Clear Filters
Clear Filters

Finding column index of the first instance of 1

3 views (last 30 days)
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.

Accepted Answer

Walter Roberson
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.
  4 Comments
Pelajar UM
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

Sign in to comment.

More Answers (1)

Jonas
Jonas on 2 May 2022
use the find() function together with a loop over each row
  1 Comment
Pelajar UM
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

Sign in to comment.

Categories

Find more on Data Type Identification in Help Center and File Exchange

Tags

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!