Objects in R:

• There are several data types:
  • numeric, character, logic, factor
• They are stored in one of many data structures
  • vectors
  • matrices
• Each element can be accessed using indices
  • numeric vectors (positive or negative)
  • logical vectors
  • character vector

Like vectores, but mixing different kinds of elements

> people <- list(c(60, 72, 57, 90, 95, 72),
+                c(1.75, 1.80, 1.65, 1.90, 1.74, 1.91),
+                c("Ali", "Deniz", "Fatma", "Emre", "Volkan", "Onur"),
+                TRUE,
+                factor(c("M","F","F","M","M","M")))

Notice that elements can have different length

• Can be indexed same as vectors
• Returns a sub-list

> people[1:2]

[[1]]
[1] 60 72 57 90 95 72

[[2]]
[1] 1.75 1.80 1.65 1.90 1.74 1.91

This is a sublist (with one element):

> people[1]

[[1]]
[1] 60 72 57 90 95 72

This is an element:

> people[[1]]

[1] 60 72 57 90 95 72

> people <- list(weight=c(60, 72, 57, 90, 95, 72),
+                height=c(1.75, 1.80, 1.65, 1.90, 1.74, 1.91),
+                names=c("Ali", "Deniz", "Fatma", "Emre", "Volkan", "Onur"),
+                valid=TRUE,
+                gender=factor(c("M","F","F","M","M","M")))

How else can we assign names?

• Can be indexed same as vectors
• Returns a sub-list

> people[1:2]

$weight
[1] 60 72 57 90 95 72

$height
[1] 1.75 1.80 1.65 1.90 1.74 1.91

This is a sublist:

> people[1]

$weight
[1] 60 72 57 90 95 72

This is an element:

> people[[1]]

[1] 60 72 57 90 95 72

> people[[1]]

[1] 60 72 57 90 95 72

> people[["weight"]]

[1] 60 72 57 90 95 72

> people$weight

[1] 60 72 57 90 95 72

Indices can also be used to change specifc parts of a list.

Try each of the following and explain the result:

> people$names <- toupper(people$names)
> people$BMI <- people$weight/people$height^2
> people$valid <- NULL

• List elements are indexed by [[]]
• sublists are indexed by []

Try these

> people[[2]]
> people[2]
> people[[2]][3]
> people[2][3]
> people[[1:3]]
> people[1:3]
> people[["weight"]]
> people$weight
> people["weight"]

> people[[2]]

[1] 1.75 1.80 1.65 1.90 1.74 1.91

> people[2]

$height
[1] 1.75 1.80 1.65 1.90 1.74 1.91

> people[[2]][3]

[1] 1.65

> people[2][3]

$<NA>
NULL

> people[[1:3]]

Error in people[[1:3]]: recursive indexing failed at level 2

> people[1:3]

$weight
[1] 60 72 57 90 95 72

$height
[1] 1.75 1.80 1.65 1.90 1.74 1.91

$names
[1] "ALI"    "DENIZ"  "FATMA"  "EMRE"   "VOLKAN" "ONUR"  

> people[["weight"]]

[1] 60 72 57 90 95 72

> people$weight

[1] 60 72 57 90 95 72

> people["weight"]

$weight
[1] 60 72 57 90 95 72

If key <- "names",

What is the diference between the following?

• people[[key]]
• people[[names]]
• people$key
• people$names

Explain

Write a list with one element for each person, representing the name, weight, height and gender.

• Bidimensional structure, like matrices
• Each column can be of a different type but same length
• All columns need a name

> ppl <- data.frame(weight=c(60, 72, 57, 90, 95, 72),
+                height=c(1.75, 1.80, 1.65, 1.90, 1.74, 1.91),
+                names=c("Ali", "Deniz", "Fatma", "Emre", "Volkan", "Onur"),
+                gender=factor(c("M","F","F","M","M","M")))

> ppl

  weight height  names gender
1     60   1.75    Ali      M
2     72   1.80  Deniz      F
3     57   1.65  Fatma      F
4     90   1.90   Emre      M
5     95   1.74 Volkan      M
6     72   1.91   Onur      M

If df is a data.frame, then df[[1]] is a vector

• All elements of a column have the same data type
• Different columns may have different types
  • In a matrix columns have all the same type
• All columns have the same size
  • In a list the elements can have any size