Convert name of day to date format in R - r

Question

I want to convert this data in format of date and create new column with the value of month-year:
month : Factor w/ 10 levels "apr","aug","dec",..: 7 7 7 7 7 7 7 7 7 7 ...
day : chr [1:41188] "mon" "mon" "mon" "mon" ...
year : num [1:41188] 2008 2008 2008 2008 2008 ...
I make a dput()
dput(head(df))
df <-
structure(list(month = structure(c(7L, 7L, 7L, 7L, 7L, 7L),
.Label = c("apr", "aug", "dec", "jul", "jun", "mar", "may",
"nov", "oct", "sep"), class = "factor"), day = c("mon", "mon",
"mon", "mon", "mon", "mon"), year = c(2008, 2008, 2008, 2008,
2008, 2008)), class = "data.frame", row.names = c(NA, -6L))
The main of problem is the month and day columns because the format is factor and character
I try the next sentences:
as.integer(factor(df$month, levels=month.abb))
And this:
match(df$month, month.abb)
I make it:
df$date<-paste(as.character(df$month), df$year)
And this worked and returns:
$ date : chr [1:41188] "may 2008" "may 2008" "may 2008" "may 2008"
How can I change to date format?

Answer 1

I'll arbitrarily pick the "first" day-of-month for each weekday that you've listed. To make it interesting, I'll change the weekdays so that we have some variability in the data.
df <-
structure(list(month = structure(c(7L, 7L, 7L, 7L, 7L, 7L),
.Label = c("apr", "aug", "dec", "jul", "jun", "mar", "may",
"nov", "oct", "sep"), class = "factor"), day = c("mon", "tue",
"wed", "fri", "sat", "sun"), year = c(2008, 2008, 2008, 2008,
2008, 2008)), class = "data.frame", row.names = c(NA, -6L))
df
# month day year
# 1 may mon 2008
# 2 may tue 2008
# 3 may wed 2008
# 4 may fri 2008
# 5 may sat 2008
# 6 may sun 2008
From here, we need to determine what the 1st day of each month is, and then find the first day-of-week that is at or after that day.
firstdow <- as.POSIXlt(paste(df$year, df$month, "01", sep = "-"), format = "%Y-%b-%d")$wday
# ?strptime says with '%u' that monday is 1
datadow <- match(df$day, c("mon", "tue", "wed", "thu", "fri", "sat", "sun"))
datadom <- (firstdow + datadow - 1) %% 7 + 1
df$date <- as.Date(paste(df$year, df$month, datadom, sep = "-"), format = "%Y-%b-%d")
df
# month day year date
# 1 may mon 2008 2008-05-05
# 2 may tue 2008 2008-05-06
# 3 may wed 2008 2008-05-07
# 4 may fri 2008 2008-05-02
# 5 may sat 2008 2008-05-03
# 6 may sun 2008 2008-05-04
And proof that this came up with the correct day-of-month to get the first day-of-week:
format(df$date, format = "%a")
# [1] "Mon" "Tue" "Wed" "Fri" "Sat" "Sun"

Answer 2

We could do 2 things:
As month.abb is a system constant, we can use it to get numeric month
Use as.yearmon from zoo package to get month and year
library(zoo)
df %>%
mutate(month = match(month, tolower(month.abb))) %>%
mutate(new_date = as.yearmon(paste(year, month), "%Y %m"))
Output:
month day year new_date
1 5 mon 2008 Mai 2008
2 5 mon 2008 Mai 2008
3 5 mon 2008 Mai 2008
4 5 mon 2008 Mai 2008
5 5 mon 2008 Mai 2008
6 5 mon 2008 Mai 2008