Slow-running code for removing rows

I have a set of code that works, but a part of it runs very slowly. Basically I'm trying to remove zero-only rows from two matrixes, and I'm able to tell that both have an empty row if the first element in one matrix is zero. Here's the actual code:

for a=1:numberofrows
    if slopes(a,1)==0
        slopes(a,:)=[];
        switchingtimes(a,:)=[];
    end
end

I'd try to do a nonzeros function and then a reshape function, but sometimes slopes have a zero or two at the end of a column.

Is there any way to speed up this code?

Answer 1

[Geoff Hayes]

Christian - what about using find as

slopes(find(slopes(:,1)==0), :) = []

We use find to return all indices of the first column of slopes whose element is a zero. We then remove all of these rows with the assignment to [] (like you have done already).

[Christian F]

Thanks! It worked way faster than the old code!

[Stephen23]

find is not required. Logical indexing is simpler and faster:

idx = slopes(:,1)==0;
slopes(idx,:) = []
switchingtimes(idx,:) = [];

https://www.mathworks.com/help/matlab/math/array-indexing.html