Grouping 2 levels of a factor in R

Question

I have a column of data that is a factor with levels A, B and C, I am interested in combining two of these levels into one factor, so it would become A and B, with B = B and C, or maybe a new variable A and D, with D = B and C. I can come up with plenty of ways to do this by looping through the column with if statements, but I feel like there should be a more elegant approach and I was wondering if someone could point me in the right direction.

More recent, better answer: http://stackoverflow.com/questions/19410108/cleaning-up-factor-levels-collapsing-multiple-levels-labels — Jack Tanner, Apr 07 '15 at 19:26

score 93 · Accepted Answer · answered Mar 07 '12 at 15:31

93

Use levels(x) <- ... to specify new levels, and to combine some previous levels. For example:

f <- factor(LETTERS[c(1:3, 3:1)])
f
[1] A B C C B A
Levels: A B C

Now combine "A" and "B" into a single level:

levels(f) <- c("A", "A", "C")
f
[1] A A C C A A
Levels: A C

answered Mar 07 '12 at 15:31

Andrie

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As simple as I could have hoped for, thank you! – asjohnson Mar 07 '12 at 15:42
3

Is there a way of doing this in dplyr? – Hatshepsut Mar 20 '16 at 06:43
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What if I have say 100 levels (0,1,2,3.... 99) and I want to get only 3 (0, 1, biggerthan1) ? – skan Nov 29 '16 at 16:57
@Hatshepsut use `levels<-`; see https://stackoverflow.com/questions/10449366/levels-what-sorcery-is-this/10491881#10491881 – Frank Sep 17 '19 at 13:11

score 23 · Answer 2 · answered Oct 16 '16 at 21:35

23

If you're using dplyr pipes you can use the forcats package.

library(forcats)
f %>% fct_collapse(A = c("A","B"))

#[1] A A C C A A
#Levels: A C

answered Oct 16 '16 at 21:35

Joe

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score 4 · Answer 3 · answered Jan 23 '17 at 01:38

4

The rockchalk library is able to combine levels. I think its great, if you want to combine B and C together in a factor do this:

library(rockchalk)
combineLevels(mydf$facVar,levs = c("B", "C"), newLabel = c("BandC") )

answered Jan 23 '17 at 01:38

debo

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Grouping 2 levels of a factor in R

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