matlab:Chapra , ROOTS [ Bracketing Method] Help needed.

% bisection.m
function bisection
% find root of x^2 - 3 on some interval

xa = 0;         xb = 10;        % search interval

for it = 1:20           % loop
    xtest = xa + (xb-xa)/2;         % mid point of interval
    fa = f(xa);                     % left-interval function value
    fb = f(xb);                     % right-interval function value
    ftest = f(xtest);               % mid-point function value
    if sign(fa)*sign(ftest)<0       % if zero in left half
        xb = xtest;                 %   take left half of interval
    elseif sign(ftest)*sign(fb)<0   % if zero in right half
        xa = xtest;                 %   take right half of interval
    elseif ftest ==0                % if zero at mid-point
        break                       %   this is the zero
    else                            % 
        error('multiple roots or no root')  % may have no zero or multiple zeros
    end    
    xit(it) = xtest;                % store mid-points
end
figure;plot(xit)                    % plot mid-points, should converge to the root


function y = f(x)                   % function we're finding the root of
y = x^2-3;