Change row order in a matrix/dataframe

Question

I need to change/invert rows in my data frame, not transposing the data but moving the bottom row to the top and so on. If the data frame was:

1 2 3  4 5 6 7 8 9 

I need to convert to

7 8 9 4 5 6 1 2 3 

I've read about sort() but I don't think it is what I need or I'm not able to find the way.

Answer 1

There probably are more elegant ways, but this works:

m <- matrix(1:9, ncol=3, byrow=TRUE)  # m[rev(seq_len(nrow(m))), ]  # Initial answer m[nrow(m):1, ]      [,1] [,2] [,3] [1,]    7    8    9 [2,]    4    5    6 [3,]    1    2    3 

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This works because you are indexing the matrix with a reversed sequence of integers as the row index. nrow(m):1 results in 3 2 1.

Answer 2

You can reverse the order of a data.frame using the dplyr package:

iris %>% arrange(-row_number()) 

Or without using the pipe-operator by doing

arrange(iris, -row_number())