Creating and indexing a list in R

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list is a type of data structure in R programming language. Unlike other data structures, especially matrix and vector, in which each element must has the same type of data, a list provides the flexibility of storing various information in one object. For example, a list may contain vectors, matrices, data frames, and even other lists and that is why many complex R statistical analysis packages often return lists for their function operations.

1. Creating a list

list() is used to create a list in R. Following example shows creating a list, which stores information of a scalar, a vector, a matrix, and a data frame.

#create a scalar, a vector, a matrix, and a data frame
> a <- "New List"
> b <- c(21, 13, 42)
> c <- matrix(1:100, nrow = 10)
> d <- c("wang", "zhang", "li")
 
> ID <- c(3, 4, 5, 6) 
> age <- c(15, 14, 18, 12)
> blood <- c("Type5", "Type6", "Type5", "Type6")
> status <- c("Poor", "Improved", "Excellent", "Poor")
> pdata <- data.frame(ID, age, blood, status)

#creating a list to contain the information of scalar, 
#vector, matrix, data frame created above
#we optinally set names 'title', 'sum', 'e' for 
#several objects in this list
> newlist <- list(title = a, sum =b, c, d, e=pdata)

#show this list
> newlist
$title
[1] "New List"

$sum
[1] 21 13 42

[[3]]
      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
 [1,]    1   11   21   31   41   51   61   71   81    91
 [2,]    2   12   22   32   42   52   62   72   82    92
 [3,]    3   13   23   33   43   53   63   73   83    93
 [4,]    4   14   24   34   44   54   64   74   84    94
 [5,]    5   15   25   35   45   55   65   75   85    95
 [6,]    6   16   26   36   46   56   66   76   86    96
 [7,]    7   17   27   37   47   57   67   77   87    97
 [8,]    8   18   28   38   48   58   68   78   88    98
 [9,]    9   19   29   39   49   59   69   79   89    99
[10,]   10   20   30   40   50   60   70   80   90   100

[[4]]
[1] "wang"  "zhang" "li"   

$e
  ID age blood    status
1  3  15 Type5      Poor
2  4  14 Type6  Improved
3  5  18 Type5 Excellent
4  6  12 Type6      Poor

> 

2. Indexing a list

The elements or objects of a list can be indexed by using double brackets with index number, double brackets with object names, or dollar symbol with object names. Following example code shows each of these usage.

#index an object of a list , using double brackets with index
#number
> newlist[[2]]
[1] 21 13 42

> newlist[[3]]
      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
 [1,]    1   11   21   31   41   51   61   71   81    91
 [2,]    2   12   22   32   42   52   62   72   82    92
 [3,]    3   13   23   33   43   53   63   73   83    93
 [4,]    4   14   24   34   44   54   64   74   84    94
 [5,]    5   15   25   35   45   55   65   75   85    95
 [6,]    6   16   26   36   46   56   66   76   86    96
 [7,]    7   17   27   37   47   57   67   77   87    97
 [8,]    8   18   28   38   48   58   68   78   88    98
 [9,]    9   19   29   39   49   59   69   79   89    99
[10,]   10   20   30   40   50   60   70   80   90   100
> 

#index an object of a list , using double brackets with 
#object name
> newlist[["sum"]]
[1] 21 13 42

> newlist[['e']]
  ID age blood    status
1  3  15 Type5      Poor
2  4  14 Type6  Improved
3  5  18 Type5 Excellent
4  6  12 Type6      Poor

#index an object of a list , using dollar symbol with 
#object name

> newlist$e 
  ID age blood    status
1  3  15 Type5      Poor
2  4  14 Type6  Improved
3  5  18 Type5 Excellent
4  6  12 Type6      Poor

> newlist$sum
[1] 21 13 42

> 

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