Replacing row in matrix with previous row depending on condition

Hi

Say I have a matrix:

A=[2,5,3;5,8,2;1,-2,5]

If a value in any of the rows are -2 the whole row should be replaced by the previous row.

So the result would be:

A=[2,5,3;5,8,2;5,8,1]

The matrix consists of 1 million rows, so I'm looking for the fastest method.

Answer 1

[the cyclist]

Here is one way:

while any(A(:)==-2)
    rowToReplace = find(any(A==-2,2));
    A(rowToReplace,:) = A(rowToReplace-1,:); 
end

This solution could probably made faster by using only logical indices, without the find command, but I felt lazy.

[Oliver Zacho]

Thanks alot, this one does that job in a decent amount of time. Lovely. Have a splendid weekend.

[the cyclist]

This is a little faster, in limited testing:

while any(A(:)==-2)
    rowToReplace = any(A==-2,2);
    A(rowToReplace,:) = A([rowToReplace(2:end); false],:);
end

The optimized version might depend on the pattern of -2's in the array.

Answer 2

[David Hill]

If you have consecutive rows containing -2, you will have to repeat until all the -2 rows have been replaced if that is your goal

[a,~]=find(A==-2);
a=unique(a);
A(a,:)=A(a-1,:);

[Oliver Zacho]

Thanks alot! Have a nice weekend. :)