# Returning peak values from a column

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Christopher on 8 Sep 2013
Hello,
I am trying to do the following. I have two column vectors:
Cp=[0;0.4;0.8;0.99;0.8;0.4;0];
Cf=[1;2;3;4;5;6;7];
I am trying to obtain a Cf column vector for the corresponding Cp column vector peak and below. So Cp peaks at 0.99 and I am trying to obtain the values for Cf from the beginning of Cp until the peak of Cp. All of the values of Cp are less than the peak value.
So
Cf_new=[1;2;3;4]
I am also trying to obtain the last half of Cf for Cp after the peak until the end. So the last half would be:
Cf_new2=[5;6;7]
Thank you

Matt J on 8 Sep 2013
[~,idx]=max(Cp);
Cf_new=Cf(1:idx);
Cf_new2=Cf(idx+1:end);

Image Analyst on 8 Sep 2013
Edited: Image Analyst on 8 Sep 2013
peakIndex = find(diff(Cp) < 0, 1, 'first')
Cf_new = Cf(1 : peakIndex)
Cf_new2 = Cf(peakIndex+1:end)
In the command window:
peakIndex =
4
Cf_new =
1
2
3
4
Cf_new2 =
5
6
7
Or you might mean "max value" instead of peak. In that case you need to use max() instead of diff(), and you need to know what you want to do if you have a run of several max values in a row.
Christopher on 8 Sep 2013
Thanks Image Analyst

Andrei Bobrov on 8 Sep 2013
Edited: Andrei Bobrov on 8 Sep 2013
t = [true;diff(Cp(:))>=0]
Cf_new = Cf(t);
Cf_new2 = Cf(~t);