The family of apply() functions in R is used to apply user-defined functions to the elements of complex structures like matrices, lists or data frames.

These functions help a lot in simplifying your code and making it more readable. Moreover, they are compatible with parallel processing as well. Let us look at each function with detailed examples.

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## The apply() function in R Programming

The apply() function in R is used in case of matrices to apply a user-specified function on the rows or columns of the matrix. The following is the general syntax for apply() function.

apply(matrix, code, f, fargs)

In the above form, `matrix`

is the matrix object which we are using the apply() function for. `Code`

represents whether we wish to apply the function for rows (code set to 1) or columns (code set to 2). `F`

represents the function we need to apply and `fargs`

are the arguments to pass to the function.

Let us first define a matrix to illustrate the apply() function.

> x <-matrix(c(4,5,6,10,12,16),nrow=2,ncol=3) > x [,1] [,2] [,3] [1,] 4 6 12 [2,] 5 10 16

Suppose that we wish to perform a very specific function upon each of these elements, like squaring first, then dividing by 3 and multiplying by 4. Let us define a function that does this.

> f <-function(x) + { + return (x^2*3/4) + } > f(5) [1] 18.75

Now, in order to apply this function to all the rows/columns in a matrix, we call the apply() function.

> apply(x, 2, f) [,1] [,2] [,3] [1,] 12.00 27 108 [2,] 18.75 75 192

The function gets conveniently applied to each element in the matrix without calling it in a loop. The apply() function in R doesn’t provide any speed benefit in execution but helps you write a cleaner and more compact code.

## sapply() and lapply() functions in R Programming

### Working with Lists

The `lapply()`

function in R is short for list apply. This works in a manner similar to the `apply()`

function above, but uses lists instead of matrices.

Let us look at an example:

> mylist <- list(c(1,2,3,4),c(10,20,30,40),c(5,5,5,5)) > lapply(mylist,mean) [[1]] [1] 2.5 [[2]] [1] 25 [[3]] [1] 5

The list **mylist** is a list of 3 vectors. We wish to apply a mean function to each one of the vectors. This is done by calling `lapply(mylist,mean)`

that returns the mean values of the three constituent vectors.

Similarly, sapply() function in R is short for for simplified apply. Instead of obtaining a separate mean value for each vector, `sapply`

returns a vector containing the mean values.

> sapply(mylist,mean) [1] 2.5 25.0 5.0

### Working with Data Frames in R

Since data frames can be treated as a special case of lists, the functions `lapply()`

and `sapply()`

work in both cases. Let us look at an example.

Let us create a data frame first and then apply a sort() function on it using the lapply() function in R.

names <- c("Adam","Antony","Brian","Carl","Doug") ages <- c(23,22,24,25,26) playerdata <- data.frame(names,ages,stringsAsFactors = FALSE) #Apply a sort function on the dataframe > lapply(playerdata,sort) $names [1] "Adam" "Antony" "Brian" "Carl" "Doug" $ages [1] 22 23 24 25 26

The function returns both the columns of the data frame in a sorted order separately.

Similarly, calling sapply() provides a compact list with each column sorted separately.

> sapply(playerdata,sort) names ages [1,] "Adam" "22" [2,] "Antony" "23" [3,] "Brian" "24" [4,] "Carl" "25" [5,] "Doug" "26"

## tapply() function

The tapply() function also belongs to the same family but **used only in case of factors**. This is best explained with an example. Suppose we have the salaries of employees in a company in the form of a vector and their respective means of transport in a factor. Suppose that we wish to calculate what is the average salary of each group using a specific means of transport.

The tapply() function in R programming can be called for this purpose using the R’s built-in mean function.

> salaries <-c(25000,30000,45000,66000,20000,50000,35000,20000,15000) > transport <-c('Bus','Car','Bus','Car','Metro','Metro','Bus','Bus','Metro') > tapply(salaries,transport,mean) Bus Car Metro 31250.00 48000.00 28333.33

As you can observe, the tapply() function in R outputs a well-formatted mean of the salaries with the means of transport as columns. Also, notice how it only accounts for unique values from the transport vector automatically.