converting time format

df.count.mon

Yes, it is a little different. Is there a way to get 'yyyy-mm' format?
Thanks.

On Wed, Feb 22, 2017 at 2:10 PM, Jim Lemon <drjimlemon at gmail.com> wrote:

Hi Lily.
Two problems. You have named the month field "mon" and then refer to
it as "month". Second, as the resolution of as.Date is days, it can't
produce a valid date without specifying the day. Thus:

df.count.mon<-data.frame(count=sample(1:24,24),
 year=rep(2014:2015,each=2),mon=rep(1:12,2))
# make each day the first day of the month
df.count.mon$time<-
 as.Date(paste(df.count.mon$year, df.count.mon$mon,1),
 '%Y %m %d')
df.count.mon
   count year mon       time
1     22 2014   1 2014-01-01
2     12 2014   2 2014-02-01
...
You will get values, but I don't think they are the ones you want.

Jim

On Thu, Feb 23, 2017 at 6:33 AM, lily li <chocold12 at gmail.com> wrote:

Hi R users,

I have a dataframe, with year, month, day, and other variables. I wanted
to
calculated monthly values of the variables. For example, there is one
variable called 'count'. I use the code below to convert daily data to
monthly data.

df.count.mon = aggregate(count ~ year+month, data= df, sum)

The new dataframe has three columns: year, month, and count. Now I want
to
add one more column as 'time', which has the format 'yyyy-mm'. I use the
code below but the new column has all NA values. What is the problem and
how to solve it?

df.count.mon$time = as.Date(paste(df.count.mon$year,
df.count.mon$month),
'%Y %m')

I had experience to add one more column with the format 'yyyy-mm-dd',
which
works, but not with monthly format. Thanks for your help.

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