[R] Count number of Fridays in a month

[Gabor Grothendieck]

On Fri, Oct 10, 2014 at 7:28 AM, Abhinaba Roy <abhinabaroy09 at gmail.com> wrote:
> Hi R helpers,
>
> I want to write a function which will
>
> 1. Count the number of fridays in the current month ( to extract month from
> given date) and also the number of fridays in the preceeding month
>
> 2. Calculate the ratio of the number of fridays in current month to the
> number of fridays in the precceding month
>
> 3. Return a integer value calculated as
>     ifelse(ratio>1,1,ifesle(ration<1,-1),0)
>
> The date which is passed is in the format *'31-may-2014'*
>
> So, given the date '31-may-2014'
>
> Number of fridays in May2014 = 5
> Number of fridays in Apr2014 = 4
>
> Ratio = 5/4 >1
> Hence, the function will return a value 1
>
> I want to call the function by passing '31-may-2014' as an argument

If d is a "Date" class variable equal to a month end date, e.g.

   d <- as.Date("31-may-2014", format = "%d-%b-%Y")

then this gives the ratio of the number of Fridays in d's month
to the number of Fridays in the prior month:

   days <- seq(as.Date(cut(d - 32, "month")), d, "day")
   ratio <- exp(diff(log(tapply(format(days, "%w") == 5, format(days,
"%Y%m"), sum))))

Now apply the formula in your point 3 to ratio and put it all in a function.

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