How can I split a data frame in R randomly?

Question

I have a data frame with ca. 1000 rows, and I want to split it randomly into 8 smaller dataframes each containing 100 element. I tried to used the sample function 8 times on the data frame, but sometimes it selects the same rows.

Answer 1

We create a grouping variable by sampleing 1 to 8 with size as the number of rows of the dataset, split the sequence of rows with the grouping variable in a list, loop through the list (lapply(...), subset the dataset and get the first 100 rows with head

lst <- lapply(split(1:nrow(df1), sample(1:8, nrow(df1), replace=TRUE, prob = rep(1/8, 8))),
           function(i) head(df1[i,],100))
sapply(lst, nrow)
#  1   2   3   4   5   6   7   8 
#100 100 100 100 100 100 100 100 

As @RHertel mentioned in the comments, we can do a second sample to get the 100 rows

lst <- lapply(split(1:nrow(df1), sample(1:8, nrow(df1), replace=TRUE, prob = rep(1/8, 8))),
       function(i) df1[sample(i, 100, replace=FALSE),])

data

set.seed(24)
df1 <- data.frame(V1= 1:1000, V2= rnorm(1000))