ATMathew ATMathew -4 years ago 84
R Question

Find the day of a week

Let's say that I have a date in R and it's formatted as follows.

date
2012-02-01
2012-02-01
2012-02-02


Is there any way in R to add another column with the day of the week associated with the date? The dataset is really large, so it would not make sense to go through manually and make the changes.

df = data.frame(date=c("2012-02-01", "2012-02-01", "2012-02-02"))


So after adding the days, it would end up looking like:

date day
2012-02-01 Wednesday
2012-02-01 Wednesday
2012-02-02 Thursday


Is this possible? Can anyone point me to a package that will allow me to do this?
Just trying to automatically generate the day by the date.

Answer Source
df = data.frame(date=c("2012-02-01", "2012-02-01", "2012-02-02")) 
df$day <- weekdays(as.Date(df$date))
df
##         date       day
## 1 2012-02-01 Wednesday
## 2 2012-02-01 Wednesday
## 3 2012-02-02  Thursday

Edit: Just to show another way...

The wday component of a POSIXlt object is the numeric weekday (0-6 starting on Sunday).

as.POSIXlt(df$date)$wday
## [1] 3 3 4

which you could use to subset a character vector of weekday names

c("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", 
    "Friday", "Saturday")[as.POSIXlt(df$date)$wday + 1]
## [1] "Wednesday" "Wednesday" "Thursday" 
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