user5987132 - 6 months ago 41

R Question

I'm trying to reverse the order of the rows from the matrix m, so instead of getting this like in the following:

`n <- rbind(m)`

n

[,1] [,2] [,3] [,4] [,5]

[1,] 1 2 3 4 5

[2,] 6 7 8 9 10

[3,] 11 12 13 14 15

[4,] 16 17 18 19 20

[5,] 21 22 23 24 25

I want to get this instead:

`[,1] [,2] [,3] [,4] [,5]`

[5,] 21 22 23 24 25

[4,] 16 17 18 19 20

[3,] 11 12 13 14 15

[2,] 6 7 8 9 10

[1,] 1 2 3 4 5

Thanks for your help! Still learning the basics!

Answer

We can combine `apply()`

and `rev()`

to the matrix `n`

and rearrange the entries:

```
n <- matrix(1:25, 5, byrow = TRUE)
apply(n,2,rev)
# [,1] [,2] [,3] [,4] [,5]
#[1,] 21 22 23 24 25
#[2,] 16 17 18 19 20
#[3,] 11 12 13 14 15
#[4,] 6 7 8 9 10
#[5,] 1 2 3 4 5
```

By using `apply()`

with the second parameter (the margin) equal to 2 we loop through all the columns of `n`

, and for each column we apply the function `rev()`

, which reverses the entries of a vector.

A more compact and faster way to obtain the same result is the solution pointed out by @Cath:

```
n[nrow(n):1,]
```

This simply reverses the order of the rows in the matrix.