[R] how to calculate "conditional" mean?

[Terry Mu]

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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