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Re: Finding peaks/max in a graph
From: |
David Bateman |
Subject: |
Re: Finding peaks/max in a graph |
Date: |
Mon, 5 Apr 2004 13:35:07 +0200 |
User-agent: |
Mutt/1.4.1i |
A for-loop is not the way to go about this. Consider the code fragment
n = 9
a = [1 2 3 4 5 4 3 2 1]';
peaks = find([a(2:n,1) - a(1:n-1,1) < 0; 1] & [1; a(1:n-1,1) - a(2:n,1) < 0]);
that will find all indexes of the peaks in the data in a single statement
D.
Daprès edA-qa mort-ora-y <address@hidden> (le 05/04/2004):
> I need to find the peaks in a graph (sampled data). By peaks I mean
> each point at which the slope changes from positive to negative (or 0
> slope, but these are usually cusps).
>
> As this is sampled data (resulting from an FFT actually), I need to find
> only a certain number of the peaks (the maximum peaks, since the noise
> in the sample yields a high number of real peaks).
>
> I wrote a function which does this, but it is extremely slow (I'm
> working with sample sets of 100K in size). Does anybody else know how I
> can achieve this any faster, or which standard functions.
>
> Purpose: To find the most predominant frequencies in a waveform (sample
> fed into FFT, then into peaks finding algorithm)
>
> Attached: The script I wrote
>
> --
> edA-qa mort-ora-y
> Idea Architect
> http://disemia.com/
>
> function p = find_peaks( data, num )
>
> cur = data(1);
> mn = cur;
> dir = 0;
>
> peaks = [];
>
> for i = 2:length(data)
> s = sign( data(i)-cur );
> if( s != dir )
> dir = s;
>
> if( dir < 0 )
> peaks = [peaks; i, data(i)];
> endif
> endif
>
> cur = data(i);
> mn = min(cur,mn);
> endfor
>
> l = rows(peaks);
> if( num > l )
> num = l;
> endif
>
> # Ensure we are using a matrix, otherwise sort treats it as a vector
> # we should populate this with the minimum value actually... (TBD)
> if( l == 1 )
> peaks = [peaks; 0, mn];
> l = 2;
> endif
>
> [ spv, spi ] = sort( peaks );
>
> p = [];
>
> for i = 0:(num-1)
> p = [p; peaks( spi(l-i,2), 1 ), peaks( spi(l-i,2), 2 ) ];
> endfor
>
> endfunction
--
David Bateman address@hidden
Motorola CRM +33 1 69 35 48 04 (Ph)
Parc Les Algorithmes, Commune de St Aubin +33 1 69 35 77 01 (Fax)
91193 Gif-Sur-Yvette FRANCE
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-------------------------------------------------------------
- Finding peaks/max in a graph, edA-qa mort-ora-y, 2004/04/05
- Re: Finding peaks/max in a graph,
David Bateman <=
- Re: Finding peaks/max in a graph, edA-qa mort-ora-y, 2004/04/05
- Re: Finding peaks/max in a graph, Joe Koski, 2004/04/05
- Re: Finding peaks/max in a graph, edA-qa mort-ora-y, 2004/04/05
- Re: Finding peaks/max in a graph, David Bateman, 2004/04/05
- Re: Finding peaks/max in a graph, Thomas Shores, 2004/04/05
- Re: Finding peaks/max in a graph, David Bateman, 2004/04/05