Reverse the levels in a factor

Simple literal reversal. Will stop with an error message if x is not a factor (or ordered) variable.

reverse(x, eol = c("Skip", "DNP"))

Arguments

x

a factor variable

eol

values to be kept at the end of the list. Does not accept regular expresssions, just literal text strings representing values.

Value

a new factor variable with reversed values

Details

Sometimes people want to reverse some levels, excluding others and leaving them at the end of the list. The "eol" argument sets aside some levels and puts them at the end of the list of levels.

The use case for the eol argument is a factor with several missing value labels, as appears in SPSS. With up to 18 different missing codes, we want to leave them at the end. In the case for which this was designed, the researcher did not want to designate those values as missing before inspecting the pattern of observed values.

Author

Paul Johnson <pauljohn@ku.edu>

Examples

## Consider alphabetication of upper and lower
x <- factor(c("a", "b", "c", "C", "a", "c"))
levels(x)
#> [1] "C" "a" "b" "c"
xr1 <- reverse(x)
xr1
#> [1] a b c C a c
#> Levels: c b a C
## Keep "C" at end of list, after reverse others
xr2 <- reverse(x, eol = "C")
xr2
#> [1] a b c C a c
#> Levels: c b a C
y <- ordered(x, levels = c("a", "b", "c", "C"))
yr1 <- reverse(y)
class(yr1)[1] == "ordered"
#> [1] TRUE
yr1
#> [1] a b c C a c
#> Levels: C < c < b < a
## Hmm. end of list amounts to being "maximal".
## Unintended side-effect, but interesting.
yr2 <- reverse(y, eol = "C")
yr2
#> [1] a b c C a c
#> Levels: c < b < a < C
## What about a period as a value (SAS missing)
z <- factor(c("a", "b", "c", "b", "c", "."))
reverse(z)
#> [1] a b c b c .
#> Levels: c b a .
z <- factor(c(".", "a", "b", "c", "b", "c", "."))
reverse(z)
#> [1] . a b c b c .
#> Levels: c b a .
## How about R NA's
z <- factor(c(".", "a", NA, "b", "c", "b", NA, "c", "."))
z
#> [1] .    a    <NA> b    c    b    <NA> c    .   
#> Levels: . a b c
reverse(z)
#> [1] .    a    <NA> b    c    b    <NA> c    .   
#> Levels: c b a .
z <- ordered(c(".", "a", NA, "b", "c", "b", NA, "c", "."))
z
#> [1] .    a    <NA> b    c    b    <NA> c    .   
#> Levels: . < a < b < c
str(z)
#>  Ord.factor w/ 4 levels "."<"a"<"b"<"c": 1 2 NA 3 4 3 NA 4 1
## Put "." at end of list
zr <- reverse(z, eol = ".")
zr
#> [1] .    a    <NA> b    c    b    <NA> c    .   
#> Levels: c < b < a < .
str(zr)
#>  Ord.factor w/ 4 levels "c"<"b"<"a"<".": 4 3 NA 2 1 2 NA 1 4
z <- ordered(c(".", "c", NA, "e", "a", "c", NA, "e", "."),
         levels = c(".", "c", "e", "a"))
reverse(z, eol = ".")
#> [1] .    c    <NA> e    a    c    <NA> e    .   
#> Levels: a < e < c < .
reverse(z, eol = c("a", "."))
#> [1] .    c    <NA> e    a    c    <NA> e    .   
#> Levels: e < c < a < .