How to randomly split data into three equal sizes?

问题

I have a dataset with 9558 rows from three different projects. I want to randomly split this dataset in three equal groups and assign a unique ID for each group, so that Project1_Project_2_Project3 becomes Project1, Project2 and Project3.

I have tried many things, and googled codes from people with similar problem as I have. I have used sample_n() and sample_frac(), but unfortunately I can't solve this issue myself :/

I have made an example of my dataset looking like this:

ProjectName <- c("Project1_Project2_Project3")
data <- data.frame(replicate(10,sample(0:1,9558,rep=TRUE)))
data <- data.frame(ProjectName, data)

And the output should be randomly split in three equal group of nrow=3186 and then assigned to the values

ProjectName Count of rows
Project1     3186
Project2     3186
Project3     3186

回答1:

IMO it should be sufficient to assign just random project names.

dat$ProjectName <- sample(factor(rep(1:3, length.out=nrow(dat)), 
                          labels=paste0("Project", 1:3)))

Result

head(dat)
#   X1 X2 X3 X4 X5 X6 X7 X8 X9 X10 ProjectName
# 1  1  1  0  1  1  1  1  0  1   0    Project1
# 2  1  1  1  1  1  1  0  0  1   0    Project1
# 3  0  0  1  1  0  0  0  1  1   1    Project1
# 4  1  1  1  0  1  0  1  1  0   1    Project3
# 5  1  0  0  1  1  1  1  0  0   1    Project1
# 6  1  0  0  0  0  1  0  1  1   1    Project3

table(dat$ProjectName)
# Project1 Project2 Project3 
#     3186     3186     3186 

Data

set.seed(42)
dat <- data.frame(replicate(10, sample(0:1, 9558, rep=TRUE)))

回答2:

Add an id to data:

data$id <- 1:nrow(data)

Take the first sample:

project1 <- dplyr::sample_frac(data, 0.33333)

Remove the used rows from data and save into project2:

project2 <- data[!(data$id %in% project1$id), ]

Sample half of the remainder:

project3 <- dplyr::sample_frac(project2, 0.5)

Finally remove those in the project3 sample from project2:

project2 <- project2[!(project2$id %in% project3$id), ]

Check all ids are unique:

# should all be FALSE
any(project1$id %in% project2$id)
any(project1$id %in% project3$id)
any(project2$id %in% project3$id)

And double-check the data frames have the right number of cases:

nrow(project1)
nrow(project2)
nrow(project3)

回答3:

I had this same problem once. This is how I did it. If you just use sample, the groups are uneven, by sampling off a vector where the groups are even worked for me.

sampleframe <- rep(1:3, ceiling( nrow( data)/3 ) ) 

data$grp <- 0
data[  , "grp"  ] <- sample( sampleframe , size=nrow( data) ,  replace=FALSE )

project1 <- data[data$grp %in% 1 ,]
project2 <- data[data$grp %in% 2 ,]
project3 <- data[data$grp %in% 3 ,]

回答4:

I like the solution in this comment to a Github gist.

You could generate the indices as suggested:

folds <- split(sample(nrow(data), nrow(data), replace = FALSE), as.factor(1:3))

Then get a list of 3 equal size data frames using:

datalist <- lapply(folds, function(x) data[x, ])

来源：https://stackoverflow.com/questions/55375807/how-to-randomly-split-data-into-three-equal-sizes