Keep more than two values in a dataframe

Question

Having a dataframe like this

df <- data.frame(ID = sample(rep(letters, each=3)), value = rnorm(n=26*3))
keep <- c("a", "d", "r", "x")

How is it possible to keep both "a" and "d"

example using something like this: df[df$ID == "a"|"d", ]

Answer 1

Use %in%

df[df$ID %in% c("a","d"), ]

Answer 2

We can use base R function which(). which() returns indexes where given condition is met.

df[which(df$ID == "a" | df$ID == "d"),]

Answer 3

An option with %chin%

library(data.table)
setDT(df)[ID %chin% c('a', 'd')]