New to MATLAB - trying to write bisection method?

Hello, I'm brand new to MATLAB and am trying to understand functions and scripts, and write the bisection method based on an algorithm from our textbook. However, I'm running into problems. Could anyone help me please?

Here is my code:

function [f] = Bisection(a,b,Nmax,TOL)
f = x^3  - x^2 + x;
i=1;
BisectA=f(a);
while i <= Nmax
    p=a+(b-a)/2;
    BisectP=f(p);
    if BisectP == 0 || (b-a)/2 < TOL        
        disp('p');
    end
    i=i+1;
    if BisectA*BisectP > 0
        a=p;
        BisectA=BisectP;
    else
        b=p;
    end
end
disp('Method failed after num2str(Nmax) iterations, Nmax=', Nmax);

Thanks.

[Walter Roberson]

What problems are you running into? Are you getting an error message? If so then at which line, and under what circumstances?

[youcef mokrane]

x=4:4.7
f=tan(x)-x
a=4
b=4.7
fa=tan(a)-a
fb=tan(b)-b
n=1
n0=5000
             while n<5000
                   p=(a+b)/2
                   fp=tan(p)-p
                  n=n+1
                     if fa*fp>0
                         a=p
                       else 
                         b=p
                    end
           end

[Walter Roberson]

Why are you bothering to do x=4:4.7 ? The default increment for the colon operator is 1, so 4:4.7 is the same as 4:1:4.7 which is just going to be 4 .

Why are you assigning to n0 when you do not use it?

Answer 1

[Walter Roberson]

Your line

f = x^3  - x^2 + x;

does not define a function. Try

f = @(x) x^3  - x^2 + x;

[AM]

Hi, I tried that but it gave me an "unexpected MATLAB expression" error. I then tried f(x) = x^3 - x^2 + x;

but I received an "undefined function or variable 'x' " message.

I have no idea what's going on?!

[Walter Roberson]

Sorry somehow my fingers missed the @. I have corrected above.

[Nick Zarantonello]

You need to use dot multiplication

[SADIA RASHEED]

function [f] = Bisection(a,b,Nmax,TOL)
 f = @(x) x^3  - x^2 + x;
 i=1;
BisectA=f(a);
while i <= Nmax
    p=a+(b-a)/2;
    BisectP=f(p);
    if BisectP == 0 || (b-a)/2 < TOL        
        disp('p');
    end
    i=i+1;
    if BisectA*BisectP > 0
        a=p;
        BisectA=BisectP;
    else
        b=p;
    end
end
disp('Method failed after num2str(Nmax) iterations, Nmax=', Nmax);

%%%%but it gives an error  function [f] = Bisection(a,b,Nmax,TOL)
|
Error: Function definitions are not permitted in this context.

plz help

[Walter Roberson]

You need to store the code in a file named Bisection.m

It is not permitted to define functions from the MATLAB command line.

In all MATLAB versions up to R2016a, it was also not permitted to define functions in "script" files -- that is, files that do not start with the word "function" or "classdef". In R2016b that is now permitted, though.

[SADIA RASHEED]

okay.Thanks

[Silverio Jr Magday]

Error in ==> Bisection at 4 BisectA=f(a);

what possible correction I can make?

[Walter Roberson]

Silverio Jr Magday:

You can make the correction of going down to the command line and calling the function by name, passing in appropriate valeus for a, b, Nmax, and TOL, such as

Bisection(-8, 14, 207, 1e-10)

You attempted to run the code by clicking on "Run", which is the same as if you had gone to the command line and commanded

Bisection

without passing in anything. Then when the code reached the first line in which it needed one of the parameters, the code failed because there was no parameter there.

[A_J Khan]

   function p = bisection(f,a,b)
      |
  Error: Function definitions are not permitted in this
  context.

I have this error with above code....???

[Walter Roberson]

You can never define a function at the command prompt: you have to store a function inside a .m file; in this case, bisection.m

In versions up to R2016a you cannot store a function inside a script file (a .m file that does not start with the word "function" or "classdef"). That changed in R2016b.

Answer 2

[Frank Cano]

Here are some Bisection method examples