Remove rows of zeroes, and extract the removed rows indices

Hi all,

I have T =

     0     0     0     0
     1     2     0     4
     5     6     0     8
     0     0     0     0
    10    11     0    12

t2=T(any(T,2),:) will give

t2 =

     1     2     0     4
     5     6     0     8
    10    11     0    12

But how can I efficiently retrieve the indices of the removed rows from T, that is: 1,4 ?? Then how can I efficiently retrieve the indices of the not removed rows from T, that is: 2,3,5 ?? Without using while, for loop etc.

Thank you in advance.

Answer 1

[Star Strider]

This works:

T = [0     0     0     0
     1     2     0     4
     5     6     0     8
     0     0     0     0
    10    11     0    12];

RemovedRows = all(T == 0, 2)
RemovedRowsIdx = find(RemovedRows)
KeptRowsIdx = find(any(T,2))

RemovedRows =

    5×1 logical array

     1
     0
     0
     1
     0

RemovedRowsIdx =

       1
       4

KeptRowsIdx =

       2
       3
       5

EDIT — Remembered ‘KeptRowsIdx’.

[Guillaume]

KeptRowsIdx = find(~RemovedRows)

would probably be faster. No point going over the whole matrix again just to find the opposite of the first scan.

[Star Strider]

@Guillaume — Excellent point, especially since I specifically created ‘RemovedRows’ to show how the code worked.

[Image Analyst]

If speed is what was needed, you shouldn't even use find() at all. I doubt he really needed the actual row numbers - that's why I didn't even bother to compute them. I think it's just the final matrix that is wanted. He'll probably never even use the row numbers again - so use logical indexing, which you already have, rather than doing the additional step to get linear indexing.

[Star Strider]

@Image Analyst —

Possibly, but that wasn’t the impression I got.

Quoting:

• ‘But how can I efficiently retrieve the indices of the removed rows from T, that is: 1,4 ??Then how can I efficiently retrieve the indices of the not removed rows from T, that is: 2,3,5 ??’

The index numbers appear to be requested.

Answer 2

[Image Analyst]

Try this:

rowsToKeep = ~all(T == 0, 2)
t2 = T(rowsToKeep, :)