在 R 中选择数据帧的行

Question

Consider the following data set.

id var1 var2
1   A   33
2   B   23
3   A   45
4   A   55
5   B   22
6   A   33
7   B   90
8   A   78
9   B   12
10  A   11

I want to take a random sample (rows) of size 5 and 4 from A and B category, respectively. Can one help me, please? Thanks!

Answer 1

You can use sample :

sample_1 <- sample(df[df$var1 == "A", ]$var2, 5)
sample_2 <- sample(df[df$var1 == "B", ]$var2, 4)

Use replace=TRUE for sampling with replacement.

Data

df <- read.table(text="id var1 var2
1   A   33
2   B   23
3   A   45
4   A   55
5   B   22
6   A   33
7   B   90
8   A   78
9   B   12
10  A   11", header=TRUE)

Answer 2

An option is to split the dataset and use sample in Map

do.call(rbind, Map(function(dat, y)
   dat[sample(seq_len(nrow(dat)), size = y),], split(df, df$var1), c(5, 4)))

data

df <- structure(list(id = 1:10, var1 = c("A", "B", "A", "A", "B", "A", 
"B", "A", "B", "A"), var2 = c(33L, 23L, 45L, 55L, 22L, 33L, 90L, 
78L, 12L, 11L)), class = "data.frame", row.names = c(NA, -10L
))