Applying User-defined Functions on Factor Levels of Dataset in R Programming - by() Function

by() function in R programming is an object-oriented wrapper function which performs the provided function on factor levels of the data set passed in the arguments of function call.

Syntax: by(data, INDICES, FUN)

Parameters:

data: represents the dataset
INDICES: represents the factor list of dataset
FUN: represents the function to be performed on factor levels

Example 1:

# Using mtcars dataset 
df <- data.frame(mtcars) 
  
# Factor levels on gear 
dffactors <- factor(mtcars$gear) 
  
# Output maximum hp of each factor i.e., gears 
by(df, dffactors, function(x){ 
  m <- max(x$hp) 
}) 

Output:

dffactors: 3
[1] 245
------------------------------------------------------------ 
dffactors: 4
[1] 123
------------------------------------------------------------ 
dffactors: 5
[1] 335

Example 2:

# Using mtcars dataset 
df <- data.frame(mtcars) 
  
# Factor levels on gear 
dffactors <- factor(mtcars$gear) 
  
# Output mean of qsec of each factor i.e., gears 
by(df, dffactors, function(x){ 
  m <- mean(x$qsec) 
}) 

Output:

dffactors: 3
[1] 17.692
------------------------------------------------------------ 
dffactors: 4
[1] 18.965
------------------------------------------------------------ 
dffactors: 5
[1] 15.64

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