Question

l <- c("a","b","c")

m is a 5×2 data frame. C1 is 1:5, C2 is a:e:

m <- data.frame(C1 = 1:5, C2 = letters[1:5], stringsAsFactors = FALSE)

I want to find n, where it contains only those rows of m where m$C2 is in the values stated in l

The resulting n is a 3×2 such that C2 is a:c, i.e.

  C1 C2
1  1  a
2  2  b
3  3  c

Answer 1

One option is to do the matching by hand with %in%:

m <- data.frame(C1 = 1:5, C2 = letters[1:5], stringsAsFactors = FALSE)
l <- c("a","b","c")

with(m, m[C2 %in% l, ])

R> with(m, m[C2 %in% l, ])
  C1 C2
1  1  a
2  2  b
3  3  c

Or alternatively via the match() function:

R> m[with(m, match(C2, l, nomatch = FALSE)), ]
  C1 C2
1  1  a
2  2  b
3  3  c

where the nomatch argument is required to get rid of the NA rows.