Split vector randomly into two sets

Question

I have a vector t with length 100 and want to divide it into 30 and 70 values but the values should be chosen randomly and without replacement. So none of the 30 values are allowed to be in the sub vector of the 70 values and vice versa.

I know the R function sample which I can use to randomly chose values from a vector with and without replacement. However, even when I use replace = FALSE I have to run the sample function twice once with 30 and once with 70 values to chose. That means that some of the 30 values might be in the 70 values and vice versa.

Any ideas?

Answer 1

Regarding my comment, what is wrong with:

vec <- 1:100
set.seed(2)
samp <- sample(length(vec), 30)

a <- vec[samp]
b <- vec[-samp]

?

To show these are separate sets with no duplicates:

R> intersect(a, b)
integer(0)

If you have duplicate values in your vector that is a different matter, but your question is unclear.

With duplicates in vec things are a bit more complicated and it depends what result you wanted to achieve.

R> set.seed(4)
R> vec <- sample(100, 100, replace = TRUE)
R> set.seed(6)
R> samp <- sample(100, 30)
R> a <- vec[samp]
R> b <- vec[-samp]
R> length(a)
[1] 30
R> length(b)
[1] 70
R> length(setdiff(vec, a))
[1] 41

So the setdiff() "fails" here as it doesn't get the length right, but then a and b contain duplicate values (but not observations! from the sample):

R> intersect(a, b)
 [1] 57 35 91 27 71 63  8 92 49 77

The duplicates (intersection) arises because the values above occurred twice in the original sample vec

Answer 2

How about this:

t <- 1:100 # or whatever your original set is
a <- sample(t, 70)
b <- setdiff(t, a)