Adding up rows in R

Question

I have a dataframe as follows

 df <- data.frame(matrix(rnorm(60), nrow=20))

and would like to find the sum of rows as follows:
Adding the first row to the 11th, second row to the 12th, third to 13th, fourth to 14th, until ten to 20th.

This is just a sample and I would like to do this to a dataframe with 150 rows and 20 columns.

Answer 1

For your example, you could use

DF <- data.frame(matrix(rnorm(60), nrow=20))
DF[1:10,] + DF[11:20,]