Vectorize for loop with recursion

Hi all,

I have a for loop, which I'm struggeling to vectorize.

I've made a short example of what I'm trying to accomplish here.

This is not my exact code but I hope it shows the dilemma.

Does anyone have a clue?

Thanks in advance!

P1 = zeros(10,1);
P2 = zeros(10,1);
P3 = zeros(10,1);
P4 = zeros(10+1,1);
x = 1;
for i=1:10
    if P4(i) > 0
        P1(i) = x;
    else
        P2(i) = x;
    end
    P3(i) = P2(i) + P1(i);
    
    P4(i+1) = P4(i) + P3(i);
end

Answer 1

[Birdman]

P1(P4>0)=x;
P2(P4<0)=x;
P3=P1+P2;
P4(2:end)=P3+P4(1:end-1);

[olf nair]

I was thinking about this solution too but unfortunatly this only picks up the main problem:

The if conditions depend on the exact state of P4, which isnt known in that vectorized solution yet.