Finding zeros and intersections

With data (x,y), the function finds "x" that correspond to y=y0.

xsol=fzero_data(x,y,y0);
"fzero.m" function (Optim toolbox) computes one zero of a defined one-variable function, while "fzero_data.m" computes all the zeros (or crossings) of data vectors x and y at y=y0. The number of solutions can be found with "length(xsol)". If the requested value y=y0 is not in the range of the data, an error will stop the program. For better accuracy the function "spline()" instead of "interp1()" can be inserted in the last loop of the function.

%%%%%%%%%% example %%%%%%%%%

theta=0:0.01:2*pi;
f=sin(2.*theta);

%find the solutions for f=0
f0=0;
theta0=fzero_data(theta,f,f0);
Nsol0=length(theta0);

%find the solutions for f=0.95
f1=0.95;
theta1=fzero_data(theta,f,f1);
Nsol1=length(theta1);

plot(theta,f,'-k',[0,theta0],f0.*ones(1,Nsol0+1),'-*r',[0,theta1],f1.*ones(1,Nsol1+1),'-ob')
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