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R Factors


Factors

Factors are used to categorize data. Examples of factors are:

  • Demography: Male/Female
  • Music: Rock, Pop, Classic, Jazz
  • Training: Strength, Stamina

To create a factor, use the factor() function and add a vector as argument:

Example

# Create a factor
music_genre <- factor(c("Jazz", "Rock", "Classic", "Classic", "Pop", "Jazz", "Rock", "Jazz"))

# Print the factor
music_genre

Result:

[1] Jazz    Rock    Classic Classic Pop     Jazz    Rock    Jazz
Levels: Classic Jazz Pop Rock
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You can see from the example above that that the factor has four levels (categories): Classic, Jazz, Pop and Rock.

To only print the levels, use the levels() function:

Example

music_genre <- factor(c("Jazz", "Rock", "Classic", "Classic", "Pop", "Jazz", "Rock", "Jazz"))

levels(music_genre)

Result:

[1] "Classic" "Jazz"    "Pop"     "Rock"   
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You can also set the levels, by adding the levels argument inside the factor() function:

Example

music_genre <- factor(c("Jazz", "Rock", "Classic", "Classic", "Pop", "Jazz", "Rock", "Jazz"), levels = c("Classic", "Jazz", "Pop", "Rock", "Other"))

levels(music_genre)

Result:

[1] "Classic" "Jazz"    "Pop"     "Rock"    "Other"
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Factor Length

Use the length() function to find out how many items there are in the factor:

Example

music_genre <- factor(c("Jazz", "Rock", "Classic", "Classic", "Pop", "Jazz", "Rock", "Jazz"))

length(music_genre)

Result:

[1] 8
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Access Factors

To access the items in a factor, refer to the index number, using [] brackets:

Example

Access the third item:

music_genre <- factor(c("Jazz", "Rock", "Classic", "Classic", "Pop", "Jazz", "Rock", "Jazz"))

music_genre[3]

Result:

[1] Classic
Levels: Classic Jazz Pop Rock
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Change Item Value

To change the value of a specific item, refer to the index number:

Example

Change the value of the third item:

music_genre <- factor(c("Jazz", "Rock", "Classic", "Classic", "Pop", "Jazz", "Rock", "Jazz"))

music_genre[3] <- "Pop"

music_genre[3]

Result:

[1] Pop
Levels: Classic Jazz Pop Rock
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Note that you cannot change the value of a specific item if it is not already specified in the factor. The following example will produce an error:

Example

Trying to change the value of the third item ("Classic") to an item that does not exist/not predefined ("Opera"):

music_genre <- factor(c("Jazz", "Rock", "Classic", "Classic", "Pop", "Jazz", "Rock", "Jazz"))

music_genre[3] <- "Opera"

music_genre[3]

Result:

Warning message:
In `[<-.factor`(`*tmp*`, 3, value = "Opera") :
  invalid factor level, NA generated
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However, if you have already specified it inside the levels argument, it will work:

Example

Change the value of the third item:

music_genre <- factor(c("Jazz", "Rock", "Classic", "Classic", "Pop", "Jazz", "Rock", "Jazz"), levels = c("Classic", "Jazz", "Pop", "Rock", "Opera"))

music_genre[3] <- "Opera"

music_genre[3]

Result:

[1] Opera
Levels: Classic Jazz Pop Rock Opera
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