R – Lists

A list in R is a generic object consisting of an ordered collection of objects. Lists are one-dimensional, heterogeneous data structures. The list can be a list of vectors, a list of matrices, a list of characters and a list of functions, and so on. 

A list is a vector but with heterogeneous data elements. A list in R is created with the use of list() function. R allows accessing elements of an R list with the use of the index value. In R, the indexing of a list starts with 1 instead of 0 like in other programming languages.  

Creating a List

To create a List in R you need to use the function called “list()”. In other words, a list is a generic vector containing other objects. To illustrate how a list looks, we take an example here. We want to build a list of employees with the details. So for this, we want attributes such as ID, employee name, and the number of employees. 

Example:

R

# R program to create a List

# The first attributes is a numeric vector
# containing the employee IDs which is created
# using the command here
empId = c(1, 2, 3, 4)

# The second attribute is the employee name
# which is created using this line of code here
# which is the character vector
empName = c("Debi", "Sandeep", "Subham", "Shiba")

# The third attribute is the number of employees
# which is a single numeric variable.
numberOfEmp = 4

# We can combine all these three different
# data types into a list
# containing the details of employees
# which can be done using a list command
empList = list(empId, empName, numberOfEmp)

print(empList)

Output:

[[1]]
[1] 1 2 3 4

[[2]]
[1] "Debi"    "Sandeep" "Subham"  "Shiba"  

[[3]]
[1] 4

Accessing components of a list

We can access components of an R list in two ways. 

• Access components by names: All the components of a list can be named and we can use those names to access the components of the R list using the dollar command.

Example:

R

# R program to access
# components of a list

# Creating a list by naming all its components
empId = c(1, 2, 3, 4)
empName = c("Debi", "Sandeep", "Subham", "Shiba")
numberOfEmp = 4
empList = list(
  "ID" = empId,
  "Names" = empName,
  "Total Staff" = numberOfEmp
  )
print(empList)

# Accessing components by names
cat("Accessing name components using $ command\n")
print(empList$Names)

Output:

$ID
[1] 1 2 3 4

$Names
[1] "Debi"    "Sandeep" "Subham"  "Shiba"  

$`Total Staff`
[1] 4

Accessing name components using $ command
[1] "Debi"    "Sandeep" "Subham"  "Shiba"  

• Access components by indices: We can also access the components of the R list using indices. To access the top-level components of a R list we have to use a double slicing operator “[[ ]]” which is two square brackets and if we want to access the lower or inner-level components of a R list we have to use another square bracket “[ ]” along with the double slicing operator “[[ ]]“.

Example:

R

# R program to access
# components of a list

# Creating a list by naming all its components
empId = c(1, 2, 3, 4)
empName = c("Debi", "Sandeep", "Subham", "Shiba")
numberOfEmp = 4
empList = list(
  "ID" = empId,
  "Names" = empName,
  "Total Staff" = numberOfEmp
  )
print(empList)

# Accessing a top level components by indices
cat("Accessing name components using indices\n")
print(empList[[2]])

# Accessing a inner level components by indices
cat("Accessing Sandeep from name using indices\n")
print(empList[[2]][2])

# Accessing another inner level components by indices
cat("Accessing 4 from ID using indices\n")
print(empList[[1]][4])

Output:

$ID
[1] 1 2 3 4

$Names
[1] "Debi"    "Sandeep" "Subham"  "Shiba"  

$`Total Staff`
[1] 4

Accessing name components using indices
[1] "Debi"    "Sandeep" "Subham"  "Shiba"  
Accessing Sandeep from name using indices
[1] "Sandeep"
Accessing 4 from ID using indices
[1] 4

Modifying components of a list

A R list can also be modified by accessing the components and replacing them with the ones which you want.

Example:

R

# R program to edit
# components of a list

# Creating a list by naming all its components
empId = c(1, 2, 3, 4)
empName = c("Debi", "Sandeep", "Subham", "Shiba")
numberOfEmp = 4
empList = list(
  "ID" = empId,
  "Names" = empName,
  "Total Staff" = numberOfEmp
)
cat("Before modifying the list\n")
print(empList)

# Modifying the top-level component
empList$`Total Staff` = 5

# Modifying inner level component
empList[[1]][5] = 5
empList[[2]][5] = "Kamala"

cat("After modified the list\n")
print(empList)

Output:

Before modifying the list
$ID
[1] 1 2 3 4

$Names
[1] "Debi"    "Sandeep" "Subham"  "Shiba"  

$`Total Staff`
[1] 4

After modified the list
$ID
[1] 1 2 3 4 5

$Names
[1] "Debi"    "Sandeep" "Subham"  "Shiba"   "Kamala" 

$`Total Staff`
[1] 5

Concatenation of lists

Two R lists can be concatenated using the concatenation function. So, when we want to concatenate two lists we have to use the concatenation operator.

Syntax:

list = c(list, list1)
list = the original list 
list1 = the new list 

Example:

R

# R program to edit
# components of a list

# Creating a list by naming all its components
empId = c(1, 2, 3, 4)
empName = c("Debi", "Sandeep", "Subham", "Shiba")
numberOfEmp = 4
empList = list(
  "ID" = empId,
  "Names" = empName,
  "Total Staff" = numberOfEmp
)
cat("Before concatenation of the new list\n")
print(empList)

# Creating another list
empAge = c(34, 23, 18, 45)

# Concatenation of list using concatenation operator
empList = c(empName, empAge)

cat("After concatenation of the new list\n")
print(empList)

Output:

Before concatenation of the new list
$ID
[1] 1 2 3 4

$Names
[1] "Debi"    "Sandeep" "Subham"  "Shiba"  

$`Total Staff`
[1] 4

After concatenation of the new list
[1] "Debi"    "Sandeep" "Subham"  "Shiba"   "34"      "23"      "18"      "45"  

Deleting components of a list

To delete components of a R list, first of all, we need to access those components and then insert a negative sign before those components. It indicates that we had to delete that component. 

Example:

R

# R program to access
# components of a list

# Creating a list by naming all its components
empId = c(1, 2, 3, 4)
empName = c("Debi", "Sandeep", "Subham", "Shiba")
numberOfEmp = 4
empList = list(
  "ID" = empId,
  "Names" = empName,
  "Total Staff" = numberOfEmp
)
cat("Before deletion the list is\n")
print(empList)

# Deleting a top level components
cat("After Deleting Total staff components\n")
print(empList[-3])

# Deleting a inner level components
cat("After Deleting sandeep from name\n")
print(empList[[2]][-2])

Output:

Before deletion the list is
$ID
[1] 1 2 3 4

$Names
[1] "Debi"    "Sandeep" "Subham"  "Shiba"  

$`Total Staff`
[1] 4

After Deleting Total staff components
$ID
[1] 1 2 3 4

$Names
[1] "Debi"    "Sandeep" "Subham"  "Shiba"  

After Deleting sandeep from name
[1] "Debi"   "Subham" "Shiba" 

Merging list

We can merge the R list by placing all the lists into a single list.

R

# Create two lists.
lst1 <- list(1,2,3)
lst2 <- list("Sun","Mon","Tue")

# Merge the two lists.
new_list <- c(lst1,lst2)

# Print the merged list.
print(new_list)

Output:

[[1]]
[1] 1

[[2]]
[1] 2

[[3]]
[1] 3

[[4]]
[1] "Sun"

[[5]]
[1] "Mon"

[[6]]
[1] "Tue"

Converting List to Vector

Here we are going to convert the R list to vector, for this we will create a list first and then unlist the list into the vector.

R

# Create lists.
lst <- list(1:5)
print(lst)

# Convert the lists to vectors.
vec <- unlist(lst)

print(vec)

Output:

[[1]]
[1] 1 2 3 4 5

[1] 1 2 3 4 5

R List to matrix

We will create matrices using matrix() function in R programming. Another function that will be used is unlist() function to convert the lists into a vector.

R

# Defining list
lst1 <- list(list(1, 2, 3),
            list(4, 5, 6))

# Print list
cat("The list is:\n")
print(lst1)
cat("Class:", class(lst1), "\n")

# Convert list to matrix
mat <- matrix(unlist(lst1), nrow = 2, byrow = TRUE)

# Print matrix
cat("\nAfter conversion to matrix:\n")
print(mat)
cat("Class:", class(mat), "\n")

Output:

The list is:
[[1]]
[[1]][[1]]
[1] 1

[[1]][[2]]
[1] 2

[[1]][[3]]
[1] 3


[[2]]
[[2]][[1]]
[1] 4

[[2]][[2]]
[1] 5

[[2]][[3]]
[1] 6


Class: list 

After conversion to matrix:
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    5    6
Class: matrix