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

41 views (last 30 days)
Jason on 19 Feb 2016
Edited: Sunetra CV on 14 Nov 2019
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!

MHN on 19 Feb 2016
Edited: MHN on 19 Feb 2016
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')
##### 2 CommentsShowHide 1 older comment
Jason on 19 Feb 2016
thank you very much!

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 CommentsShowHide 1 older comment
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)

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?