有没有一种快速的方法可以多次过滤data.frame的行？

Question

I have a dataframe xd from which I wish to filter data for id=1,2, but with 1 and 2 both repeated twice.

set.seed(12)
xd <- data.frame(id = sort(sample(3,20, rep=TRUE)), y = rnorm(20))
fxd <- subset(xd, subset = id %in% c(1,2,1,2)) # doesn't work
str(fxd)

However, this doesn't work because it only selects id=1 and id=2 only once. Is there any quick way of getting around it?

Answer 1

The subset argument of function subset expects logical expression, meaning you can select from rows by mentioning TRUE/FALSE for each row.

If you want to replicate selection then an option is to use which . Which returns row-number that can be replicated. Hence, option can be as:

set.seed(12)
xd <- data.frame(id = sort(sample(3,20, rep=TRUE)), y = rnorm(20))

fxd <- xd[rep(which(xd$id %in% c(1,2)), each = 2),]

fxd
#      id           y
# 1     1 -0.77771958
# 1.1   1 -0.77771958
# 2     1 -1.29388230
# 2.1   1 -1.29388230
# 3     1 -0.77956651
# 3.1   1 -0.77956651
# 4     1  0.01195176
# 4.1   1  0.01195176
# 5     1 -0.15241624
# 5.1   1 -0.15241624
# 6     1 -0.70346425
# 6.1   1 -0.70346425
# 7     1  1.18887916
# 7.1   1  1.18887916
# 8     1  0.34051227
# 8.1   1  0.34051227