R Basic

R head() and tail() Functions

For data exploration, quickly inspecting the data, verifying the structure, or debugging large datasets, we need some kind of tool that helps prevent excessive data flooding in the console. That’s where head() and tail() generic functions come into play.

head() function

The head() function displays the first n parts of the vector, matrix, table, or data frame. If you don’t pass the count “n” or rows to be displayed, it will display six rows/elements by default.

Syntax

head(data, n = number, ...)

Parameters

Name	Value
data	An input object (data frame, vector, matrix, or list)
n	Number of rows/elements to be returned. Positive n returns the first n elements. Negative n returns all elements except the last abs(n) elements. It helps us exclude rows/elements from the end.
…	They are additional arguments. For example, if you use matrices, you can use nrow or ncol.

Return value

For vectors, head() returns a subset of the first n elements.
For data frames/matrices, it returns a subset of the first n rows.
For lists, it returns the first n elements of the list.

If the “n” argument exceeds the input object’s length, it will return the whole object.

Basic usage with a data frame

Let’s define a data frame and use the head() function to quickly inspect the data frame.

# Creating a data frame.
df <- data.frame(
  col1 = c(1, 2, 3, 4, 5, 6, 7),
  col2 = c(8, 9, 10, 11, 12, 13, 14),
  col3 = c(15, 16, 17, 18, 19, 20, 21)
)

head(df)

Selecting the first n rows

# Create a data frame.
df <- data.frame(
  col1 = c(1, 2, 3, 4, 5, 6, 7),
  col2 = c(8, 9, 10, 11, 12, 13, 14),
  col3 = c(15, 16, 17, 18, 19, 20, 21)
)

# Selecting first 3 rows
head(df, 3)

Selecting the first n values in the specific column

# Create a data frame.
df <- data.frame(
  col1 = c(1, 2, 3, 4, 5, 6, 7),
  col2 = c(8, 9, 10, 11, 12, 13, 14),
  col3 = c(15, 16, 17, 18, 19, 20, 21)
)

# Selecting first 4 rows of column 3
head(df$col2, 4)

Negative “n” in Data Frames

If you pass the negative “n”, it will start excluding the rows from the end of the data frame. For example, if you want to exclude the last five rows from a data frame, you should pass n = -5.

# Create a data frame.
df <- data.frame(
  col1 = c(1, 2, 3, 4, 5, 6, 7),
  col2 = c(8, 9, 10, 11, 12, 13, 14),
  col3 = c(15, 16, 17, 18, 19, 20, 21)
)

# excluding last 2 rows
head(df, -5)

It will return the following output:

   col1  col2   col3
1   1     8      15
2   2     9      16

The above output shows that it removed the last five rows and displayed the first two rows.

Usage with the dataset

df <- datasets::USArrests

head(df, 5)

Output

Usage with vector

rv <- 1:5

cat("First three values of vector", "\n")

head(rv, 3)

Usage with a matrix

Let’s create a matrix of 7×3 and fetch the first four rows of the Matrix.

rv <- 1:21

mtrx <- matrix(rv, nrow = 7, ncol = 3)

cat("Using head() function to get first 4 rows", "\n")

head(mtrx, 4)

Usage with a list

Let’s define a list and quickly view the first 2 elements of the list:

# Creating a list
demo_list <- list(a = 1:3, b = 4:6, c = 7:9, d = 10:12)

demo_list
# $a
# [1] 1 2 3

# $b
# [1] 4 5 6

# $c
# [1] 7 8 9

# $d
# [1] 10 11 12


# Get first 2 elements
head(demo_list, n = 2)
# $a
# [1] 1 2 3

# $b
# [1] 4 5 6

tail() function

The tail() function views the last n part of a vector, matrix, table, or data frame.

It complements the head() function that returns the first n rows of a data frame.

Syntax

tail(data, n, ...)

Parameters

Name	Value
data	An input object (data frame, vector, matrix, or list).
n	Number of rows/elements to be returned. Positive n returns the last n elements. Negative n returns all elements except the first abs(n) elements. It helps us exclude rows/elements from the start.
…	They are additional arguments. For example, if you use matrices, you can use nrow or ncol.

Return value

For vectors, head() returns a subset of the last n elements.
For data frames/matrices, it returns a subset of the last n rows.
For lists, it returns the last n elements of the list.

If the “n” argument exceeds the input object’s length, it will return the whole object.

Viewing the last rows of the data frame

By default, it will return the last six rows of the data frame if you have not passed a number of rows/elements.

# Create a data frame.
df <- data.frame(
  col1 = c(1, 2, 3, 4, 5, 6, 7),
  col2 = c(8, 9, 10, 11, 12, 13, 14),
  col3 = c(15, 16, 17, 18, 19, 20, 21)
)

# Applying tail() function to df
tail(df)

Selecting the last n rows from a data frame

To select the last three rows of a data frame, pass three as the second argument.

# Create a data frame.
df <- data.frame(
  col1 = c(1, 2, 3, 4, 5, 6, 7),
  col2 = c(8, 9, 10, 11, 12, 13, 14),
  col3 = c(15, 16, 17, 18, 19, 20, 21)
)

# Selecting last 3 rows using tail() function
tail(df, 3)

Get the last n values in the specific column

Let’s implement the above diagram in R code.

# Create a data frame.
df <- data.frame(
  col1 = c(1, 2, 3, 4, 5, 6, 7),
  col2 = c(8, 9, 10, 11, 12, 13, 14),
  col3 = c(15, 16, 17, 18, 19, 20, 21)
)

# Selecting last 4 rows of column 3
tail(df$col3, 4)

Excluding rows

If you pass the negative n, it will start excluding rows from the start. For example, tail(df, -5) will exclude the first five rows from the data frame.

# Create a data frame.
df <- data.frame(
  col1 = c(1, 2, 3, 4, 5, 6, 7),
  col2 = c(8, 9, 10, 11, 12, 13, 14),
  col3 = c(15, 16, 17, 18, 19, 20, 21)
)

print(df)

# excluding first 5 rows
tail(df, n = -5)

Output

   col1  col2   col3
6   6     13     20
7   7     14     21

As you can see, it removes the first five rows and displays the last two rows.

Usage with built-in dataset

Let’s import the ChickWeight dataset.

df <- datasets::ChickWeight

Use the tail() function to get the last five rows of the dataset.

df <- datasets::ChickWeight
tail(df, 5)

Output

     weight Time Chick Diet
574   175   14    50    4
575   205   16    50    4
576   234   18    50    4
577   264   20    50    4
578   264   21    50    4

Usage with vector

rv <- 1:10
rv
cat("Last five integers of vector", "\n")
tail(rv, 5)

Output

[1] 1 2 3 4 5 6 7 8 9 10
Last five integers of vector
[1] 6 7 8 9 10

Usage with a matrix

rv <- 1:21

mtrx <- matrix(rv, nrow = 7, ncol = 3)

cat("Using tail() function to get last 3 rows", "\n")

tail(mtrx, 3)

Usage with a list

# Creating a list
demo_list <- list(a = 1:3, b = 4:6, c = 7:9, d = 10:12)

demo_list
# $a
# [1] 1 2 3

# $b
# [1] 4 5 6

# $c
# [1] 7 8 9

# $d
# [1] 10 11 12


# Get last 2 elements
tail(demo_list, n = 2)
# $c
# [1] 7 8 9

# $d
# [1] 10 11 12

That’s it!

Krunal Lathiya

Krunal Lathiya is a seasoned Computer Science expert with over eight years in the tech industry. He boasts deep knowledge in Data Science and Machine Learning. Versed in Python, JavaScript, PHP, R, and Golang. Skilled in frameworks like Angular and React and platforms such as Node.js. His expertise spans both front-end and back-end development. His proficiency in the Python language stands as a testament to his versatility and commitment to the craft.