Returns a reversed version of its argument. Where rev() treats every
object as one long vector, revX()
reverses the order along the dimensions of a multidimensional object,
so that a matrix, table, array or data frame comes
back with its rows, its columns, or both, in the opposite order and its
dimnames moved along with the data. Which dimensions are turned around is
chosen with margin.
Usage
revX(x, ...)
# Default S3 method
revX(x, margin = 1L, ...)
# S3 method for class 'array'
revX(x, margin = seq_along(dim(x)), ...)
# S3 method for class 'matrix'
revX(x, margin = seq_along(dim(x)), ...)
# S3 method for class 'table'
revX(x, margin = seq_along(dim(x)), ...)
# S3 method for class 'data.frame'
revX(x, margin = 1:2, ...)Arguments
- x
a vector, matrix, table, array or data frame to be reversed.
- ...
further arguments, passed on to the method dispatched on. This is how
marginis handed over; arguments beyond it are ignored with a warning.- margin
the dimensions to reverse,
1for the rows,2for the columns, and so on, each at most once. Defaults to all dimensions ofx, which for a vector means1.
Value
an object of the same class and dimensions as x, with the order of
the elements along margin reversed.
Details
A vector has one dimension and is simply reversed, as by rev(), with
margin = 1 accepted so that calling code need not know whether its
argument has dimensions. For everything else, margin names the dimensions to be reversed, 1 for the
rows, 2 for the columns, and so on for the higher dimensions of an array;
the default reverses all of them. The values in the object are not
rearranged relative to their labels: reversing an object twice along the
same margin returns the original.
margin names each dimension at most once; repeating one says nothing and
is refused, as is any value outside the dimensions of x.
The additional arguments of the generic are the way margin reaches the
methods. Anything else is ignored with a warning, rather than being dropped
silently although it was meant to change the result.
Examples
tab <- matrix(c(1, 11, 111,
2, 22, 222,
3, 33, 333),
byrow=TRUE, nrow=3,
dimnames=list(mar1=1:3, mar2=c("a","b","c")))
revX(tab, margin=1)
#> mar2
#> mar1 a b c
#> 3 3 33 333
#> 2 2 22 222
#> 1 1 11 111
revX(tab, margin=2)
#> mar2
#> mar1 c b a
#> 1 111 11 1
#> 2 222 22 2
#> 3 333 33 3
# reverse both dimensions
revX(tab, margin=c(1, 2))
#> mar2
#> mar1 c b a
#> 3 333 33 3
#> 2 222 22 2
#> 1 111 11 1
# the dimnames travel with the data, so this is not a transposition
revX(tab, margin=c(1, 2))["3", "a"] == tab["3", "a"]
#> [1] TRUE
## [1] TRUE
# reverse a 3-dimensional array
aa <- array(c(tab, 2 * tab), dim = c(3, 3, 2),
dimnames = c(dimnames(tab), list(mar3 = c("A", "Z"))))
# reverse rows
revX(aa, 1)
#> , , mar3 = A
#>
#> mar2
#> mar1 a b c
#> 3 3 33 333
#> 2 2 22 222
#> 1 1 11 111
#>
#> , , mar3 = Z
#>
#> mar2
#> mar1 a b c
#> 3 6 66 666
#> 2 4 44 444
#> 1 2 22 222
#>
# reverse columns
revX(aa, 2)
#> , , mar3 = A
#>
#> mar2
#> mar1 c b a
#> 1 111 11 1
#> 2 222 22 2
#> 3 333 33 3
#>
#> , , mar3 = Z
#>
#> mar2
#> mar1 c b a
#> 1 222 22 2
#> 2 444 44 4
#> 3 666 66 6
#>
# reverse the third dimension
revX(aa, 3)
#> , , mar3 = Z
#>
#> mar2
#> mar1 a b c
#> 1 2 22 222
#> 2 4 44 444
#> 3 6 66 666
#>
#> , , mar3 = A
#>
#> mar2
#> mar1 a b c
#> 1 1 11 111
#> 2 2 22 222
#> 3 3 33 333
#>
# reverse all dimensions
revX(aa)
#> , , mar3 = Z
#>
#> mar2
#> mar1 c b a
#> 3 666 66 6
#> 2 444 44 4
#> 1 222 22 2
#>
#> , , mar3 = A
#>
#> mar2
#> mar1 c b a
#> 3 333 33 3
#> 2 222 22 2
#> 1 111 11 1
#>
# same as
revX(aa, margin = 1:3)
#> , , mar3 = Z
#>
#> mar2
#> mar1 c b a
#> 3 666 66 6
#> 2 444 44 4
#> 1 222 22 2
#>
#> , , mar3 = A
#>
#> mar2
#> mar1 c b a
#> 3 333 33 3
#> 2 222 22 2
#> 1 111 11 1
#>
# data frames are reversed by rows, by columns or both
d <- data.frame(a = 1:3, b = 4:6)
revX(d, 1)
#> a b
#> 3 3 6
#> 2 2 5
#> 1 1 4
revX(d, 2)
#> b a
#> 1 4 1
#> 2 5 2
#> 3 6 3