Cumulative Sum for Column of Ones and Zeros

I have a column of ones and zeros, and I want to create a second column that takes a cumulative sum of the ones. When a 0 is encountered, the cumulative sum should go to zero, and repeat. For example,

A = [1 1 1 1 1 0 0 1 1]

B = [1 2 3 4 5 0 0 1 2]

Basically, I would like to count the number of consecutive zeros in the first column vector until a zero is reached, and then restart at the next group of ones. I need to output this as a column vector. I have tried using a for loop, but it takes minutes to calculate only a few rows and begins to return zeros after a few rows. Code is below (please excuse the inefficient way I have initialized the vectors):

T1 = [1:43801]'
Tdecl = zeros(size(T1));  
UM = randi([0 1],43801,1)

for i = 1:43801
    if UM(i)==1
        Tdecl(i) = Tdecl(i)+1
    else
        Tdecl(i) = 0
    end
end

[Jos (10584)]

You also need to take the previous element into account

if UM(1) == 1, Tdecl(1) = 1 ; end
for i=2:43801
    if UM(i)==1,
        Tdecl(i) = Tdecl(i-1) + 1 ;
    end
end

but note that runindex (see my answer below) is much faster.

Answer 1

[Jos (10584)]

A perfect job for runindex:

A = [0 1 1 1 0 0 1 1 1 1 0 0 0 1 1 1 1 1 1 0]
B = runindex(A)
B(A==0) = 0

My function RUNINDEX can be downloaded from the File Exhchange:

https://uk.mathworks.com/matlabcentral/fileexchange/56131-runindex

The code is short and very fast, using a nice index trick and cumsum.

[Kevin Zhang]

Hi Jos,

Thanks for the reply, I managed to figure this out myself - I neglected to use semicolons in my loop and it was causing huge issues.

[Ronni Isham]

Could you please upload your final solution?