Reorder factor levels by day of the week in R

Question

I have the following data.frame in R:

> daily
        DoW         Duration
1    Friday 14.0000000000000
2    Monday 21.0000000000000
3  Saturday 12.0000000000000
4  Thursday 28.0000000000000
5   Tuesday 12.0000000000000
6 Wednesday 91.0000000000000
7    Sunday 20.0000000000000

I'd like to change the order of the factor levels so that the weeks are in (US) day-of-week order.

It looks like I can do this in a slow, puzzling way with relevel(). But this only takes 1 numeric argument and moves it to the top. So, relevel(daily$DoW, 7), moves Sunday to the top, but the rest remain unordered (which means I need to relevel it in reverse order).

Doable, but there must be a better way, right?

(Time series solution also acceptable.)

Solution 1

You need to specify the levels in factor and then use order with indexing:

daily$DoW <- factor(daily$DoW, levels= c("Sunday", "Monday", 
    "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"))

daily[order(daily$DoW), ]

Solution 2

Instead of a factor, what you want is an Ordered.Factor.

This line of R code converts your DoW variable to an "Ordered Factor":

daily$DoW <- ordered(daily$DoW, levels=c("Monday", "Tuesday", "Wednesday", "Thursday", 
"Friday", "Saturday", "Sunday"))

Now when you use table, plot or any other functions on Dow it will be the order you specified above.

Comments

[Tyler Rinker]

I checked to see if R has a days of the week constant built in like it does months and it doesn't appear to. Figured it would.

[Mittenchops]

This almost worked---I implemented the final line to be an assignment, daily <- daily[order(daily$DoW),] But I notice that when I make a histogram out of it with ggplot(daily, aes(x=DoW, y=Duration)) + geom_histogram(), the factors are listed in the correct order on the x axis, but not on the y axis---that is, the y values still correspond to the old positions of the x values. ie, Sunday is 14, instead of 20.

[Mittenchops]

Yeah, maybe my assignment was incorrect, but when I just type daily now, the index is still listed as 7,2,5,6,4,1,3, even though the DoW column now begins with Sunday.