R help - Mar 2010 - FW: How to assign week numbers to a time-series (additional question)

I want to thank everyone who has provided me advice on how to solve this
problem, it was all very helpful. However, I am still left with one issue
remaining. All of the methods provided still require me to input the origin
date, which to lead to correct week number assignment, must be the last Saturday
prior to the survey start date of May 01 or May 01 if that date happens to be a
Saturday. I would prefer a method that is robust to whether or not May 01 is a
Saturday and does not require me to input any data other than the initial
dataframes.  This program will be used to produce sampling schedules for many
subsequent years, most of which will not begin on a Saturday. Does anyone know
how this could be done? Please see the attached partial copy of my dataframe. 
The survey runs from May 01 to Oct 31.

Thanks so much,

Mike

From: Hosack, Michael
Sent: Friday, March 05, 2010 4:24 PM
To: 'R-help at r-project.org'
Subject: How to assign week numbers to a time-series

Hello everyone,

My progress has stalled on finding a way of creating a somewhat complicated
variable to add to my existing dataframe and I am hoping one of you could help
me out. The dataframe below contains only a fraction of the data of my complete
dataframe, but all of the variables. What I want to do is add another variable
named 'WEEK' to this dataframe that is assigned 1 for row 1 and remains
1 until the first SAT (i.e. Saturday) under variable 'DOW' (day of week)
occurs, at which point variable 'WEEK' is now assigned 2. 'WEEK'
should continue to be assigned 2 until the following SAT under variable
'DOW' at which variable 'WEEK' will now be assigned 3, and so
on. In this scheme, weekdays are such that SAT=1, SUN=2, MON=3,.....FRI=7. I am
basically trying to assign week numbers to potential sampling days in a survey
season for use in a program that will generate a fisheries creel survey
schedule. I should note that if element 1 happens to have DOW=SAT (that is the
case this year, since the first day of our survey 05/01 is a Saturday), then
WEEK 1 begins on day 1 (05/01/2010) and WEEK 2 will begin on the first SAT under
variable DOW. I hope I explained this clearly enough, if not let me know. If
this sent twice, I apologize.

Mike

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