The Lists in R Programming
A list is an ordered collection of objects (components). Lists are referred to as the recursive vectors. In a list we can store different modes or types of data.
We could create a list to represent our employee, Joe, this way:
j<- list(name="Joe", salary=55000, unionms=T)
R doesn’t provide multi line or block comments. You must start each line of a multi line comment with #. For debugging purposes, you can also surround code that you want the interpreter to ignore with the statement if(FALSE){...}.Changing the FALSE to TRUE allows the code to be executed.
■ Assigning a value to a nonexistent element of a vector, matrix, array, or list expands that structure to accommodate the new value. For example, consider the following:
> x <- c(8, 6, 4)
> x[7] <- 10
> x
[1] 8 6 4 NA NA NA 10
The vector x has expanded from three to seven elements through the assignment. x<- x[1:3] would shrink it back to three elements.
■ R doesn’t have scalar values. Scalars are represented as one-element vectors.
■ Indices in R start at 1, not at 0. In the vector earlier, x[1] is 8.
■ Variables can’t be declared. They come into existence on first assignment.
A list is an ordered collection of objects (components). Lists are referred to as the recursive vectors. In a list we can store different modes or types of data.
A list allows you to gather a variety of (possibly unrelated) objects under one name. A list may contain a combination of vectors,matrices, data frames, and even other lists.
Lists are important R structures for two reasons. First,they allow you to organize and recall disparate information in a simple way. Second,the results of many R functions return lists.
Creating a List :
We create a list using the list() function
mylist <- list(object1, object2, ...)
here the objects are any of the structures (vectors,matrices..). Optionally, you can name the objects in a list:
here the objects are any of the structures (vectors,matrices..). Optionally, you can name the objects in a list:
mylist <- list(name1=object1, name2=object2, ...)
Example 1:
let’s consider an employee database. For each employee, we wish to store the name, salary , and a Boolean indicating union membership. Since we have three different modes here—character, numeric,and logical. So that we can use the list here to store different modes of this data.j<- list(name="Joe", salary=55000, unionms=T)
#The result of the list displays all its components
You can also specify elements of the list by indicating a component number or a name within double brackets. In this example, mylist[[2]] and mylist[["ages"]] both refer to the same four-element numeric vector. For named components,mylist$ages would also work.
> mylist[[2]]
[1] 25 26 18 39
> mylist[["ages"]]
[[1] 25 26 18 39
> j
$name
[1] "Joe"
$salary
[1] 55000
$unionms
[1] TRUE
Also we can create the list without specifying the object names.
j<- list("Joe",55000,T)
>j
[[1]]
[1] "Joe"
[[2]]
[1] 55000
[[3]]
[1] TRUE
It could be ideal to use the names instead of numeric indices.
We can access the specific object of a list as follows..
> j$sal
[1] 55000
Since Lists are also a kind of vectors, they can be created via vector():
> z <- vector(mode="list")
> z[["abc"]] <- 3
> z
$abc
[1] 3
Example 2:
In this example, you create a list with four components: a string, a numeric vector, a matrix, and a character vector. You can combine any number of objects and save them as a list.
> g <- "My Sample List"
> h <- c(25, 26, 18, 39)
> j <- matrix(1:10, nrow=5)
> k <- c("one", "two", "three")
> mylist <- list(title=g, ages=h, j, k)
> mylist
$title
[1] "My Sample List"
$ages
[1] 25 26 18 39
[[3]]
[,1] [,2]
[1,] 1 6
[2,] 2 7
[3,] 3 8
[4,] 4 9
[5,] 5 10
[[4]]
[1] "one" "two" "three"
[1] 25 26 18 39
> mylist[["ages"]]
[[1] 25 26 18 39
Notes:
The period (.) has no special significance in object names. The dollar sign ($) has a some what analogous meaning to the period in other object-oriented languages and can be used to identify the parts of a data frame or list. For example, A$x refers to variable x in data frame A.■ Assigning a value to a nonexistent element of a vector, matrix, array, or list expands that structure to accommodate the new value. For example, consider the following:
> x <- c(8, 6, 4)
> x[7] <- 10
> x
[1] 8 6 4 NA NA NA 10
The vector x has expanded from three to seven elements through the assignment. x<- x[1:3] would shrink it back to three elements.
■ R doesn’t have scalar values. Scalars are represented as one-element vectors.
■ Indices in R start at 1, not at 0. In the vector earlier, x[1] is 8.
■ Variables can’t be declared. They come into existence on first assignment.
Thanks, TAMATAM
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