# Finding column index of the first instance of 1

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Pelajar UM on 2 May 2022
Commented: Walter Roberson on 3 May 2022
I have a logical array like this:
I want to extrat the colum index of the cells where the first instance of 1 is detected. Like this:
For example, you see here that the first two rows show 4, because that is where 1 is first detected.

Walter Roberson on 2 May 2022
C = sum(cumprod(~X, 2),1) + 1;
C will be one more than the number of columns for any row that has no 1.
Pelajar UM on 3 May 2022
Edited: Pelajar UM on 3 May 2022
Thanks @Torsten. The second line gives a warning:
Unexpected use of '[' in a scalar context.
And it appears to break the loop already in k=1.
Instead we can use
if isempty(idx) == 1
break
end
But checking idx in every loop takes longer.
Method 1: 22 seconds
Method 2: 528 seconds
Walter Roberson on 3 May 2022
isempty(idx)

### More Answers (1)

Jonas on 2 May 2022
use the find() function together with a loop over each row
Pelajar UM on 2 May 2022
Edited: Pelajar UM on 2 May 2022
Like this?
Doesn't work, because it doesn't find the first instance. It finds all the indices that meet this condition.
for p=1:n %n is the length of the logical array X
G(p,:)=find (X(p,:));
end