How to convert a vector to a matrix in R starting from the bottom to top-CodePudding

I have a vector x = c(1,2,3,1,2,0,1,0,0). I want to convert it to a matrix starting from the bottom to the top. That is, I need to fill out the entries of the matrix starting from the last entry of each column.

I tried the following:

x <- c(1,2,3,1,2,0,1,0,0) 
M2 <- matrix(x, 3, 3) 
M2
     [,1] [,2] [,3]
[1,]    1    1    1
[2,]    2    2    0
[3,]    3    0    0

However, I need it as follows:

M2
     [,1] [,2] [,3]
[1,]    3    0    0
[2,]    2    2    0
[3,]    1    1    1

CodePudding user response：

We could use apply and the rev function:

apply(M2, 2, rev)

    [,1] [,2] [,3]
[1,]    3    0    0
[2,]    2    2    0
[3,]    1    1    1

CodePudding user response：

> x=c(1,2,3,1,2,0,1,0,0)
> m=matrix(x,3,3);m[nrow(m):1,]
     [,1] [,2] [,3]
[1,]    3    0    0
[2,]    2    2    0
[3,]    1    1    1

CodePudding user response：

Operating directly on x:

y = x[seq_along(x)   rep_len(c(2, 0, -2), length(x))]
matrix(y, 3)
     [,1] [,2] [,3]
[1,]    3    0    0
[2,]    2    2    0
[3,]    1    1    1