[R] Extract month and year as one column

[Gabor Grothendieck]

On Fri, Mar 13, 2015 at 11:36 AM, Kumsaa <waddeessa at gmail.com> wrote:
> How could I extract both month and year from a date? I know how to
> separately extract both using lubridate package:
>
> df$month <- month(df$date)
> df$year<- year(df$date)
>
> I wish to extract year and month as one column
>
>> dput(mydf)
> structure(list(date = structure(c(14975, 14976, 14977, 14978,
> 14979, 14980, 14981, 14982, 14983, 14984, 14985, 14986, 14987,
> 14988, 14989, 15340, 15341, 15342, 15343, 15344, 15345, 15346,
> 15347, 15348, 15349, 15350, 15351, 15352, 15353, 15354), class = "Date"),
>     temp = c(6.5140544091004, 3.69073712745884, 3.04839429519466,
>     9.16988228171461, -1.17176248610603, 2.88216040747883,
> 4.98853844809017,
>     4.07520306701834, 9.82902813943658, 2.79305715971987, 8.04721677924611,
>     7.50667729759095, 2.91055000121842, 1.65559895014064, 4.8019596483372,
>     16.2567986804179, 13.3352908067145, 16.6955807821108, 6.28373374879922,
>     6.97181051627531, 5.74282686202818, 4.37018386569785, 12.5725962512824,
>     4.6583055309578, 8.76457542037641, 10.7070862034423, 12.84023567151,
>     5.78620621848167, 5.98643374478599, 13.0993210289842)), .Names =
> c("date",
> "temp"), row.names = c(NA, 30L), class = "data.frame")
>

The zoo package has a "yearmon" class that represents dates as year
and month with no day:

> library(zoo)
> transform(mydf, yearmon = as.yearmon(date))
        date      temp  yearmon
1 2011-01-01  6.514054 Jan 2011
2 2011-01-02  3.690737 Jan 2011
3 2011-01-03  3.048394 Jan 2011
4 2011-01-04  9.169882 Jan 2011
5 2011-01-05 -1.171762 Jan 2011
6 2011-01-06  2.882160 Jan 2011
etc.