MATLAB: Bisection script running endlessly

bisectionendlessinfiniteloop

I'm trying to develop a script to obtain the root of a function using the bisection method. This is what I have:
function [ TC ] = bisect(xl, xu, es, osf)
format long
xl = xl + 273.15;
xu = xu + 273.15
xr = (xl + xu)/2;
ea = inf; 
ii = 1; 
while ea > es;
 xr_vec = zeros(1,ii) 
    if fTa(xl, osf)*fTa(xr, osf) < 0 
        xu = xr
        ii = (ii-1) + 1; 
        xr = (xl + xu)/2;
        xr_vec(ii) = xr;
        if ii > 1; 
            xr = (xl + xu)/2;
        xr_vec(ii) = xr; 
        ea = (xr_vec(ii) - xr_vec(ii-1))/(xr_vec(ii))*100;
        end
    else xl = xr
    ii = (ii-1)+1;
    xr = (xl + xu)/2;
    xr_vec(ii) = xr;
    if ii > 1;
        xr = (xl + xu)/2;
        xr_vec(ii) = xr;
        ea = (xr_vec(ii) - xr_vec(ii-1))/(xr_vec(ii))*100;
    end
    end
end
TC = xr - 273.15;
end
function f = fTa(Ta, osf)
f = -139.34411 + (1.575701e5)/(Ta) - (6.642308e7)/(Ta^2) + (1.243800e10)/(Ta^3) - (8.621949e11)/(Ta^4) - log(osf);
end
Command window input: bisect(0, 0.4, 0.05, 8)
  • TC = temperature in Celsius
  • TA = temperature in Kelvin
  • osf = oxygen concentration (just another input variable)
  • xl = specified lower guess
  • xu = specified upper guess
  • xr = average of xl and xu
  • ea = the relative % approximate error
  • es = specified error boundary
I'm putting all of the xr values in a vector so that ea can be calculated easily. The fTa function at the bottom is being called in the bisect function.
Every time I run it, I keep getting an endless loop with recurring numbers, and I'm not sure where it's going wrong.
I appreciate any help, Thank you

Best Answer

In two places you have
ii = (ii-1) + 1;
That leaves ii unchanged at 1, since (1-1)+1 is 1. Your code for ii > 1 will never be invoked.
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