Local Maximas in Matlab

Question

In Matlab, by the function max ()， I can only get one single maximum element of a vector, even if there can be several local maximum elements. I was wondering how to get values of all local maximas in MATLAB.

Answer 1

Local maximums will always have this characteristic:

1) The prior point will always be less. 2) The following point will always be less.

So you could do something like this:

% Example input vector a
a = [0 1 2 1 2 3 2 1 0 1 2 1 5 2 1 4 1];

% Shift vectors so that prior point and next point lines up with
% current point. 
prior_points   = a(1:(length(a) - 2));
current_points = a(2:(length(a) - 1));
next_points    = a(3:length(a));

% Identify indices where slope is increasing from prior to current
% and decreasing from current to next.
prior_slope_positive = ((current_points  - prior_points)    > 0);
next_slope_negative  = ((next_points     - current_points)  <= 0);

% This will return indices of local maximas.  Note that the result
% is incremented by one since current_points was shifted once.
b = find(prior_slope_positive & next_slope_negative) + 1;

Note that this example does not include the first and last points as potential local maximas.

The local maximas in vector a are: Index Value 3 2 6 3 11 2 13 5 16 4

Therefore, at the conclusion vector b will be equal to: [3 6 11 13 16]