# Finding the number of rows to the next row containing a 1

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andyc100 2020 年 8 月 27 日

Hi
I have a column vector of 1s and 0s and I want to find find the number of rows to the next row containing a 1. For example:
A = [0 0 0 1 0 1 1 1 0 0 0 0 0 1]';
I would like the code to return
B = [3 2 1 0 1 0 0 0 5 4 3 2 1 0]';
Is there a vectorized way that this can be done?
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Bruno Luong 2020 年 8 月 28 日

As I'm slightly surprised by the performance of the for-loop (I would expext it's good but not THAT good), I then try to see how far it from a MEX implementation. And I'm stunned, it's almost as fast (even faster for smaller array).
t Rik = 0.651531 [s]
t Bruno = 0.379362 [s]
t Stephen = 0.104442 [s]
t MEX = 0.073168 [s]
The code (benchmark + Cmex) is in the attacheh file for those who wants to play with.
I must congratulat TMW for improving the for-loop performance over many years.

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### 採用された回答

Rik 2020 年 8 月 27 日

It took some time, but here is a solution that should also work for large matrices.
clc,clear
format compact
A = [0 0 0 1 0 1 1 1 0 0 0 0 0 1]';
% 3 2 1 0 1 0 0 0 5 4 3 2 1 0
B=A;
if pad,B(end+1)=1;end %this method requires the last position to be a 1
B=flipud(B);
C=zeros(size(B));
C(B==1)=[0;diff(find(B))];
C=ones(size(B))-C;
out=cumsum(C)-1;
out=flipud(out);
%only for display:
[A out]
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andyc100 2020 年 8 月 27 日
This is not a concern for me as there will always be 1s in my implementation. Can always just do a check for not all zeros in the vector before running the code I guess.

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### その他の回答 (3 件)

Binbin Qi 2020 年 8 月 27 日
A = [0 0 0 1 0 1 1 1 0 0 0 0 0 1]';
C = find(A);
D = (1:length(A)) - C;
D(D>0) = D(D>0) + inf';
min(abs(D))'
ans =
3
2
1
0
1
0
0
0
5
4
3
2
1
0
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Andrei Bobrov 2020 年 8 月 27 日
+1

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Bruno Luong 2020 年 8 月 27 日

As much as I love vectorization, this problem is a typical case where the for-loop method is easier, faster, more readable.
This code is ready for 2D array, it works along the first dimension independently.
A=rand(30000,1000)>0.7;
tic
Al=logical(A);
B=zeros(size(A));
b=B(1,:);
for k=size(B,1):-1:1
b = b+1;
b(Al(k,:))=0;
B(k,:)=b;
end
toc
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andyc100 2020 年 8 月 27 日
Thank you Burno. This actually works very fast too and I like that it works for multiple columns. I might end up using it if I end up needing multiple columns.

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Bruno Luong 2020 年 8 月 28 日

Now I just discover CUMSUM has direction option, this is based on Rik's cumsum method, but avoid the double flipping.
B = ones(size(A));
i1 = find(A);
B(i1) = 1-diff([i1;size(A,1)+1]);
B = cumsum(B,'reverse');
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Rik 2020 年 8 月 28 日
Cool, I didn't remember that was an option. Turns out that is already an option as far back as R2015a. The release notes no longer allow you to look back further than R2015a, even if you try modifying the url. All I know is that it isn't possible in R2011a.

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