Finding the first occurance using interp1
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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
采纳的回答
Use only the first two data X,Y data points in the interpolation.
6 个评论
Jason
2018-2-27
Torsten
2018-2-27
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.
Matt J
2018-2-27
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
2018-2-27
It could happen that Y is increasing, couldn't it ?
更多回答（1 个）
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)
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