How can i alternating signs in every line in a loop

alpha6=zeros(1,6)

for i = 1:90
   alpha6(:,:,i)=i;
   alpha6(:,2:2:end)= (alpha6(:,2:2:end))*-1
   
end

[Adam Danz]

By "every line" do you mean every column, every row, or something else?

Answer 1

[Adam Danz]

To flip the sign of every second column of a matrix, starting with column 2,

M(:,2:2:end) = -1*M(:,2:2:end);

Note that this may not alternate signs if some values are negative to begin with. If you want to ensure that every second column starting with col 2 is negative and the other columns are positive,

M(:,2:2:end) = -1*abs(M(:,2:2:end));