MATLAB: Find X-value given Y

find xinterp1interpolationintersectionlog10logarithmic

Hello
I've been doing some research regarding this but I can't seem to find a solution for when I want to find the X-value given Y.
I have the following data:
>> X
ans =
            20          50         100         200         500        1000        2000        5000       10000       20000
>> Y
ans =
     -0.1755   -0.1755   -0.9151   -2.8534   -8.4043  -13.1515  -20.0000  -27.5350  -33.9794  -40.0000
Where the X is plotted on a logarithmic scale. I tried interp1 with X and Y switched but that dosent work since its not strictly monotonic increasing.
Does anyone have a solution on this? Say I want to find the value of X given Y=-5

Best Answer

The ‘x’ and ‘y’ vectors were not the same sizes, so I added 25 to ‘x’, then added a small amount to the duplicated element in ‘y’ and did the interpolation:
x = [25  50         100         200         500        1000        2000        5000       10000       20000];
y = [ -0.1755   -0.1755   -0.9151   -2.8534   -8.4043  -13.1515  -20.0000  -27.5350  -33.9794  -40.0000];
dy = diff([0 y]);
dyix = find(dy == 0);
y(dyix) = y(dyix-1)+1E-8;
xint = interp1(y, x, -10);              % Find The Value Of ‘x’ Corresponding To y=-10’
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