMO,G2,A,b,x,Epsi,ny,Vc,c2,D2,Q)

But then on this line,

ci = cifun(T,TM,TMO,G2,A,b,x,Epsi,ny,Vc,c2,D2,Q)

You only pass in 13. MATLAB doesn't care about the names, just the number and order. When you call your function as above, MATLAB goes through and assigns the first passed argument to the first declared parameter. So it gives the value you passed in as T to the parameter y inside the function. Then it assigns the second passed argument to the second declared parameter. So it gives the value you passed in as TM to the parameter T inside the function... and so on until you see that the value you passed in as Q goes to the parameter D2 inside the function, leaving Q inside the function undefined.

So if your function declaration says it takes N arguments and you only pass N-1, the Nth parameter will be undeclared inside the function. If you try to then use this parameter, you will see the same error you saw.

Since you declare y as syms inside the function, you are back to the first problem you had. That you are overwriting the value as soon as it is passed in. So take the y out of the function declaration line!

Best Answer

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