Implementing Bisection Method in Matlab: Troubleshooting Error Message

[renolovexoxo]

Here is the code I have, but I keep getting the error message: Undefined function 'f' for input arguments of type 'double'.

I don't know what I have that is causing this. Does anybody see what's wrong with my code?

MaxIt = 1000;
epsilon = 10^-5;
a=1;
b=2;
c = (b+a)/2;
NumIt = 0;
while NumIt< MaxIt && abs(f(c))>epsilon
if f(a)*f(c) < 0
b = c;
else
a = c;
end
NumIt = NumIt + 1;
c = (b+a)/2;
end
end
function y = f(x)
y = exp(x)-2^-x+2*cos(x)-6;
end
Undefined function 'f' for input arguments of type 'double'.

 

[D H]

As written, you would need to store your function f(x) in a separate Matlab file named f.m. You can't have local functions in a Matlab script file. You can have local functions in a Matlab function file, so one way to avoid having a separate file for that tiny function is to turn your script into a Matlab function.

This is such a short and simple function that may not even want to do that. Another option is to use an anonymous function instead. Define f(x) via f = @(x) exp(x)-2^-x+2*cos(x)-6; You'll need to put this near the top of your script rather than at the bottom.

 

[renolovexoxo]

Okay, that worked great! Thank you.

Last question, just because I have a hard time in Matlab.
When I'm looking at the solution, do I want to display c or f(c) to get the root? It would be c, correct?

 

[D H]

c, of course. f(c) will be close to zero -- in this case. Try this, however:

• Change MaxIt to 1024 or so (adjust downward if you get divide by zero).
• Change the initial value of a from +1 to -1.
• Change your function to f = @(x) 1/x;

It's worthwhile to print either f(c) or NumIt (or both) to see if the algorithm truly did converge on a zero.

 

[blue_raver22]

It seems like the issue is that the function 'f' is not defined properly. In order for the code to run, the function 'f' needs to be defined with an input argument of type 'double'. In this case, the input argument should be 'x'. Make sure to define the function properly before running the code again. Additionally, check for any typos or syntax errors in the code. If the issue persists, try debugging by printing out the values of 'a', 'b', and 'c' to see if they are being updated correctly in the while loop.