The Ruby Way, Second Edition: Solutions and Techniques in Ruby Programming (2nd Edition)

7.5. Finding the Nth Weekday in a Month

Sometimes for a given month and year, we want to find the date of the third Monday in the month, or the second Tuesday, and so on. The code in Listing 7.1 makes that calculation simple.

If we are looking for the nth occurrence of a certain weekday, we pass n as the first parameter. The second parameter is the number of that weekday (0 meaning Sunday, 1 meaning Monday, and so on). The third and fourth parameters are the month and year, respectively.

Listing 7.1. Finding the Nth Weekday

def nth_wday(n, wday, month, year) if (!n.between? 1,5) or (!wday.between? 0,6) or (!month.between? 1,12) raise ArgumentError end t = Time.local year, month, 1 first = t.wday if first == wday fwd = 1 elsif first < wday fwd = wday - first + 1 elsif first > wday fwd = (wday+7) - first + 1 end target = fwd + (n-1)*7 begin t2 = Time.local year, month, target rescue ArgumentError return nil end if t2.mday == target t2 else nil end end

The peculiar-looking code near the end of the method is put there to counteract a long-standing tradition in the underlying time-handling routines. You might expect that trying to create a date of November 31 would result in an error of some kind. You would be mistaken. Most systems would happily (and silently) convert this to December 1. If you are an old-time UNIX hacker, you may think this is a feature; otherwise, you may consider it a bug.

We will not venture an opinion here as to what the underlying library code ought to do or whether Ruby ought to change that behavior. But we don't want to have this routine perpetuate the tradition. If you are looking for the date of, say, the fifth Friday in November 2000, you will get a nil value back (rather than December 1, 2000).

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