% 
clear
close all

load invdat.dat
x = invdat(:,1);
y = invdat(:,2);

n = length(x);
ystar = 0.25;
xstar = [];
numxstar = 0;
    
for i = 1:n-1
    if( (y(i)-ystar)*(y(i+1)-ystar) < 0)
      numxstar = numxstar+1;
      xs = x(i-1:i+2);
      ys = y(i-1:i+2);
      S = spline(ys,xs);
      xstar(numxstar) = ppval(S,ystar);
    end
 end

hold on
plot(x,y,'b-*')
plot(xstar,ystar*ones(size(xstar)),'ko')