Random sampling in R within Categorical variable

0
votes

Suppose I have a data frame with categorical variable of n classes and a numerical variable. I need to randomize the numerical variable within each category. For example , consider the following table:

Col_1           Col_2      
   A               2        
   A               5           
   A               4           
   A               8        
   B               1   
   B               4        
   B               9          
   B               7

When I tried sample() function in R, it threw the result considering both the categories. Is there any function where I can get this kind of output? (with or without replacement, doesn't matter)

Col_1           Col_2      
 A               8        
 A               4           
 A               2           
 A               5        
 B               9  
 B               7       
 B               4          
 B               1
2
rpermutationsampling

2 Answers

4
votes

You could sample row numbers within groups. In base R, we can use ave

df[with(df, ave(seq_len(nrow(df)), Col_1, FUN = sample)), ]

#  Col_1 Col_2
#2     A     5
#4     A     8
#1     A     2
#3     A     4
#7     B     9
#5     B     1
#8     B     7
#6     B     4

In dplyr, we can use sample_n

library(dplyr)
df %>% group_by(Col_1) %>% sample_n(n())

data

df <- structure(list(Col_1 = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 
2L), .Label = c("A", "B"), class = "factor"), Col_2 = c(2L, 5L, 
4L, 8L, 1L, 4L, 9L, 7L)), class = "data.frame", row.names = c(NA, -8L))
3
votes

Here's a dplyr solution:

library(dplyr)
set.seed(2)
dat %>%
  group_by(Col_1) %>%
  mutate(Col_2 = sample(Col_2)) %>%
  ungroup()
# # A tibble: 8 x 2
#   Col_1 Col_2
#   <chr> <int>
# 1 A         2
# 2 A         4
# 3 A         5
# 4 A         8
# 5 B         7
# 6 B         9
# 7 B         1
# 8 B         4

A data.table method:

library(data.table)
datDT <- as.data.table(dat)
set.seed(2)
datDT[, Col_2 := sample(Col_2), by = "Col_1"]
datDT
#    Col_1 Col_2
# 1:     A     2
# 2:     A     4
# 3:     A     5
# 4:     A     8
# 5:     B     7
# 6:     B     9
# 7:     B     1
# 8:     B     4

Data

dat <- read.table(header = TRUE, stringsAsFactors = FALSE, text = "
Col_1 Col_2
A 2
A 5
A 4
A 8
B 1
B 4
B 9
B 7")