Explain apply() Function in R with Examples

The apply() function in R is used to apply a function to the rows or columns of a data frame or matrix. This function simplifies complex operations on matrices and data frames, making it easier to perform repetitive tasks without explicit loops.

R apply() Function

The apply() function in R is versatile and can be used with various functions to process rows or columns of a matrix, or data frame. The MARGIN parameter specifies whether to apply the function to rows (MARGIN = 1) or columns (MARGIN = 2), and the FUN parameter specifies the function to apply.

Syntax of apply() Function

Following is the syntax of the apply() function.


# Synatx of apply() function
apply(X, MARGIN, FUN, ...)

Parameters

x: X is a matrix or data frame, representing the data on which the function will be performed.
MARGIN: An integer vector indicating which margins to apply the function to. 1 indicates rows, 2 indicates columns, and c(1, 2) indicates rows and columns.
FUN: Is the function you want to use

Return Value

The return value is a matrix, vector, or array obtained by applying the specified function to the margins of X.

Use apply() Function to Matrix Rows in R

Let’s create the matrix and use the apply() function on each row to get the result in a vector. To do this, pass the given matrix into the apply() function along with the MARGIN parameter set to 1 (indicating rows), and specify the desired function using the FUN parameter. This will apply the specified function to each row and return the result as a vector.


# Use apply() function to matrix rows
# Create a matrix
mat <- matrix(1:12, nrow = 3)
print("Given matrix:")
print(mat)
print("Get the type of given matrix:")
print(class(mat))
result <- apply(mat, MARGIN = 1, FUN = mean)
print("After applying apply function to matrix:")
print(result)

The apply() function has returned a numeric vector containing the means of each row in a given matrix.

Yields below output.

Use apply() to Columns of Matrix in R

To apply this function to the columns of the matrix, you can set the MARGIN parameter to 2 along with the FUN parameter. This will apply the specified function to each column of the given matrix, resulting in a vector. Let’s set the MARGIN parameter to 2 and FUN to mean, and pass them into the apply() function to get the means of each column of the matrix.


# Use apply() function to matrix columns
# Create a matrix
mat <- matrix(1:12, nrow = 3)
print("Given matrix:")
print(mat)
print("Get the type of given matrix:")
print(class(mat))
result <- apply(mat, MARGIN = 2, FUN = mean)
print("After applying apply function to matrix:")
print(result)
print("Get the type of result:")
print(class(result))

Yields below output.

Apply Function to Whole Matrix

Similarly, you can use the apply() function to process an entire matrix by specifying the MARGIN parameter with 1:2 (where 1 is for rows and 2 is for columns). Let’s apply a custom function to the entire matrix, processing each element, and return the result as a matrix.


# Apply function to whole matrix
# Create a matrix
mat <- matrix(1:12, nrow = 3)
result <- apply(mat, 1:2, function(x) x - 5)
print("After applying apply function to matrix:")
print(result)

# Output:
# [1] "After applying apply function to matrix:"
#      [,1] [,2] [,3] [,4]
# [1,]   -4   -1    2    5
# [2,]   -3    0    3    6
# [3,]   -2    1    4    7

The apply() function has returned a matrix where 5 has been subtracted from each element.

Apply Custom Function to Matrix Rows

Alternatively, you can use the apply() function to apply a custom function to the rows of a matrix. First, define the custom function, and then pass it to the apply() function along with the MARGIN and FUN parameters. This function will apply the custom function to each element in a row-wise manner and return the result as a matrix.


# Applying a Custom Function to Matrix Rows
# Define a custom function
cus_fun <- function(x) {
  return(x * 2)
}
# Use apply() function to matrix rows
# Create a matrix
mat <- matrix(1:12, nrow = 3)
result <- apply(mat, MARGIN = 1, FUN = cus_fun)
print("After applying custom function to matrix:")
print(result)

# Output:
# [1] "After applying custom function to matrix:"
#     [,1] [,2] [,3]
# [1,]    2    4    6
# [2,]    8   10   12
# [3,]   14   16   18
# [4,]   20   22   24

The apply() function has returned a matrix where each element is doubled.

Apply Function to Data Frame

You can also manipulate data frame rows, columns, or the entire data frame using the apply() function. Let’s create a data frame and use this function to operate over the rows of the data frame, obtaining the result in the form of a vector.


# Apply function to data frame rows
# Create a data frame
df <- data.frame(
  A = c(1, 2, 3),
  B = c(4, 5, 6),
  C = c(7, 8, 9)
)

print("Given data frame:")
print(df)
result <- apply(df, MARGIN = 1, FUN = mean)
print("After applying apply function to data frame:")
print(result)
print("Get the type of result:")
print(class(result))

# Output:
# [1] "Given data frame:"
#   A B C
# 1 1 4 7
# 2 2 5 8
# 3 3 6 9

# [1] "After applying apply function to data frame:"
# [1] 4 5 6
# [1] "Get the type of result:"
# [1] "numeric"

This function has returned a numeric vector where the elements are the means of each row of the data frame.

Conclusion

The apply() function is a versatile and powerful tool in R, essential for performing operations on matrices or data frames. By allowing users to specify the margins and functions to apply, it provides a flexible way to process and manipulate data efficiently. Its ability to handle custom functions and its memory efficiency make it a crucial function for data analysis in R.

Happy learning !!