## Finding the first occurance using interp1

### Jason (view profile)

on 27 Feb 2018
Latest activity Edited by Matt J

on 27 Feb 2018

### Matt J (view profile)

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

on 27 Feb 2018
Edited by Matt J

### Matt J (view profile)

on 27 Feb 2018

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

Matt J

### Matt J (view profile)

on 27 Feb 2018
i=find(Y(1:end-1)>=0.2 & Y(2:end)<=0.2,1);
Y20=interp1(Y(i:i+1),X(i:i+1),0.2);
Torsten

### Torsten (view profile)

on 27 Feb 2018
It could happen that Y is increasing, couldn't it ?
Matt J

### Matt J (view profile)

on 27 Feb 2018
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 (view profile)

on 27 Feb 2018
Edited by Sean de Wolski

### Sean de Wolski (view profile)

on 27 Feb 2018

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

### Jason (view profile)

on 27 Feb 2018
Thankyou for your answer. Im sorry I can't accept both. Matt came first.