优雅的方式从数据框中删除稀有因子水平

Question

我想按因子对数据帧进行子集化.我只想保留高于特定频率的因子水平.

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

此代码创建数据框:

   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

而且我想降低重复次数少于5次的因子水平.我开发了一个for循环,它正在工作:

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

但是,存在更快更漂亮的解决方案吗？

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

library(data.table)
setDT(df)[, variable[.N >= 5], by = factor]

##    factor         V1
## 1:      a -0.8204684
## 2:      a  0.4874291
## 3:      a  0.7383247
## 4:      a  0.5757814
## 5:      a -0.3053884
## 6:      b  1.5117812
## 7:      b  0.3898432
## 8:      b -0.6212406
## 9:      b -2.2146999
## 10:     b  1.1249309

Answer 3

关于什么

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