Random sampling in R within Categorical variable

Question

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

Answer 1

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))