MATLAB: Find value of x from curve

find value of x from curve

hi, i had values of y from 0 to 2pi, related to x values from 0 to 10 with step 0.5; i.e., i calculated the value of y using program for the values of x=0:0.5:10. i want to find the value of x from the plot(x,y), for cer
x=0:0.5:10;
y=0  0.383135237593798  0.762305633253970  1.12500381866696  1.47841592230188  1.82360426305799  2.16799972102249  2.50162151353647  2.83129834831420  3.15388959500014  3.47113990999891  3.77183336056194  4.06431023145887  4.35173946610087  4.63997932479654  4.92354827732318  5.20849294256939  5.49434987763603  5.77926587166893  5.77926587166893  6.28318530717959
find x for values of y=0 1.57079632679490 3.14159265358979 4.71238898038469;
thanks a lot for any helps

Best Answer

Use interp1, interpolating on the y values, reversing the x and y arguments. Your data contain two identical y-values, so it is necessary to cheat a bit and add a very small value (I chose 1E-15) to make them monotonically increasing in y. I sorted them first to be certain I wasn’t missing any of the repeated values:
x = 0:0.5:10;
y = [0  0.383135237593798  0.762305633253970  1.12500381866696  1.47841592230188  1.82360426305799  2.16799972102249  2.50162151353647  2.83129834831420  3.15388959500014  3.47113990999891  3.77183336056194  4.06431023145887  4.35173946610087  4.63997932479654  4.92354827732318  5.20849294256939  5.49434987763603  5.77926587166893  5.77926587166893  6.28318530717959];
yq = [0 1.57079632679490 3.14159265358979 4.71238898038469];    % Query Points
xy = [x' y'];                                       % Create MAtrix For Sorting
[xys, idx] = sortrows(xy,2);                        % Sort
dup = find(diff([xys(:,2)]) == 0)+1;                % Find Non-Increasing Values
xys(dup,2) = xys(dup,2) + [1:length(dup)]*1E-15;    % Add ‘1E-15’ To Duplicates
xq = interp1(xys(:,2), xys(:,1), yq);               % Interpolate To Find ‘x’ Values
figure(1)
plot(x, y, 'g')
hold on
plot(xq, yq, 'mp')
hold off
The ‘xq’ variable holds the x values you want that correspond to the y values in ‘yq’. The plot shows them.
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