Question

I have a data frame df with an ID column eg A,B,etc. I also have a vector containing certain IDs:

L <- c("A", "B", "E")

How can I filter the data frame to get only the IDs present in the vector? Individually, I would use

subset(df, ID == "A")

but how do I filter on a whole vector?

Answer 1

You can use the %in% operator:

> df <- data.frame(id=c(LETTERS, LETTERS), x=1:52)
> L <- c("A","B","E")
> subset(df, id %in% L)
   id  x
1   A  1
2   B  2
5   E  5
27  A 27
28  B 28
31  E 31

If your IDs are unique, you can use match():

> df <- data.frame(id=c(LETTERS), x=1:26)
> df[match(L, df$id), ]
  id x
1  A 1
2  B 2
5  E 5

or make them the rownames of your dataframe and extract by row:

> rownames(df) <- df$id
> df[L, ]
  id x
A  A 1
B  B 2
E  E 5

Finally, for more advanced users, and if speed is a concern, I’d recommend looking into the data.table package.