来自R中数据帧的随机样本

Question

I have the following data frame:

id<-c(1,1,2,3,3)
date<-c("23-01-08","01-11-07","30-11-07","17-12-07","12-12-08")
df<-data.frame(id,date)
df$date2<-as.Date(as.character(df$date), format = "%d-%m-%y")

id     date      date2
1   23-01-08 2008-01-23
1   01-11-07 2007-11-01
2   30-11-07 2007-11-30
3   17-12-07 2007-12-17
3   12-12-08 2008-12-12

Now I want to extract a random sample of ids and not the rows. In fact I am looking for a way to randomly pick two of the ids and extract all records related to them. For instance if it randomly pick ids 2 and 3 the output data frame should look like:

id     date      date2
2   30-11-07 2007-11-30
3   17-12-07 2007-12-17
3   12-12-08 2008-12-12

Any helps would be appreciated.

Answer 1

You can randomly pick two IDs using sample()

chosen <- sample(unique(df$id), 2)

and then extract those records

subset(df, id %in% chosen)

Answer 2

First you have to generate the sample indexes:

s_ids=sample(unique(df$id),2)

now that you have that you select the proper records in your df

new_df=df$[df$id %in% s_ids,]

Answer 3

You can use sample function.

set.seed(2)
df[match(sample(unique(df$id),2),df$id),]

sample() function will generate random indexes and then you can match them back to your df data frame rows and get the rest of the data. For more information check ?sample

Answer 4

Or using dplyr

library(dplyr)
df %>% 
    filter(id %in% sample(unique(id),2))
#  id     date      date2
#1  2 30-11-07 2007-11-30
#2  3 17-12-07 2007-12-17
#3  3 12-12-08 2008-12-12

Or

df %>%
     select(id) %>%
     unique() %>%
     sample_n(2) %>%
     semi_join(df, .)
#  id     date      date2
#1  1 23-01-08 2008-01-23
#2  1 01-11-07 2007-11-01
#3  2 30-11-07 2007-11-30

Answer 5

Using sqldf:

library(sqldf)
a <- sqldf("SELECT DISTINCT id FROM df  ORDER BY RANDOM(*) LIMIT 2")
sqldf("SELECT * FROM df WHERE id IN a")

Ouput:

  id     date      date2
1  1 23-01-08 2008-01-23
2  1 01-11-07 2007-11-01
3  3 17-12-07 2007-12-17
4  3 12-12-08 2008-12-12