Interpolate Data and find the matching x-Value

I have got an arry with 151937x2 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.

Answer 1

[Star Strider]

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

[Gabriel Bischof]

Thank you, it is working, but i dont understand how it works.

Could you discribe it to me please?

[Star Strider]

I will do my best!

The fsolve function is a root-finder (or zero-finder), so I used it here to find an ‘x’ value such that the interp1 funciton returns a value for that interpolation that is equal to the ‘yval’ argument.

So fsolve tries different ‘x’ values until it finds one that makes the interp1 result equal to ‘yval’.

[Gabriel Bischof]

I understand. Thank you a lot, this was a great help.

[Star Strider]

As always, my pleasure!