Lists

In this lesson, we will learn all about lists and how to create them.

We'll cover the following

Previously, we learned about vectors, which are containers where each element has the same type. However, R language provides programmers with the ability to put elements of different types into a single container.

How cool is that! And this is exactly where lists come in the picture.

A list is basically a vector in which all the elements can be of a different type.

Creating Lists

Lists can be created using the list() function.

It works similar to the c() function.

In this function, list the contents you want to add as arguments to the list() function.

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myList <- list(1, 1+1i, "a", TRUE)
print(myList)

Lists have the capability of containing other lists within themselves. Therefore, lists are recursive. We can test this using the is.recursive() function which returns true if the object is recursive and false otherwise.

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myList <- list(1, 1+1i, "a", TRUE)
is.recursive(myList)

Inserting Elements in a List

We can insert an element in the list using the c() method that we used for inserting elements in a vector.

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myList <- list(1, 1+1i, "a", TRUE)
myList <- c("s", myList) 
cat("Appending 's' at the start of the vector: \n")
print(myList)

Accessing and Modifying Lists

We can fetch an element at a specific index by using the square brackets [] around the specified index.

Indexing starts at 11, which means that the first element of the list is at index 1.

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myList <- list(1, 1+1i, "a", TRUE)
print(myList[1])

In the above code, we access the first list element of a list.

Illustration of list object
Illustration of list object

In the above illustration, each element in a list can be another list, so to obtain a single element use double square brackets[[]] instead.

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myList <- list(1, 1+1i, "a", TRUE)
print(myList[[1]])

We can modify a single element also using the double square brackets.

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myList <- list(1, 1+1i, "a", TRUE)
myList[[1]] = 100
print(myList[[1]])

Named Entries in Lists

List entries can be named:

For example:

myList <- list( 
         integerVar = 1:3,
         numericVar = 0.5,
         characterVar = c('a', 'b') )

The entries in named lists can be accessed by their name instead of their indexes as well.

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myList <- list( 
  integerVar = 1:3,
  numericVar = 0.5,
  characterVar = c('a', 'b') )
print(myList['integerVar']) # prints the name as well as the value
print(myList[['integerVar']]) # prints only the value

As we have seen in this lesson, lists can be particularly useful because they can store objects of different lengths and various classes.