Cumulative Sum for Column of Ones and Zeros

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Kevin Zhang
Kevin Zhang 2017-12-5
评论： Ronni Isham 2019-1-2
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
  1 个评论
Jos (10584)
Jos (10584) 2017-12-5
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.

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采纳的回答

Jos (10584)
Jos (10584) 2017-12-5
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.
  2 个评论
Kevin Zhang
Kevin Zhang 2017-12-7
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.

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