Unregcognized function or variable 'xotold'

Im trying to get this function to print out the xopt and fopt but i dont know what other inputs I need to put in. How could i fix the error on xoptold?

format short
x = linspace(0,10);
xl = 0; xu = 10;
g = @(x) (2*x) ./ (4+0.8*x + x^2 + 0.2*x^3);
[xopt, fopt] = MinParabolicInterpolation(g, xl, xu)
%%Unregcognized function or variable 'xotold'
%error in MinParabolicInterpolation (line 29)

plot(x,g,fopt,'*g')
xlabel('c(mg/L)');
ylabel('g (d-1)');
title('Yeast Growth Rate versus Antibiotic Concentration');

function [xopt,fopt]=...
    MinParabolicInterpolation(func,xlow,xhigh,es,maxit,varargin)
    
    if nargin<3,error('at least 3 input arguments required'),end
    if nargin<4||isempty(es), es=0.0001;end
    if nargin<5||isempty(maxit), maxit=50;end
    iter = 0;
    x1 = xlow; x3 = xhigh;
    x2 = (x1 + x3) / 2;
    f1 = func(x1,varargin{:});
    f2 = func(x2,varargin{:});
    f3 = func(x3,varargin{:});
    if f2<f1 && f2<f3,   xoptold = x2; end
    while(1)

        xopt=x2-0.5*((x2-x1)^2*(f2-f3)-(x2-x3)^2* ...
        (f2-f1))/((x2-x1)*(f2-f3)-(x2-x3)*(f2-f1));
        fopt = func(xopt,varargin{:});
        iter = iter + 1;
        if xopt > x2
            x1 = x2;
            f1 = f2;
        else
            x3 = x2;
            f3 = f2;
        end
        
        x2 = xopt; f2 = fopt;
        if xopt~=0,ea=abs((xopt - xoptold) / xopt) * 100;end
        xoptold = xopt;
        if ea<=es || iter>=maxit,break,end
    end
end

Answer 1

[Walter Roberson]

if f2<f1 && f2<f3,   xoptold = x2; end

You only set xtoptold if that condition is true, but later you use it even if the condition was false

[Gerardo]

how would I fix it so that it would run correctly?

[Walter Roberson]

Put an else on the

if f2<f1 && f2<f3,   xoptold = x2; end

and refuse to run the while loop if the else is encountered.