pacomet pacomet - 4 months ago 26
R Question

Change row order in a matrix/dataframe

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

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

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.