From: Rob Biedenharn <Rob@...>
Date: 2007-01-23T07:26:01+09:00
Subject: Re: find monday of first week of the month


On Jan 22, 2007, at 4:25 PM, Daniel Liebig wrote:

> Hi,
>
> im pretty new to Ruby and i tried to find out how to calculate the  
> date for the first monday of the first week of the month (i.e. this  
> would be 2007-01-29 for Feb. 2007)
> My approach is to get the weekday of the first day of the month and  
> calculate back from there. By getting the weekday i can tell how  
> far to calculate back (if necessary at all). If i'm calcualating  
> back from janurary, i also have to switch back the year.
>
> So this is what i came out with:
>
>     year = 2007
>     month = 2
>
>     # get the weekday of the first day of the month
>     first_wd = Date.new(year, month, 1).wday
>
>     # if weekday is sunday
>     if first_wd == 0
>       date_modifier = - 6
>
>     #if weekday is monday
>     elsif first_wd == 1
>       date_modifier = 1
>
>     # any other day
>     else
>       date_modifier = - ( first_wd - 1 )
>     end
>
>     #if i have to count back, swith month (and year) back
>     if date_modifier < 0
>       if month > 1
>         month -= 1
>       else
>         month = 12
>         year -= 1
>       end
>     end
>
>     # get date with calculated modifier
>     date_start = Date.new(year, month, date_modifier )
>
> Ok, this works, but it seems pretty complicated to me. Is there a  
> smarter (and more readable) way to do this? As far as i know Ruby  
> right now, it's pretty unusual if an "easy" problem like this needs  
> more than three lines of code ;).
>
> Thx for any help
> Regards
> Daniel
>

Your attempt isn't too far off from the Time#beginning_of_week from  
ActiveSupport
      # File vendor/rails/activesupport/lib/active_support/core_ext/ 
time/calculations.rb, line 128
128:         def beginning_of_week
129:           days_to_monday = self.wday!=0 ? self.wday-1 : 6
130:           (self - days_to_monday.days).midnight
131:         end

Working just with a Date, that gives you:

 >> feb=Date.new(2007,2,1)
=> #<Date: 4908265/2,0,2299161>
 >> feb.to_s
=> "2007-02-01"
 >> feb -= (feb.wday != 0 ? feb.wday-1 : 6)
=> #<Date: 4908259/2,0,2299161>
 >> feb.to_s
=> "2007-01-29"

What about January 2008?

 >> jan08 = Date.new(2008,1,1)
=> #<Date: 4908933/2,0,2299161>
 >> jan08 -= (jan08.wday !=0 ? jan08.wday-1 : 6)
=> #<Date: 4908931/2,0,2299161>
 >> jan08.to_s
=> "2007-12-31"

You shouldn't have too much trouble wrapping that up into a nice  
little method of your own.

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com