[R] Subsetting a zooreg object using window / subset

[Achim Zeileis]

On Thu, 22 Sep 2011, Gabor Grothendieck wrote:

> On Thu, Sep 22, 2011 at 6:48 AM, Wesley Roberts <wroberts at csir.co.za> wrote:
>> Dear R users,
>>
>>
>> I am currently working in subsetting a zooreg() object using either window or subset. I have a solution but it may be a bit cumbersome when I start working with actual data. Your inputs would be greatly appreciated.
>>
>> Example: I have a zooreg() object that starts in 1997 and ends in 2001. This object contains daily data for the 4 years
>>
>> aa<-zooreg(1:1825,start=as.Date("1997-01-01"))
>>
>> My aim is to subset the data according to seasons (Southern Hemisphere) for continuous years: December - January - February (DJF-Summer: 1997-2001), March - April - May (MAM-Autumn: 1997-2001), June - July - August (JJA-Winter: 1997-2001), September - October - November (SON-Spring: 1997-2001) thereby analysing the seasons data only for all years. The example below is only for DJF but I would like to replicate the analysis for each season.
>>
>> My solution so far uses subset to select the monthly data for each year and then rbind() the results.
>>
>> bb <- subset(aa, index(aa)>=as.Date("1998-12-01")&index(aa)<=as.Date("1999-02-28"))
>> cc <- subset(aa, index(aa)>=as.Date("1999-12-01")&index(aa)<=as.Date("2000-02-28"))
>> dd <- subset(aa, index(aa)>=as.Date("2000-12-01")&index(aa)<=as.Date("2001-02-28"))
>>
>> ee<- rbind(bb,cc,dd)
>>
>> The method above appears to do the job just fine except that I have around 30 locations (catchments) each with varying data availability and some with over 20 years worth of data. Ideally I would like to combine the second set of commands into a single command where I specify the start and end year and the months that I am interested in.
>>
>
> This gives the season (1 = djf, 2 = mam, 3 = jja, 5 = son):
>
> seas <- as.numeric(format(as.yearqtr(as.yearmon(time(aa)) + 1/12), "%q"))

An alternative route might be to go via the "mon" component of POSIXlt, 
e.g.,

   as.POSIXlt(time(aa))$mon %in% c(11, 0, 1)

instead of

   seas == 1

etc.

> and this picks out djf:
>
> aa[seas == 1]
>
>
> -- 
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