MATLAB: Interpolate Data and find the matching x-Value

interpolationMATLAB

I have got an arry with 151937×2 double wicht discribes a Curve.
I am trying to find the x-Value to the matching y- Value for example:
data=[1   2,
      2   3,
      3   3,
      4   5,
      7   9]
I have tryed it with:
      interp1(data(:,1), data(:,2),y-Value ,'PCHIP')
But the function returns the according y-Value, logically. So I tryed to swap x and y datas. The Problem is that the y-Values arent unique.
I hope someone can help me.

Best Answer

Try this: 
data=[1   2,
      2   3,
      3   3,
      4   5,
      7   9];
xval = @(yval) fsolve(@(x) interp1(data(:,1), data(:,2), x, 'linear') - yval, median(data(:,1)));
that with these calls to it: 
Out3 = xval(3)
Out6 = xval(6)
Out8 = xval(8)
produces: 
Out3 =
     3.0000
Out6 =
     4.7500
Out8 =
     6.2500
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