Factor levels in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a factor in R?

A factor in R is a data structure used to represent categorical data. It stores the data as integer codes with corresponding character labels called levels.

beginner

How do you create a factor with specific levels in R?

Use the factor() function with the levels argument. For example: factor(c("low", "high", "medium"), levels = c("low", "medium", "high")) creates a factor with levels ordered as low, medium, high.

intermediate

What happens if you assign a value not in the levels to a factor?

R will convert that value to NA (missing value) because it is not recognized as one of the factor's predefined levels.

beginner

How can you see the levels of a factor variable in R?

Use the levels() function. For example, levels(my_factor) returns a character vector of the factor's levels.

intermediate

Why is the order of factor levels important?

The order of factor levels affects how R orders and compares the categories. For example, in plots or summary statistics, the order controls the display and sorting.

What does the levels() function return when used on a factor?

A The data type of the factor

B The numeric codes of the factor

C The length of the factor

D A character vector of the factor's categories

The levels() function returns the character vector of all categories (levels) defined for the factor.

What happens if you create a factor with levels = c("low", "medium") but include "high" in the data?

A "high" becomes NA because it's not in the levels

B "high" is automatically added to levels

C An error is thrown

D "high" is ignored silently

Values not in the specified levels are converted to NA in factors.

How do you create an ordered factor in R?

A Use factor() with ordered = TRUE

B Use levels() function

C Use as.character()

D Use numeric()

Setting ordered = TRUE in factor() creates an ordered factor where levels have a meaningful order.

Why might you want to set the order of factor levels explicitly?

A To convert factors to numeric

B To speed up calculations

C To control the order of categories in plots and summaries

D To remove missing values

Explicit order controls how categories appear in outputs like plots and tables.

Which function converts a character vector to a factor in R?

A factor()

B Both A and B

C levels()

D as.factor()

Both factor() and as.factor() convert character vectors to factors.

Explain what factor levels are and why they matter in R.

Think about how R stores categories and how order affects results.

Describe how to create a factor with custom levels and what happens if data contains values outside those levels.

Consider the factor() function and the levels parameter.