基本创造因素

因子是在 R 中表示分类变量的一种方式。因子在内部存储为整数向量。提供的字符向量的唯一元素称为因子的级别。默认情况下，如果用户未提供级别，则 R 将在向量中生成一组唯一值，按字母数字顺序对这些值进行排序，并将它们用作级别。

 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"

如果要更改级别的顺序，则可以选择手动指定级别：

levels(factor(charvar, levels = c("n","c")))

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

因素有很多属性。例如，级别可以给标签：

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

可以分配的另一个属性是该因子是否有序：

> 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

当不再使用因子级别时，可以使用 droplevels() 函数将其删除：

> 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