Error in Bisection Method

I used a code for bisection method, which supposed to be working, unfortunately it´s not and I do not know what is the problem. This code also includes user defined precision and a counter for number of iterations. The error I´m getting is for the last line in the code: Undefined function or variable 'c'.

a=-5;
b=0;
tol = input('What precision would you like? ','s')
fa=a^3-20+exp(a);
fb=b^3-20+exp(b);
counter=1
while abs(b-a) > tol
  c=(a+b)/2;
  fx = c*c-2;
  if fa*fc<0
      b=c;
      fb=fc;
  elseif fb*fc<0
      a=c;
      fa=fc;
  else
      break
  end
  fprintf('Just finished iteration #%d\n', counter);
  counter=counter+1;
end
x=c;

Answer 1

[John D'Errico]

NEVER hard code a function. Outside of the loop, you appear to be tring to solve the function

x^3 - 20 + exp(x) == 0

Inside of the loop, you are trying to solve the problem

x^2 - 2 == 0

So even if this ever somehow incorrectly did terminate, you would get garbage for a result.

Learn how to use a function handle. Or learn how to define a function.

The real reason why your code failed? I think you don't understand how to use input. READ THE HELP!

tol = input('What precision would you like? ','s')
What precision would you like? 0.001
tol =
    '0.001'

So I provided a tolerance of 0.001. But did MATLAB understand me? Because when you used input, you told MATLAB to accept the result as a string!

whos tol
Name      Size            Bytes  Class    Attributes
tol       1x5                10  char

So tol is the character string '0.001'. tol s NOT a number. Is there a good reason why you used input in that way? NO.

READ THE HELP FOR TOL.

help tol