Elegant way to drop rare factor levels from data frame

Question

I want to subset a dataframe by factor. I only want to retain factor levels above a certain frequency.

df <- data.frame(factor = c(rep("a",5),rep("b",5),rep("c",2)), variable = rnorm(12))

This code creates data frame:

   factor    variable
1       a -1.55902013
2       a  0.22355431
3       a -1.52195456
4       a -0.32842689
5       a  0.85650212
6       b  0.00962240
7       b -0.06621508
8       b -1.41347823
9       b  0.08969098
10      b  1.31565582
11      c -1.26141417
12      c -0.33364069

And I want to drop factor levels which repeated less than 5 times. I developed a for-loop and it is working:

for (i in 1:length(levels(df$factor))){
  if(table(df$factor)[i] < 5){
    df.new <- df[df$factor != names(table(df$factor))[i],] 
  }
}

But do quicker and prettier solutions exists?

Answer 1

require(dplyr)

df %>% group_by(factor) %>% filter(n() >= 5)
#factor   variable
#1       a  2.0769363
#2       a  0.6187513
#3       a  0.2426108
#4       a -0.4279296
#5       a  0.2270024
#6       b -0.6839748
#7       b -0.3285610
#8       b  0.2625743
#9       b -0.9532957
#10      b  1.4526317

Answer 2

What about

df.new <- df[!(as.numeric(df$factor) %in% which(table(df$factor)<5)),]