MATLAB: Bisection method not returning any values.

bisection

So, I've written the following code for the bisection method, however it doesn't seem to be returning any values.

function [r, n] = bisection(a,b,f,tol)
if (f(a)*f(b) > 0)
    error('Invalid choice of interval');
end
r = 0;
n = 0;
while ((b - a)/2 > tol)
    n = n + 1;
    r = (a + b)/2;
    if(f(r) == 0)
        break;
    elseif (f(a)*f(b) < 0)
        b = r;
    else
        a = r;
    end
end

I'm using it with inputs of (0,1,q,0.5) I also tried tol as 0.1 too. q is some anonymous function that I defined, in this case it's q(x) = 3x – 1. Like, I type the command Bisection(0,1,q,0.5) and it executes without returning an error but doesn't return any numerical values whatsoever. Any ideas why this is?

Best Answer

Works fine for me. It returns the wrong value, but you can fix that by changing f(a)*f(b) < 0 to f(a)*f(r) < 0.

Related Solutions

MATLAB: What is wrong with the bisection method code

You don't show us how you are calling this function, so it is hard to advise you what to correct for that.

For the code itself, some comments:

This line gives fopg1 as an input, but your code uses f:

function [z] = bisect1(a,b,fopg1,tol)

So maybe change this line to:

function [z] = bisect1(a,b,f,tol)

I assume the following line is a typo:

    elseif (f(a)*f9=(r) < 0)

and should be this instead:

    elseif (f(a)*f(r) < 0)

Your function returns a z value but you never set z in your code. Maybe you meant to use r as the return value? E.g.,

function [r] = bisect1(a,b,f,tol)

Finally, you never make use of the tol input. You should have some type of check to see when you meet the tolerance and then exit the function. This will avoid unnecessary calculations and also avoid a potential infinite loop. E.g., if you were to use your code to try to find the value of pi by looking for the root of sin(x) near 3, your current code would end up in an infinite loop. That's because sin(pi) will not be exactly 0 (because of floating point effects), and the (a+b)/2 calculation will eventually degrade and return just a again and you will end up repeating that last iteration over and over again, never quitting and never meeting your current exit criteria. In addition to a tol check, maybe check to see if n gets too large and exit on that condition as well (maybe with an error).

MATLAB: Find roots through iterative method

You can use FZERO, e.g.,

   >> f=@(x) exp(x)-3*x.^2
   >> [xroot,res]=fzero(f,.1)

You will need to provide an initial guess of each of the three roots, which you can obtain by plotting the function f(x)=e^x-3*x^2

Best Answer

Related Solutions

MATLAB: What is wrong with the bisection method code

MATLAB: Find roots through iterative method

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