Fix issues with strings using factors

• Fix those problems with a factor.
• Start by creating a list of the valid levels:

month_levels <- c(
  "Jan", "Feb", "Mar", "Apr", "May", "Jun", 
  "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
)

month_levels

##  [1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec"

• Now create a factor:

y1 <- factor(x1, levels = month_levels)

sort(y1)

## [1] Jan Mar Apr Dec
## Levels: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

• If you omit the levels, they’ll be taken from the data alphabetically:

factor(x1)

## [1] Dec Apr Jan Mar
## Levels: Apr Dec Jan Mar