I want to change the order of the levels of a factor so that a specific level comes right after another level, but I'm struggling how to do it efficiently.
Let's assume that we want to change the level of the following factor so that "20" comes right after "10". So I tried this and succesfully got the expected result:
library(tidyverse)
sample_factor <- factor(1:30)
trial_factor1 <- sample_factor %>% fct_relevel("20", after=which(levels(.)=="10"))
levels(trial_factor1)
#> [1] "1" "2" "3" "4" "5" "6" "7" "8" "9" "10" "20" "11" "12" "13" "14"
#> [16] "15" "16" "17" "18" "19" "21" "22" "23" "24" "25" "26" "27" "28" "29" "30"
However, if the order of the initial factor is reversed, it doesn't work:
trial_factor2 <- sample_factor %>% fct_rev() %>% fct_relevel("20", after=which(levels(.)=="10"))
levels(trial_factor2)
#> [1] "30" "29" "28" "27" "26" "25" "24" "23" "22" "21" "19" "18" "17" "16" "15"
#> [16] "14" "13" "12" "11" "10" "9" "20" "8" "7" "6" "5" "4" "3" "2" "1"
This is probably because, in this case, "20" is initially positioned before "10".
In addition, if I also try to change the order so that "30" comes right after "20" (expected factor levels: ..., 10, 20, 30, ...), the result gets worse:
trial_factor3 <- sample_factor %>% fct_rev() %>% fct_relevel("20", after=which(levels(.)=="10")) %>%
fct_relevel("30", after=which(levels(.)=="20"))
levels(trial_factor3)
#> [1] "29" "28" "27" "26" "25" "24" "23" "22" "21" "19" "18" "17" "16" "15" "14"
#> [16] "13" "12" "11" "10" "9" "20" "8" "30" "7" "6" "5" "4" "3" "2" "1"
Created on 2022-08-18 by the reprex package (v2.0.1)
In my real situation, I want to change the order of levels multiple times (more than 5 times) and I don't clearly know the initial order of the factor levels, so I find it really difficult to change the order flexibly.
I really appreciate your help in advance!
Seems like fct_relevel is designed to work positionally, make this level the nth level (where n = after + 1... sort of a strange design/naming decision), but you want to work based only on the level names (labels).
We can write our own version that does this, translating the after label to position, and accounting for the before/after after-label trouble. (Also, why use 30 levels in a sample when 5 will do nicely?)
fct_relevel_label = function(.f, level, after_label) {
lev = levels(.f)
move = which(lev == level)
target = which(lev == after_label)
after = if(move <= target) {target - 1} else {target}
fct_relevel(.f, level, after = after)
}
factor(1:5) %>% fct_relevel_label("2", after_label = "4") %>% levels
# [1] "1" "3" "4" "2" "5"
factor(1:5) %>% fct_rev() %>% fct_relevel_label("2", after_label = "4") %>% levels
# [1] "5" "4" "2" "3" "1"