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Applying User-defined Functions on Factor Levels of Dataset in R Programming – by() Function
• Last Updated : 01 Jun, 2020

`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
 245
------------------------------------------------------------
dffactors: 4
 123
------------------------------------------------------------
dffactors: 5
 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
 17.692
------------------------------------------------------------
dffactors: 4
 18.965
------------------------------------------------------------
dffactors: 5
 15.64```

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