Styjun

I try to figure out how dose the sample function work when I try to random select 10 samples in each group from the data frame.

I have a data frame with 5 columns and 7000 rows. I split the dataset into around 200 groups. Then I want to random select 10 samples from each group. Some groups have less 10 samples. So when I try to sample them, I set replace=T. However, when I check the output, I found out some groups which were more then 10 sample in the group have repeat samples.

I am not sure how to fix this?

 mydata2<- split(mydf,mydf$Group)
 names(mydata2)<-paste0("mydata2",1:length(levels(mydf$Group))) 
 mysample<-Map(function(x) x[sample((1:nrow(x)),size=10,replace=T),], 
 mydata2)

Let's first understand the concept of replace.

By default replace is FALSE in sample. So when you do

sample(1:5, 2)
#[1] 4 1

As replace is default set to FALSE you are not allowed to repeat elements. It will randomly select 2 unique elements from 1:5.

But now when you do

sample(1:5, 6)

it results in an error stating

Error in sample.int(length(x), size, replace, prob) :
cannot take a sample larger than the population when 'replace = FALSE'

which means you are trying to take 6 unique element from 1:5 which is not possible because you have only 5 unique elements in them. So when you set replace = TRUE you say that elements are allowed to be repeated.

sample(1:5, 6, replace = TRUE)
#[1] 3 3 5 3 1 1

Here, we see with replace = TRUE element 3 is repeated whereas in first example how many times you run it, it will never repeat elements.

So now, I hope the concept of replace is clear to you. Now you want to repeat rows only when there are less than 10 rows in the list. So you can add a condition accordingly,

lapply(mydata2, function(x) x[sample(1:nrow(x) ,size=10, replace=nrow(x) < 10), ])

This will be TRUE only when the number of rows in the dataframe is less than 10.

Using reproducible example from mtcars

table(mtcars$cyl)

# 4 6 8 
#11 7 14 

We see cyl = 6 has 7 rows which is less than 10.

mydata2 <- split(mtcars, mtcars$cyl)

lapply(mydata2, function(x) x[sample(1:nrow(x) ,size=10, replace=nrow(x) < 10), ])


#$`4`
# mpg cyl disp hp drat wt qsec vs am gear carb
#Fiat X1-9 27.3 4 79.0 66 4.08 1.935 18.90 1 1 4 1
#Merc 230 22.8 4 140.8 95 3.92 3.150 22.90 1 0 4 2
#Toyota Corolla 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1
#Honda Civic 30.4 4 75.7 52 4.93 1.615 18.52 1 1 4 2
#Toyota Corona 21.5 4 120.1 97 3.70 2.465 20.01 1 0 3 1
#Lotus Europa 30.4 4 95.1 113 3.77 1.513 16.90 1 1 5 2
#Volvo 142E 21.4 4 121.0 109 4.11 2.780 18.60 1 1 4 2
#Fiat 128 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1
#Datsun 710 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1
#Merc 240D 24.4 4 146.7 62 3.69 3.190 20.00 1 0 4 2

#$`6`
# mpg cyl disp hp drat wt qsec vs am gear carb
#Valiant 18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1
#Valiant.1 18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1
#Mazda RX4 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4
#Ferrari Dino 19.7 6 145.0 175 3.62 2.770 15.50 0 1 5 6
#Valiant.2 18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1
#Mazda RX4.1 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4
#Mazda RX4 Wag 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4
#Merc 280C 17.8 6 167.6 123 3.92 3.440 18.90 1 0 4 4
#Hornet 4 Drive 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1
#Merc 280 19.2 6 167.6 123 3.92 3.440 18.30 1 0 4 4

#$`8`
# mpg cyl disp hp drat wt qsec vs am gear carb
#Dodge Challenger 15.5 8 318.0 150 2.76 3.520 16.87 0 0 3 2
#Camaro Z28 13.3 8 350.0 245 3.73 3.840 15.41 0 0 3 4
#Ford Pantera L 15.8 8 351.0 264 4.22 3.170 14.50 0 1 5 4
#Merc 450SLC 15.2 8 275.8 180 3.07 3.780 18.00 0 0 3 3
#AMC Javelin 15.2 8 304.0 150 3.15 3.435 17.30 0 0 3 2
#Pontiac Firebird 19.2 8 400.0 175 3.08 3.845 17.05 0 0 3 2
#Duster 360 14.3 8 360.0 245 3.21 3.570 15.84 0 0 3 4
#Cadillac Fleetwood 10.4 8 472.0 205 2.93 5.250 17.98 0 0 3 4
#Hornet Sportabout 18.7 8 360.0 175 3.15 3.440 17.02 0 0 3 2
#Lincoln Continental 10.4 8 460.0 215 3.00 5.424 17.82 0 0 3 4

In the output we can see the rows are repeated only for cyl = 6 and not for anybody else.

Perhaps only use replace=T when the group is smaller than the minimum size? This flag allows duplicates in the sample resulting in the behavior you're observing.

replace - If this is true a sample may contain an element several times while another element might not occur at all.

http://www.rexamples.com/14/Sample()