Data Structures

Data types are the fundamental buildings. Data structures organize these data types. ## Common data structures in R

• Vector
  A one-dimensional sequence of values, all of the same type

• Matrix
  A two-dimensional array of values, all of the same type

• List
  A collection of objects that can be of different types and structures

• Data frame
  A table-like structure where each column is a vector; different columns may have different types

• Factor
  A special object used to represent categorical data

Factor

• A type of data structure to indicate categories

• Stores both levels and the order of the levels

• It is more than a character vector (which does not store the order information)

• The factor levels affect the behavior of the downstream function.

Penguins dataset

library(ISLR2) 
str(Credit)

## 'data.frame':    400 obs. of  11 variables:
##  $ Income   : num  14.9 106 104.6 148.9 55.9 ...
##  $ Limit    : num  3606 6645 7075 9504 4897 ...
##  $ Rating   : num  283 483 514 681 357 569 259 512 266 491 ...
##  $ Cards    : num  2 3 4 3 2 4 2 2 5 3 ...
##  $ Age      : num  34 82 71 36 68 77 37 87 66 41 ...
##  $ Education: num  11 15 11 11 16 10 12 9 13 19 ...
##  $ Own      : Factor w/ 2 levels "No","Yes": 1 2 1 2 1 1 2 1 2 2 ...
##  $ Student  : Factor w/ 2 levels "No","Yes": 1 2 1 1 1 1 1 1 1 2 ...
##  $ Married  : Factor w/ 2 levels "No","Yes": 2 2 1 1 2 1 1 1 1 2 ...
##  $ Region   : Factor w/ 3 levels "East","South",..: 2 3 3 3 2 2 1 3 2 1 ...
##  $ Balance  : num  333 903 580 964 331 ...

Working with factors

• factor creates a new factor with specified levels

• table, str inspect a factor

• levels extracts and levels <- set the levels

• reorder(factor, numbers, FUN) reorders a factor according to a summary statistic FUN of numbers.

• Keep in mind character, numeric variables, and factors are treated different in R.

Factors in R functions

Many R functions behave nicely when applied on a factor.

The order of factor levels can affect statistical interpretations and visualizations.