Summary  
This chapter explains how to retrieve the distinct levels of a factor, create ordered factors, and define a custom ordering of factor levels.  

General domain of usage  
Statistical data analysis

**Levels** in a factor represent the set of distinct categories that the factor can take.

Definition

## Viewing Levels
The `levels()` function shows all unique values stored in a factor.

curr_f <- factor(c('USD', 'EUR', 'AUD', 'NOK', 
                   'CHF', 'EUR', 'AUD', 'EUR'))

# Display all levels
levels(curr_f)

## Ordered Factors
In some cases, categories have a natural order (e.g., "short" < "medium" < "tall"). Factors can be declared as ordered by setting `ordered = TRUE`.

sizes <- c('short', 'tall', 'medium',
           'medium', 'short', 'tall')

# Ordered factor (alphabetical order)
factor(sizes, ordered = TRUE)

## Custom Ordering
By default, R orders levels in alphabetical order, which may not always match the intended hierarchy. You can define a specific order by passing a vector of levels in the desired sequence.

sizes <- c('short', 'tall', 'medium',
           'medium', 'short', 'tall')
order <- c('short', 'medium', 'tall')

# Ordered factor (correct order)
factor(sizes, ordered = TRUE, levels = order)

This ensures the order matches your intended meaning.

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Levels in Factors

Viewing Levels

Example

Ordered Factors

Example

Custom Ordering

Example

Solution

Awesome!

Levels in Factors

Viewing Levels

Example

Ordered Factors

Example

Custom Ordering

Example

Solution