[R] Creating a "shifted" month (one that starts not on the first of each month but on another date)

[Dimitri Liakhovitski]

Thanks a lot, Jim.
This is exactly what I was looking for!
Dimitri

On Thu, May 19, 2011 at 12:11 PM, jim holtman <jholtman at gmail.com> wrote:
> try this:
>
> mydf<-data.frame(mydate=seq(as.Date("2011-01-01"), length = 92, by = "day"))
> # add 'day' to the dataframe
> mydf$day <- format(mydf$mydate, "%d")
> mydf$newfactor <- cumsum(mydf$day == '20')
> mydf
>
> On Thu, May 19, 2011 at 10:51 AM, Dimitri Liakhovitski
> <dimitri.liakhovitski at gmail.com> wrote:
>> Hello!
>> I have a data frame with dates. I need to create a new "month" that
>> starts on the 20th of each month - because I'll need to aggregate my
>> data later by that "shifted" month.
>> I wrote the code below and it works. However, I was wondering if there
>> is some ready-made function in some package - that makes it
>> easier/more elegant?
>> Thanks a lot!
>>
>> # Example data:
>> mydf<-data.frame(mydate=seq(as.Date("2011-01-01"), length = 92, by = "day"))
>> (mydf)
>>
>> ### Creating a new variable that has one value before
>> ### the 20th of each month and next value after it
>>
>> mydf$daynum<-as.numeric(format(mydate,"%d"))
>> library(zoo)
>> mydf$yearmon<-as.yearmon(mydf$mydate)
>> (mydf); str(mydf)
>>
>> mydf$newfactor<-NA
>> for(i in unique(mydf$yearmon)){ # looping through "yearmon" (important
>> because true data has many years of data)
>>        tempdf<-mydf[mydf$yearmon == i,]
>>        which.month<-which(unique(mydf$yearmon)==i)
>>        tempdf$newfactor[tempdf$daynum<20]<-which.month
>>        tempdf$newfactor[tempdf$daynum>19]<-(which.month+1)
>>        mydf[mydf$yearmon == i,]<-tempdf
>> }
>> (mydf)
>>
>> --
>> Dimitri Liakhovitski
>> Ninah Consulting
>> www.ninah.com
>>
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>> and provide commented, minimal, self-contained, reproducible code.
>>
>
>
>
> --
> Jim Holtman
> Data Munger Guru
>
> What is the problem that you are trying to solve?
>



-- 
Dimitri Liakhovitski
Ninah Consulting
www.ninah.com