[R] summing values by week - based on daily dates - but with some dates missing

[Dimitri Liakhovitski]

Dear everybody,

I have the following challenge. I have a data set with 2 subgroups,
dates (days), and corresponding values (see example code below).
Within each subgroup: I need to aggregate (sum) the values by week -
for weeks that start on a Monday (for example, 2008-12-29 was a
Monday).
I find it difficult because I have missing dates in my data - so that
sometimes I don't even have the date for some Mondays. So, I can't
write a proper loop.
I want my output to look something like this:
group   dates   value
group.1 2008-12-29  3.0937
group.1 2009-01-05  3.8833
group.1 2009-01-12  1.362
...
group.2 2008-12-29  2.250
group.2 2009-01-05  1.4057
group.2 2009-01-12  3.4411
...

Thanks a lot for your suggestions! The code is below:
Dimitri

### Creating example data set:
mydates<-rep(seq(as.Date("2008-12-29"), length = 43, by = "day"),2)
myfactor<-c(rep("group.1",43),rep("group.2",43))
set.seed(123)
myvalues<-runif(86,0,1)
myframe<-data.frame(dates=mydates,group=myfactor,value=myvalues)
(myframe)
dim(myframe)

## Removing same rows (dates) unsystematically:
set.seed(123)
removed.group1<-sample(1:43,size=11,replace=F)
set.seed(456)
removed.group2<-sample(44:86,size=11,replace=F)
to.remove<-c(removed.group1,removed.group2);length(to.remove)
to.remove<-to.remove[order(to.remove)]
myframe<-myframe[-to.remove,]
(myframe)



-- 
Dimitri Liakhovitski
Ninah Consulting
www.ninah.com