From help-octave-request at octave dot org Tue Apr 13 16:32:53 2004
Subject: Re: Finding peaks/max in a graph
From: "John W. Eaton" <jwe at octave dot org>
To: help at octave dot org
Cc: David dot Bateman at motorola dot com
Date: Tue, 13 Apr 2004 04:04:53 -0500

David Bateman David dot Bateman at motorola dot com wrote:

:> a = [1 2 3 4 5 4 3 2 1 2 3 2 1]';  n = length(a);
:> peaks = find([a(2:n,1) - a(1:n-1,1) < 0; 1] & [1; a(1:n-1,1) - a(2:n,1)  <0])
   peaks =
	5
	11

Just a small remark:

1) the method outlined by David fails if  multiple adjacent peaks
    occur in vector a.  Try e.g. the following vector a with a 'double' - peak:
:> a = [1 2 5 5 2 1]';  n = length(a); 
:> peaks = find( [a(2:n,1)-a(1:n-1,1) < 0;1] & [1;a(1:n-1,1)-a(2:n,1)<0] )
	peaks = [](0x1)
:> PEAKSVAL= a( peaks )
PEAKSVAL = [](0x1)

--- Workaround ---
 '<=' comparison operators must be used !
:> a = [1 2 5 5 2 1]';  n=length(a); 
:> peaks=find([a(2:n,1)-a(1:n-1,1) <= 0;1] & [1;a(1:n-1,1)-a(2:n,1)<=0] )
peaks =
  3
  4
:> PEAKSVAL = a( peaks )
PEAKSVAL =
  5
  5



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