how to find the column number of first zero element of each row

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Hi,all. could you please tell me how to find the column number of first zero element of each row. eg. for the matrix in the picture, I want the output will be outputmatrix=[3;4;4;5;5;6;6;6;6;6;1;1;1;1;1]. Thank you!

Accepted Answer

MHN
MHN on 19 Feb 2016
Edited: MHN on 19 Feb 2016
This is your answer:
a =[ -1 3 0 0 0 0
2 3 4 5 0 0
1 2 1 0 0 0]
b = abs(a); % it is a trick which forces min to do not consider your negative numbers as minimum
[~,ind] = min(b')

More Answers (2)

Adam
Adam on 19 Feb 2016
Edited: Adam on 19 Feb 2016
sum( abs( cumprod( yourMatrix, 2 ) ) > 0, 2 ) + 1;
should work I think. I tested in on a small matrix I made and it seems to give the correct answer. There may be simpler methods though!
The logic behind it is as follows:
  • cumprod is the cumulative product, run along each row (dimension 2) which will always be 0 for values after the first 0, hence the result of this will be a matrix in which only the elements before the first 0 on a row will be non-zero.
  • abs will ensure that all non-zero elements are positive in order for the following check to work correctly.
  • taking the sum of all the elements greater than 0 will give you the index of the last non-zero element.
  • Adding 1 at the end will give the element of the first 0 which should always be the column immediately after the last non-zero.
If need be you can use a small tolerance e.g. 1e-6 to hard set elements below that absolute tolerance to 0 before doing this as values that are not precisely 0 will likely cause problems.
  2 Comments
Walter Roberson
Walter Roberson on 19 Feb 2016
sum(cumprod(~YourMatrix, 2), 2) + 1
Note: this will be incorrect / misleading for any row for which there is no 0 (in which case no desired result has been defined by the poster)

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Sunetra CV
Sunetra CV on 14 Nov 2019
Edited: Sunetra CV on 14 Nov 2019
will this work if all the elements of my matrix are decimals?

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