[R] extracting months from a data

[Dalthorp, Daniel]

How about the following, which is both faster and simpler than either the
as.Date(...) or sub(...) solutions discussed earlier:

substring(x,first=nchar(x)-2)

require(microbenchmark)
microbenchmark(format(as.Date(paste0(x,"-2016"),format='%d-%b-%Y'),'%b'))
# 59.3 microseconds on my computer
microbenchmark(sub( "^\\d+-([A-Za-z]{3})$", "\\1", x ))
# 17.4 microseconds
microbenchmark(substring(x,first=nchar(x)-2))
# 3.6 microseconds

-Dan

On Wed, Mar 9, 2016 at 5:08 PM, Jeff Newmiller <jdnewmil at dcn.davis.ca.us>
wrote:

> How about slower? That is objective.
>
> I use dates all the time so I am quite familiar with what they are good
> for. However, I prefer to avoid inventing information such as which year
> the date should have included unless I have to based on knowledge of the
> data source. It is not good to mislead the consumer of output about the
> existence of year information if it wasn't there to begin with.
> --
> Sent from my phone. Please excuse my brevity.
>
> On March 9, 2016 4:49:18 PM PST, "Dalthorp, Daniel" <ddalthorp at usgs.gov>
> wrote:
>>
>> Good point about 29-Feb...fixed in the following:
>>
>> format(as.Date(paste0(x,"-2016"),format='%d-%b-%Y'),'%b')
>>
>> # Also: The date functions can be used to easily calculate passage of
>> time and offer good flexibility for formatting output.
>>
>> -Dan
>>
>> P.S. "harder to understand" is in the eye of the beholder (as is
>> "recommended").
>>
>>
>>
>> On Wed, Mar 9, 2016 at 4:39 PM, Jeff Newmiller <jdnewmil at dcn.davis.ca.us>
>> wrote:
>>
>>> Still not recommended. That takes more steps, is harder to understand,
>>> and will break when given "29-Feb" as input.
>>> --
>>> Sent from my phone. Please excuse my brevity.
>>>
>>> On March 9, 2016 4:15:31 PM PST, "Dalthorp, Daniel" <ddalthorp at usgs.gov>
>>> wrote:
>>>>
>>>> Or:
>>>>
>>>> x <- c( "3-Oct", "10-Nov" )
>>>> format(as.Date(paste0(x,rep("-1970",length(x))),format='%d-%b-%Y'),'%b')
>>>>
>>>> # the 'paste0' appends a year to the text vector
>>>> # the 'as.Date' interprets the strings as dates with format
>>>>  10-Jun-2016 (e.g.)
>>>> # the 'format' returns a string with date in format '%b' (which is just
>>>> the name of the month)
>>>>
>>>> On Wed, Mar 9, 2016 at 3:52 PM, Jeff Newmiller <
>>>> jdnewmil at dcn.davis.ca.us> wrote:
>>>>
>>>>> Your dates are incomplete (no year) so I suggest staying away from the
>>>>> date functions for this. Read ?regex and ?sub.
>>>>>
>>>>> x <- c( "3-Oct", "10-Nov" )
>>>>> m <- sub( "^\\d+-([A-Za-z]{3})$", "\\1", x )
>>>>>
>>>>> --
>>>>> Sent from my phone. Please excuse my brevity.
>>>>>
>>>>> On March 9, 2016 10:14:25 AM PST, KMNanus <kmnanus at gmail.com> wrote:
>>>>> >I have a series of dates in  format 3-Oct, 10-Oct, 20-Oct, etc.
>>>>> >
>>>>> >I want to create a variable of just the month.  If I convert the date
>>>>> >to a character string, substr is ineffective because some of the dates
>>>>> >have 5 characters (3-Oct) and some have 6 (10-Oct).
>>>>> >
>>>>> >Is there a date function that accomplishes this easily?
>>>>> >
>>>>> >Ken
>>>>> >kmnanus at gmail.com
>>>>> >914-450-0816 (tel)
>>>>> >347-730-4813 (fax)
>>>>> >
>>>>> >
>>>>> >
>>>>> >______________________________________________
>>>>> >R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>>>>> >https://stat.ethz.ch/mailman/listinfo/r-help
>>>>> >PLEASE do read the posting guide
>>>>> >http://www.R-project.org/posting-guide.html
>>>>> >and provide commented, minimal, self-contained, reproducible code.
>>>>>
>>>>>         [[alternative HTML version deleted]]
>>>>>
>>>>> ______________________________________________
>>>>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>>>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>>>> PLEASE do read the posting guide
>>>>> http://www.R-project.org/posting-guide.html
>>>>> and provide commented, minimal, self-contained, reproducible code.
>>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> Dan Dalthorp, PhD
>>>> USGS Forest and Rangeland Ecosystem Science Center
>>>> Forest Sciences Lab, Rm 189
>>>> 3200 SW Jefferson Way
>>>> Corvallis, OR 97331
>>>> ph: 541-750-0953
>>>> ddalthorp at usgs.gov
>>>>
>>>>
>>
>>
>> --
>> Dan Dalthorp, PhD
>> USGS Forest and Rangeland Ecosystem Science Center
>> Forest Sciences Lab, Rm 189
>> 3200 SW Jefferson Way
>> Corvallis, OR 97331
>> ph: 541-750-0953
>> ddalthorp at usgs.gov
>>
>>


-- 
Dan Dalthorp, PhD
USGS Forest and Rangeland Ecosystem Science Center
Forest Sciences Lab, Rm 189
3200 SW Jefferson Way
Corvallis, OR 97331
ph: 541-750-0953
ddalthorp at usgs.gov

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