r - 根据 R 中的条件重命名因子水平

Question

我想将计数小于 n 的所有因素组合成一个名为“Else”的因素

例如，如果 n = 3，那么在下面的 df 中，我想将“c”、“d”和“e”组合为“Else”：

df = data.frame(x=c(1:10), y=c("a","a","a","b","b","b","c","d","d","e"))

我首先得到一个包含所有低计数值的 df：

library(plyr)
lowcounts = ddply(df, "y", function(z){if(nrow(z)<3) nrow(z) else NULL})

我知道我可以手动更改这些，但实际上我有几十个级别，所以我需要自动化。

我只想选择并重命名级别（df）中的级别 %in% lowcount，其余部分保持不变，但不知道如何继续。

Answer 1

另一种选择：

#your dataframe
df = data.frame(x=c(1:10), y=c("a","a","a","b","b","b","c","d","d","e"))

#which levels to keep and which to change
res <- table(df$y)
notkeep <- names(res[res < 3])
keep <- names(res)[!names(res) %in% notkeep]
names(keep) <- keep

#set new levels
levels(df$y) <- c(keep, list("else" = notkeep))

df
#    x    y
#1   1    a
#2   2    a
#3   3    a
#4   4    b
#5   5    b
#6   6    b
#7   7 else
#8   8 else
#9   9 else
#10 10 else

Answer 2

为什么不这样呢？

library(data.table)
dt <- data.table(df)
dt[,ynew := ifelse(.N < 3, "else",as.character(y)), by = "y"]