Identify pre-sign changes

Assume I have a sequence of +1 and -1 only, randomly chosen..

If I want to identify the sequence point where the pre sign changes, i want to know the number of a second sequence which has the same length...

My code below does not work.. It figures out when the numbers change from positive to negative but after the sequence continues have negative numbers MATKAB still attaches the numbers from the second sequence..

BTW: Im new to MATLAB

thank you for any advice!!!

best Jan

%%%%%%%%%%%%%%%%%%%%%%% xx1 contains only -1 and +1 price contains stock prices which should should be implemented to xx2

for n = 1:length(x1)-1

    for k = 2:length(x1)

        if xx1(k) ~= xx1(n)
           xx2(n) = 0;
        else
            xx2(n) = price(n);
        end

    end

end

Answer 1

[Orion]

I'm not sure to get your problem.

something like

% here changes of sign appear at positions : 3,4,5 and 8
x1 = [1 1 -1 1 -1 -1 -1 1];

% find changes
MySignChangingIndex = find(x1(1:end-1)~=x1(2:end))+1; % gives MySignChangingIndex = [3 4 5 8];

% calculates xx2
xx2 = zeros(size(price));
xx2(MySignChangingIndex) = price(MySignChangingIndex);

[John D'Errico]

Simpler is just...

find(diff(x1))

which will identify sign changes, since it just searches for any change at all.