# by() Function in R

by() function in R applies a function to specified subsets of a data frame.First parameter of by() function, takes up the data and second parameter is by which the function is applied and third parameter is the function.

#### Syntax of by() function in R:

by(data, data\$byvar, FUN)

 data an R object, normally a data frame, possibly a matrix. data\$byvar a factor or a list of factors by which the function is applied FUN a function to be applied to the subsets of data.

#### Example of by() function in R:

Lets use the iris and mtcars data set to demonstrate  R by() function. If we want to find the mean of sepal.length of the different species, we can use by() function.

```# by() function in R with mean

mydata <- iris
by(mydata\$Sepal.Length,list(mydata\$Species),mean)
```

in the above example, mean of sepal.length is calculated for distinct Species, so the output will be

: setosa

[1] 5.006

——————————————————————————-

: versicolor

[1] 5.936

——————————————————————————-

: virginica

[1] 6.588

#### by() function in R with more than one list:

Lets use mtcars table to demonstrate one more example.

```# by() function in R with more than one list

by(mydata\$hp,list(mydata\$gear,mydata\$cyl),mean)
```

in the above example, mean of hp is calculated for distinct gear and cyl combination, so the output will be

: 3

: 4

[1] 97

——————————————————————————-

: 4

: 4

[1] 76

——————————————————————————-

: 5

: 4

[1] 102

——————————————————————————-

: 3

: 6

[1] 107.5

——————————————————————————-

: 4

: 6

[1] 116.5

——————————————————————————-

: 5

: 6

[1] 175

——————————————————————————-

: 3

: 8

[1] 194.1667

——————————————————————————-

: 4

: 8

[1] NA

——————————————————————————-

: 5

: 8

[1] 299.5

Mean of hp with gear =5 and cyl=8 is 299.5 and so on.

## Author

• With close to 10 years on Experience in data science and machine learning Have extensively worked on programming languages like R, Python (Pandas), SAS, Pyspark.