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R – Lists

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  • Last Updated : 27 Oct, 2021
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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 a list with the use of the index value. In R, the indexing of a list starts with 1 instead of 0 like 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 a 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 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 list using indices. To access the top-level components of a 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 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 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 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)
empAgeList = list(
  "Age" = empAge
)
 
# Concatenation of list using concatenation operator
empList = c(empList, empAgeList)
 
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
$ID
[1] 1 2 3 4

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

$`Total Staff`
[1] 4

$Age
[1] 34 23 18 45

Deleting components of a list

To delete components of a 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 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 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 

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