calculation x-value, given the y-value of a discrete function

I want to calculate the x-value of a given function in a given y, but my function is only available in discrete points how can I do that?

This is the function I made to make the function, what I basicly want to do is finding the x-value where 'plot(squeeze(chi2(1:20,15,10)))' intersects with minchi.

Thanks in advance

C=10;
x0=15;
a=9;
xen =linspace(0,60);
%adding noise
xen =linspace(0,60,100)';
y= C./ sqrt((xen.^2-x0^2).^2+a*xen.^2);
naruisy = y + randn(100,1)*0.01;%amplitude = *0.04
plot(xen,y)
hold on
plot(xen,naruisy)
chi2 = zeros(20,20,20);

minchi2 = 100;
for a = linspace(1,20,20)
    for b = linspace(1,20,20)
        %fprintf('huidige index:%i\n', b)
        for c = linspace(1,20,20)
            somelementen = [];
            ymetparam = c./ sqrt((xen.^2-b^2).^2+a*xen.^2);
            somelementen = (naruisy- ymetparam).^2;
%             plot(xen,naruisy)
%             hold on
%             plot(xen,ymetparam,'g')
%             hold off
%             pause(0.2)
            chi2(a,b,c)=sum(somelementen)/(length(somelementen)-3);
            if chi2(a,b,c) < minchi2
                minchi2 = chi2(a,b,c);
                minparam = [a,b,c];
                %minchi2
            end
        end
    end
end
plot(squeeze(chi2(9,15,1:20)))
hold on
minchi2
xplot=linspace(0,20,100);
yplot=xplot-xplot + minchi2;
plot(xplot,yplot)
hold on
yplot=xplot-xplot + 2*minchi2;
hold on
plot(xplot,yplot)
%%
%bij c_best en b_best de a bekijken
plot(squeeze(chi2(1:20,15,10)))
hold on
minchi2
xplot=linspace(0,20,100);
yplot=xplot-xplot + minchi2;
plot(xplot,yplot)
hold on
yplot=xplot-xplot + 2*minchi2;
hold on
plot(xplot,yplot)

Answer 1

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Read about interp1, knnsearch.