Finding the first occurance using interp1

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

Answer 1

[Matt J]

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

[Jason]

Hi, but its not always the 1st two data points. sometime my real data is very similar to the black curve.

[Torsten]

Then go through your data from the beginning, note when the Y-value is first greater or equal to 0.2 (say at position I in the Y-array) and then call interp1 as

Y20 = interp1(Y(1:I),X(1:I),0.2,'linear')

Best wishes

Torsten.

[Jason]

Like this:

[~,I] = min(abs(Y - 0.2))

[Matt J]

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]

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

[Matt J]

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

Answer 2

[Sean de Wolski]

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]

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