Factors are one way to represent categorical variables in R. A factor is stored internally as a **vector of integers**. The unique elements of the supplied character vector are known as the *levels* of the factor. By default, if the levels are not supplied by the user, then R will generate the set of unique values in the vector, sort these values alphanumerically, and use them as the levels.

```
charvar <- rep(c("n", "c"), each = 3)
f <- factor(charvar)
f
levels(f)
> f
[1] n n n c c c
Levels: c n
> levels(f)
[1] "c" "n"
```

If you want to change the ordering of the levels, then one option to to specify the levels manually:

```
levels(factor(charvar, levels = c("n","c")))
> levels(factor(charvar, levels = c("n","c")))
[1] "n" "c"
```

Factors have a number of properties. For example, levels can be given labels:

```
> f <- factor(charvar, levels=c("n", "c"), labels=c("Newt", "Capybara"))
> f
[1] Newt Newt Newt Capybara Capybara Capybara
Levels: Newt Capybara
```

Another property that can be assigned is whether the factor is ordered:

```
> Weekdays <- factor(c("Monday", "Wednesday", "Thursday", "Tuesday", "Friday", "Sunday", "Saturday"))
> Weekdays
[1] Monday Wednesday Thursday Tuesday Friday Sunday Saturday
Levels: Friday Monday Saturday Sunday Thursday Tuesday Wednesday
> Weekdays <- factor(Weekdays, levels=c("Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"), ordered=TRUE)
> Weekdays
[1] Monday Wednesday Thursday Tuesday Friday Sunday Saturday
Levels: Monday < Tuesday < Wednesday < Thursday < Friday < Saturday < Sunday
```

When a level of the factor is no longer used, you can drop it using the `droplevels()`

function:

```
> Weekend <- subset(Weekdays, Weekdays == "Saturday" | Weekdays == "Sunday")
> Weekend
[1] Sunday Saturday
Levels: Monday < Tuesday < Wednesday < Thursday < Friday < Saturday < Sunday
> Weekend <- droplevels(Weekend)
> Weekend
[1] Sunday Saturday
Levels: Saturday < Sunday
```

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