Finding cluster of positive and negative numbers in an array, then the maximum and minimum value in each cluster

Say for example, I have

a = [ 2 -2 1 4 5 -3 -2 1 -1 2 5 -2 6 -7 1 3 -4]

What I want is to group the elements as

[2], [-2], [1 4 5], [-3 -2], [1], [-1], [2 5], [-2], [6], [-7], [1, 3], [-4]

Next, I want to do this

[2], [-2], [5], [-3], [1], [-1], [5], [-2], [6], [-7], [3], [-4]

Finally I want to extract index position of these values in the original array aa.

[1 2 5 6 8 9 11 12 13 14 16 18]

How do I vectorize this process?

[Dyuman Joshi]

If I am understanding correct, you want min values for negative group and max values for positive groups?

Answer 1

[Bruno Luong]

NOTE: max of [-3 -2] is -2 not -3. Your array has 17 elements, cannot have element index of 18.

a = [ 2 -2 1 4 5 -3 -2 1 -1 2 5 -2 6 -7 1 3 -4];

lgt = runlengthencoder(a > 0);
c = mat2cell(a, 1, lgt);

[maxa, subidx]=cellfun(@max, c)

maxa = 1×12

2 -2 5 -2 1 -1 5 -2 6 -7 3 -4

subidx = 1×12

1 1 3 2 1 1 2 1 1 1 2 1

 idx=cumsum([0, lgt(1:end-1)])+subidx

idx = 1×12

1 2 5 7 8 9 11 12 13 14 16 17

 
% Do the same thinh with min
...

function [len, v, gr, subidx] = runlengthencoder(X)
% [len, v, gr, subidx] = runlengthencoder(X)
% Run-length encoder
%
% INPUT
%   X is (1 x n) row vector, column is also allowed
% OUTPUTS:
%   len: integer arrays (1 x m)
%   v: (1 x m) ordering subset of X, such that two adjadcent elements are differents
%       and X = replelem(v, len)
%   gr: 1:m (group number)
%   subidx: (1 x n) integer, interior indexes of X with in the group
%
% See also: runlengthdecoder
if ~isrow(X)
    X = reshape(X, 1, []);
end
n = size(X,2);
if n > 0
    b = [true, diff(X)~=0];
    ij = find([b, true]);
    len = diff(ij);
    v = X(b);
    if nargout >= 3
        gr = repelem(1:length(len),len);
        if nargout >= 4
            subidx = ones(1,n);
            subidx(ij(2:end-1)) = 1-len(1:end-1);
            subidx = cumsum(subidx, 2);
        end
    end
else
    [len, v, gr, subidx] = deal([]);
end
end % runlengthencoder