[R] convert weekly time series data to monthly

[Richard Saba]

I have weekly time series data with year, month, day, and price variables.
The input data set for the weekly series takes the following form: 

 Year	month	day 	price
1990	8	20	119.1		
1990	8	27	124.5
1990	9	3	124.2
1990	9	10	125.2
1990	9	17	126.6
1990	9	24	127.2
1990	10	1	132.1
1990	10	8	133.3
1990	10	15	133.9
1990	10	22	134.5
1990	10	29	133.9
..	...	...	...
...	...	....	....
2008	3	3	313.7
2008	3	10	320
2008	3	17	325.7
2008	3	24	322.4 


I would like to collapse the data into monthly averages to merge with a
monthly series. The input data set for the monthly series takes the
following form: 

M-Y		Year	Month	Change
Aug-1990	1990	8	-226.871
Sep-1990	1990	9	-896.333
Oct-1990	1990	10	111.419
Nov-1990	1990	11	-364.2
Dec-1990	1990	12	-527.645
Jan-1991	1991	1	-70.935
Feb-1991	1991	2	231.214
Mar-1991	1991	3	-239 

...	...	.....	..	....


The merged data set should be of class(ts).
I can perform the conversions outside of R  and then import but I would
rather perform all conversions within R. I have looked through the zoo and
Rmetrics packages but without success. 

Any help will be appreciated.
Thanks,

Richard Saba
Department of Economics
Auburn University
Email:  sabaric at auburn.edu
Phone:  334 844-2922