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comp.lang.ruby

find monday of first week of the month

Daniel Liebig

1/22/2007 9:22:00 PM

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
7 Answers

Daniel Berger

1/22/2007 10:02:00 PM

0

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)

The first Monday in February is on January 29th? Hm, wait, I think I
understand what you're trying to say.

> 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:

<snip>

require 'date'

date = Date.new(2007, 2)
p date.strftime("%Y-%m-%d") # 2007-02-01
date -= 1 until date.mday == 1
p date.strftime("%Y-%m-%d") # 2007-01-29

Regards,

Dan

lrlebron@gmail.com

1/22/2007 10:03:00 PM

0

You may want to try the chronic library

require 'chronic'
date_start = Chronic.parse("Monday of first week of February")


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

Daniel Berger

1/22/2007 10:08:00 PM

0


lrlebron@gmail.com wrote:
> You may want to try the chronic library
>
> require 'chronic'
> date_start = Chronic.parse("Monday of first week of February")

Oh, now that is cool. :)

Dan

James Gray

1/22/2007 10:11:00 PM

0

On Jan 22, 2007, at 4:05 PM, Daniel Berger wrote:

> require 'date'
>
> date = Date.new(2007, 2)
> p date.strftime("%Y-%m-%d") # 2007-02-01
> date -= 1 until date.mday == 1
> p date.strftime("%Y-%m-%d") # 2007-01-29

If you start with a Sunday you will end up with a Monday before
that. Not sure that's what was intended.

James Edward Gray II

William James

1/22/2007 10:13:00 PM

0


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

def first_monday( year, month )
date = Date.new( year, month, 1 )
date - ( date.wday - 1 )
end

Rob Biedenharn

1/22/2007 10:26:00 PM

0


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://agileconsult...
Rob@AgileConsultingLLC.com

Daniel Berger

1/23/2007 2:04:00 AM

0

James Edward Gray II wrote:
> On Jan 22, 2007, at 4:05 PM, Daniel Berger wrote:
>
>> require 'date'
>>
>> date = Date.new(2007, 2)
>> p date.strftime("%Y-%m-%d") # 2007-02-01
>> date -= 1 until date.mday == 1
>> p date.strftime("%Y-%m-%d") # 2007-01-29
>
> If you start with a Sunday you will end up with a Monday before that.
> Not sure that's what was intended.

Whoops.

date = date.mday == 0 ? date += 1 : date -= 1 until date.mday == 1

Regards,

Dan