crangos crangos - 2 months ago 9
R Question

Reorder levels of a factor without changing order of values

I have data frame with some numerical variables and some categorical

factor
variables. The order of levels for those factors is not the way I want them to be.

numbers <- 1:4
letters <- factor(c("a", "b", "c", "d"))
df <- data.frame(numbers, letters)
df
# numbers letters
# 1 1 a
# 2 2 b
# 3 3 c
# 4 4 d


If I change the order of the levels, the letters no longer are with their corresponding numbers (my data is total nonsense from this point on).

levels(df$letters) <- c("d", "c", "b", "a")
df
# numbers letters
# 1 1 d
# 2 2 c
# 3 3 b
# 4 4 a


I simply want to change the level order, so when plotting, the bars are shown in the desired order - which may differ from default alphabetical order.

Answer

Use the levels argument of factor:

df <- data.frame(f = 1:4, g = letters[1:4])
df
#   f g
# 1 1 a
# 2 2 b
# 3 3 c
# 4 4 d

levels(df$g)
# [1] "a" "b" "c" "d"

df$g <- factor(df$g, levels = letters[4:1])
# levels(df$g)
# [1] "d" "c" "b" "a"

df
#   f g
# 1 1 a
# 2 2 b
# 3 3 c
# 4 4 d