1. apply
Description: “Returns a vector or array or list of values obtained by applying a function to margins of an array or matrix.”
# create a matrix of 10 rows x 2 columns
m <- matrix(c(1:10, 11:20), nrow = 10, ncol = 2)
# mean of the rows
apply(m, 1, mean)
[1] 6 7 8 9 10 11 12 13 14 15
# mean of the columns
apply(m, 2, mean)
[1] 5.5 15.5
# divide all values by 2
apply(m, 1:2, function(x) x/2)
[,1] [,2]
[1,] 0.5 5.5
[2,] 1.0 6.0
[3,] 1.5 6.5
[4,] 2.0 7.0
[5,] 2.5 7.5
[6,] 3.0 8.0
[7,] 3.5 8.5
[8,] 4.0 9.0
[9,] 4.5 9.5
[10,] 5.0 10.0
https://nsaunders.wordpress.com/2010/08/20/a-brief-introduction-to-apply-in-r/