Output argument not assigned during call

I know this has been asked before, but I still can't figure out what's wrong. I'm not a coder and am only doing this because of a class I'm forced to take, so apologies in advance if I'm missing something obvious.

I have this on one script file:

function [x, e] = bisection(f, l, r)%, n)

% Inputs: f -- an inline function
%         l,r -- left and right bounds of the interval
%         
% Outputs: x -- the estimated solution of f(x) = 0
%          e -- an upper bound on the error

format long
c = f(l); d = f(r);
if c*d > 0.0
    error('Function has same sign at both endpoints')
end

disp('x, y')

%for i=1:n
while f(r) - f(l) > 0.1 
    x = (l + r)./2;
    y = f(x);
    disp([x, y])

      if y == 0.0     % solved the equation exactly
          e = 0;
          break      % jumps out of the for loop
      end

      if c*y < 0
          r=x;
      else
          l=x;
      end
  end

e = (r-l)./2;

and then call it using the following

f = @(a) 402 + a.*cosh(215./a) - 757.7;

[x, e] = bisection(f, 1, 200);

but then get "Output argument "x" (and maybe others) not assigned during call to "bisection"."

any pointers?

edit: I got it to work by changing the while loop to a for i:n loop, and then adding the 'n' in as an input. However, I'm trying to keep the function going until the range of values is less than 0.1 apart, so I'm still not sure how to do that

Answer 1

[Greg]

The output variable x is only set (assigned a value) inside the while loop. What happens if your test condition ( f(r) - f(l) > 0.1 ) never evaluates true to enter the loop?

[Andres Gomez]

Thanks for the answer! I just noticed that and ended up changing the while loop to "for i:n" and specifying 'n' in the function input. However, I still want the function to run until the range of values is less than 0.1 apart (hence why I had the while loop at first). Should I have to add an 'else' for when f(l) - f(r) < 0.1?

[Greg]

Depending on the code that calls bisection (specifically what it does with the outputs), I would just use x = NaN before the loop.