CountBio

Lists

A list is an ordered collection of objects known as its components . Different objects like arrays, matrices, vectors and dataframes can be included in a list. Each object inside a list and its elements in turn can be accessed by appropriate symbol and indexing.

Creating lists

To create a list of objects, we should call the list() function and pass the object names to it. This function returns a list of objects.

We now create five vectors of different data types and put them together as a list using "list" command as shown below.

 >  expt_name <- c("Experiment-A","Experiment-B","Experiment-C", "Experiment-D", "Experiment-E")
 >  sample_length <- c(12.5, 32.6, 16.7, 20.6, 7.5)
 >  sample_weight <- c(1122, 1123, 1124, 1125, 1126)
 >  sample_category <- c('A','S','P','K','G')
 >  lab_name <- c("IBAB_LAB")

While creating a list with these objects, it is useful to give each one of them an explicit name:

 >  alis <- list(name=expt_name, length=sample_length, weight=sample_weight, 
                         category=sample_category,lab=lab_name)
 >  
 >  alis

$name
[1] "Experiment-A" "Experiment-B" "Experiment-C" "Experiment-D" "Experiment-E"

$length
[1] 12.5 32.6 16.7 20.6  7.5

$weight
[1] 1122 1123 1124 1125 1126

$category
[1] "A" "S" "P" "K" "G"

$lab
[1] "IBAB_LAB"

To view the structure of a list

In case the structure is very large, it is better to view the structure of a list using str() command. This command also displays the number of elements in each object.

 >   str(alis)

List of 5
 $ name    : chr [1:5] "Experiment-A" "Experiment-B" "Experiment-C" "Experiment-D" ...
 $ length  : num [1:5] 12.5 32.6 16.7 20.6 7.5
 $ weight  : num [1:5] 1122 1123 1124 1125 1126
 $ category: chr [1:5] "A" "S" "P" "K" ...
 $ lab     : chr "IBAB_LAB"

Accessing the elements of a list by name

The individual components in a list can be accessed by attaching the object name and list name by '$' symbol:

 >  names(alis)

 >  alis$name

 >  alis$category

 >  alis$name[2:4]

In another format, the elements of a component can be printed by giving its name in square bracket after list name:

 >  alis["name"]

$name
[1] "Experiment-A" "Experiment-B" "Experiment-C" "Experiment-D" "Experiment-E"

Accessing the components of a list by index

To access the components of a list by index, the index is placed inside double bracket. See below:

 >  alis[[1]]

 >  alis[[5]]

Once the object in a list is accessed by index in double bracket, the name of list with index in double bracket serves as a name of object. We can then use appropriate indexing to get its individual elements. For example, alis[[1]] is a vector. We can access its elements by indexing:

 >  alis[[1]][4]

 >  alis[[1]][2:5]

To get the number of components in a list

The number of components in a list can be obtained using the length() function, as we did with vectors:

 >  length(alis)

To append a component to a list

A new element can be appended to a list by name. We create a list with 2 elements called "name1" and "name2.

 >  blis = list(name1 =c(1,2,3,4,5), name2="aaaa")
 >  
 >  blis

$name1
[1] 1 2 3 4 5

$name2
[1] "aaaa"

We will now append a new component called "name3" to the list:

 >  blis$name3 = "aaaaaaa"
 >  
 >  blis

$name1
[1] 1 2 3 4 5

$name2
[1] "aaaa"

$name3
[1] "aaaaaaa"