[R] number of Mondays

[Peter Dalgaard]

Carlos Hernandez wrote:
> dear All,
> i'm trying to calculate the number of Mondays, Tuesdays, etc that each
> month within a date range has. I have time series data that spans 60
> months and i want to calculate the number of Mondays, Tuesdays, Wed, etc
> of each month. (I want to control for weekly seasonality but my data is
> monthly).
> 
> Is there an easy way to to this in R? or is there a package i could use?
> i did some quick search in the help files and R sites but could not find
> any answers.
> 
> i appreciate any hint you could give,

This is where POSIXlt objects are useful:

> unlist(unclass(as.POSIXlt(ISOdate(1959,3,11))))
  sec   min  hour  mday   mon  year  wday  yday isdst
    0     0    12    11     2    59     3    69     0

Which means that I was born on a Wednesday (wday==3) in March (mon==2)
(some of the fields count from 0 and others, like mday, from 1;
presumably some UNIX vendor back in the Stone Age got their
implementation turned into a standard...).

This allows you to do stuff like:


> dd <- seq(Sys.Date(),as.Date("2009-3-11"),1)
> dd <- as.POSIXlt(dd)
> with(dd, table(mon,wday))
   wday
mon 0 1 2 3 4 5 6
  0 2 2 2 2 3 3 3
  1 4 4 4 4 4 4 4
  2 2 2 2 2 1 1 1

which I think is pretty much what you were looking for.


> thanks.
> 
> Carlos
> 
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