在 R 编程中将用户定义的函数应用于数据集的因子水平 – by()函数

R 编程中的by()函数是一个面向对象的包装函数，它在函数调用的参数中传递的数据集的因子级别上执行提供的函数。

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

示例 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)
})

输出：

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

示例 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)
})

输出：

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