Vector with various IFs, adding up and counting down

I need to translate an Excel formula to whatsapp.

I have one vector X: [ 5 2 -4 -6 -2 1 4 2 -3 -6 -1 8 9 ]

Now, vector y needs to accumulate if vector x is negative and the opposite when positive until back to zero: [ 0 0 -4 -10 -12 -11 -7 -5 -8 -14 -15 -7 0 ]

I have been looking through Matlab functions but don't come far. Is there any easy way to do this on Matlab?

Many thanks in advance!

Answer 1

[Paolo]

 X = [ 5 2 -4 -6 -2 1 4 2 -3 -6 -1 8 9 ];
 indx = find(X<0);
 Y = [zeros(1,indx(1)-1) cumsum(X(indx(1):end))];
 Y(Y>0) = 0;

Y =

Columns 1 through 11

     0     0    -4   -10   -12   -11    -7    -5    -8   -14   -15

Columns 12 through 13

    -7     0

[dirk-jAn]

Thanks Paolo. This answer indeed solves perfectly the example I gave. However, if the series are longer and come back into negative after some positive values, the positive values should not be added up, so a negative value should directly lead to a decrease below 0, e.g.:

X: [ 5 2 -4 -6 -2 1 4 2 -3 -6 -1 8 9 5 -6 -11]

should not be

[ 0 0 -4 -10 -12 -11 -7 -5 -8 -14 -15 -7 0 0 0 -10 ] as it is now, but should be:

[ 0 0 -4 -10 -12 -11 -7 -5 -8 -14 -15 -7 0 0 -6 -17]

In other words, positive numbers between series of negatives should be treated the same as the positive numbers at the very start.

Again thanks in advance for solving this.

[dirk-jAn]

I think it is solved:

X = [ 5 2 -4 -6 -2 1 4 2 -3 -6 -1 8 9 5 -6 -11];

X(X(1)>0)=0;

Y=X;

 for n=2:length(X)
     if sum(Y(n-1:n)) > 0
         Y(n)=0;
     else
         Y(n)=sum(Y(n-1:n));
     end
 end