make_matrix <- function (element, nrow, ncol) { M <- matrix(0,nrow,ncol) for (i in 1:nrow) for (j in 1:ncol) M[i,j] <- element(i,j) M } away_from_mean <- function (vec, d) { m <- mean(vec) vec [ vec <= m-d | vec >= m+d ] } iris [iris$Petal.Length / iris$Petal.Width >= 4, ] sum_both_pos_rows <- function (A, B) { A_pos <- rowSums(A) > 0 B_pos <- rowSums(B) > 0 A[A_pos & B_pos,] + B[A_pos & B_pos,] }