Sampling using an id variable

Question

I know how to do ordinary random sampling using R:

mysample <- mydata[sample(1:nrow(mydata), 100),]

However, I want to sample by id variables. Let me explain - my dataset looks like this:

id var1 var2 ...
1  5.1  1.2
1  4.7  0.9
2  3.3  1.6
3  3.4  5.7
4  7.9  1.3

Now, I want to take a random sample of, say, 2, by id numbers. Let's say the random sample yields id 1 and 4, then my sample would look like this:

id var1 var2 ...
1  5.1  1.2
1  4.7  0.9
4  7.9  1.3

In other words, I'm sampling 2 id numbers, but I'm actually getting 3 cases.

How can I accomplish this in R?

Answer 1

Your data:

mydata <- read.table(text = "id var1 var2 
1  5.1  1.2
1  4.7  0.9
2  3.3  1.6
3  3.4  5.7
4  7.9  1.3", header = TRUE)

Sample two id values:

set.seed(1)
ids <- sample(unique(mydata$id), 2) # important: the UNIQUE id numbers
# [1] 2 4

Extract subset:

mydata[mydata$id %in% ids, ]
#   id var1 var2
# 3  2  3.3  1.6
# 5  4  7.9  1.3