Why won't my code run.

My code keeps giving me the error

Error using bisec (line 5) Not enough input arguments.

I don't understand what am doing wrong.

function m = bisec(f, low, high, tol)
disp('Here Bisection Method Starts: ');

% evaluating both intervals
y1 = feval(f, low);
y2 = feval(f, high);
i = 0;

% error handling
if y1 * y2 > 0
    disp('sorry not correct values given...');
    m = 'Error'
    return
end

% start bisection method until nearer to zero
while (abs(high - low) >= tol)
    i = i + 1;
    % Getting new value to test for root
    m = (high + low)/2;
    y3 = feval(f, m);
    if y3 == 0
        return
    end

      % Update limits
      if y1 * y3 > 0
          low = m;
          y1 = y3;
      else
          high = m;
      end
  end

% Printing final results
w = feval(f, m);
fprintf('\n x = %f produces f(x) = %f \n %i iterations\n', m, y3, i-1);
fprintf(' Approximation with tolerance = %f \n', tol);

Answer 1

[James Tursa]

How are you calling bisec? From the error message, it looks like you are not providing all of the inputs. E.g., if you only did bisec(@myFunction) then that would only be one input, and then bisec would complain when it tried to use low because it wasn't input to the function.

A nit: If y1 or y2 start out exactly 0, your code doesn't detect this immediate solution.

[me]

I don't understand how what you mean. If that was what I did wrong how would I fix it.

[James Tursa]

Show us exactly what you type in to call your bisec function. Are you just pressing the run button from the editor? That will run the function without any inputs (and also give you the error). E.g., type this in at the command line:

bisec(@sin,-.1,.2,1e-4)

[me]

thank you. when I input that into the command line I get a solution. thank you. btw how can i write an if statement with two conditions

i am trying to say that if If a>=b or sign(f(a))*sign(f(b))==1 then we get an error

[me]

sorry I ment low>=high or sign(y1)*sign(y2)==1 then we get an error

[me]

also how do i call the function if f=sin(x)+x

[James Tursa]

You don't need to test for low>high since your code will work regardless of the order they are input (try it!). But testing for low==high makes sense. Note that if low==high then sign(y1)==sign(y2) will also be true, so that implies sign(y1)*sign(y2)==1. So the 2nd test covers the case for low==high. So maybe something like this:

if( sign(y1)*sign(y2) == 1 )
    error('Invalid inputs. low==high or signs of function results are the same');
end

But keep in mind my earlier comment about y1==0 or y2==0 being detected for immediate solution. Also note that I used the "error" function. This is the normal way for a function to stop with an error as it forces the calling function to either stop or to deal with the error. Doing "m = 'Error'" as you are currently doing simply returns a value back to the caller that may in fact get used downstream by the caller as an actual valid result, producing garbage results. It relies on a user actually looking at the screen to tell that something went wrong. This is not a good way for your function to report an error since it makes unsafe assumptions about the caller.

To use f=sin(x)+x you could do this (anonymous function handle):

f = @(x)sin(x)+x

Then pass in f as the first argument. E.g., see this link:

http://www.mathworks.com/help/matlab/matlab_prog/anonymous-functions.html?searchHighlight=anonymous%20function