Changing every other row in a matrix in R

Question

Would anyone be able to show me a way of changing every second row in a column of a matrix to have the opposite sign to the one it currently has (eg changing a 4 to -4, or a -4 to 4)? I can do it with a for loop, but I was hoping there might be a more efficient method for doing it.

Answer 1

If you have an even number of rows, try

c(1,-1)*M

Where M is your matrix.

Otherwise, use

(2*(row(M) %% 2)-1)*M

EDIT: Carl Witthoft's suggestion:

rep(c(1,-1),length=nrow(M))*M

works with any matrix.

Answer 2

最终答案： rep(c(1,-1),length=nrow(M))*M