[R] how to calculate "conditional" mean?

[Gabor Grothendieck]

In my previous post the format should have been "%m %d".

Terry Mu <muster <at> gmail.com> writes:

: 
: thank you,
: 
: one more question:
: 
: How can I list only values for given condition?
: for example, I want to see only data from febuary, i.e. month=2?
: 
: On Sun, 5 Dec 2004 16:26:11 +0000 (UTC), Gabor Grothendieck
: <ggrothendieck <at> myway.com> wrote:
: > Terry Mu <muster <at> gmail.com> writes:
: > 
: > :
: > : a data set like this:
: > 
: > 
: > :
: > : year   month    day    count
: > : 2001     1            1         10
: > : 2001     1            2          11
: > : ....
: > : 2004     7            17        8
: > : ....
: > :
: > : basically it is a count of of some numbers everyday through a few years
: > :
: > : now I'd like to get the mean of the count for every day.
: > :
: > : I thought I can do this using for and ifelse,
: > : but is there a simple way to do this? I wish a function like
: > : mean(count, 'only when year, month, day are equal') could do this.
: > 
: > You want to represent your dates as objects of the Date class
: > and then use tapply.  (See ?as.Date, ?Date, ?tapply, ?paste, ?with.
: > Also look up ?aggregate and ?by and read about dates in R News 4/1.)
: > Assuming DF is your data frame:
: > 
: >    Dates <- with(DF,
: >        as.Date( paste(year, month, day, sep="-") )
: >    )
: >    tapply(DF$count, Dates, mean)
: > 
: > 
: > 
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: 
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