solving nonlinear equations

Question

I want to solve two nonlinear equations in MATLAB so i did the following:

part of my script

c=[A\u;A\v];
% parts of code are omitted.
x0=[1;1];
sol= fsolve(@myfunc,x0);

the myfunc function is as follows

function F = myfunc(x)

  F=[ x(1)*c(1)+x(2)*c(2)+x(1)*x(2)*c(3)+c(4)-ii;

     x(1)*c(5)+x(2)*c(6)+x(1)*x(2)*c(7)+c(8)-jj];

end

i have two unknowns x(1) and x(2)

my question is How to pass a values(c,ii,jj) to myfunc in every time i call it?

or how to overcome this error Undefined function or method 'c' for input arguments of type 'double'.

thanks

my question is clear or not?! – HforHisham Dec 16 '12 at 22:02 — HforHisham, Dec 16 '12 at 22:02

Apteronotus · Accepted Answer · 2012-12-17T22:31:42.263

1

Edit: The previous answer was bogus and not contributing at all. Hence has been deleted. Here is the right way.

In your main code create a vector of the coefficients c,ii,jj and a dummy function handle f_d

coeffs = [c,ii,jj];
f_d = @(x0) myfunc(x0,coeffs);      % f_d considers x0 as variables
sol = fsolve(f_d,x0);

Make your function myfunc capable of taking in 2 variables, x0 and coeffs

function F = myfunc(x, coeffs)
c = coeffs(1:end-2);
ii = coeffs(end-1);
jj = coeffs(end);

F(1) = x(1)*c(1)+x(2)*c(2)+x(1)*x(2)*c(3)+c(4)-ii;
F(2) = x(1)*c(5)+x(2)*c(6)+x(1)*x(2)*c(7)+c(8)-jj;

I think that should solve for x0(1) and x0(2).

Edit: Thank you Eitan_T. Changes have been made above.

edited Dec 17 '12 at 22:31

answered Dec 16 '12 at 22:50

Apteronotus

56
4

this means that i solve for 12 variable! which is wrong, i only have two unknowns. any way thanks for your answer – HforHisham Dec 16 '12 at 23:06
I was too hasty in writing that. Let me write the code out properly and post it. I know you are saying that 2 equations for 12 variables is inconsistent. Let me write it again. – Apteronotus Dec 16 '12 at 23:19
@Apteronotus `x0` is defined outside `myfunc`. Inside `myfunc` that should be `x`. – Eitan T Dec 17 '12 at 13:12
By the way, if `x` and `c` are vectors, you can use a scalar product instead of explicitly writing that sum: `F(1) = x * c(1:4) - coeffs(end - 1);` and `F(2) = x * c(5:8) - coeffs(end);`. However, you should pay attention to vector dimensions and transpose if necessary. – Eitan T Dec 17 '12 at 13:13

score 1 · Answer 2 · answered Dec 18 '12 at 06:56

There is an alternative option, that I prefer, if a function handle is not what you are looking for.

Say I have this function:

function y = i_have_a_root( x, a )
    y = a*x^2;
end

You can pass in your initial guess for x and a value for a by just calling fsolve like so:

a = 5;
x0 = 0;
root = fsolve('i_have_a_root',x0,[],a);

Note: The [] is reserved for fsolve options, which you probably want to use. See the second call to fsolve in the documentation here for information on the options argument.

solving nonlinear equations

2 Answers2