Finding the first occurance using interp1

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Jason 2018-2-27

编辑： Matt J 2018-2-27

Hello. I have some data (red curve) and I'm trying to find the X value at which Y = 0.2.

I have used

Y20 = interp1(Y,X,0.2,'linear')

which works well, but finds the last occurrance.

How can I find the first occurrance (i.e. around x=9)

Thanks

Matt J 2018-2-27

编辑：Matt J 2018-2-27

Use only the first two data X,Y data points in the interpolation.

Torsten 2018-2-27

It could happen that Y is increasing, couldn't it ?

Matt J 2018-2-27

编辑：Matt J 2018-2-27

Not according to the posted figure, but even if it could, I think the extension is an exercise I'll leave for the OP.

Sean de Wolski 2018-2-27

编辑：Sean de Wolski 2018-2-27

Use cummax and cummin to find the the first set of points that cross 0.2. Then interp just them.

x = 1:10
y = sin(x)
plot(x,y)
yval = 0.2;
idx = find(cummin(y)<0.2 & cummax(y)>0.2, 1, 'first')
interp1(y([idx-1 idx]), x([idx-1, idx]), 0.2)

Jason 2018-2-27

Thankyou for your answer. Im sorry I can't accept both. Matt came first.

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