[R] extract week from date

[Douglas Bates]

Douglas Bates <bates at cs.wisc.edu> writes:

> For example
> 
> > as.character(unique(cheese[,1]))
>  [1] "5/4/99"  "5/5/99"  "5/7/99"  "5/11/99" "5/12/99" "5/14/99" "5/18/99"
>  [8] "5/19/99" "5/21/99" "5/25/99" "5/27/99" "5/28/99" "8/3/99"  "8/4/99" 
> [15] "8/6/99"  "8/10/99" "8/11/99" "8/12/99" "8/13/99" "8/17/99" "8/18/99"
> [22] "8/19/99" "8/20/99" "8/24/99" "8/25/99" "8/27/99" "8/31/99"
> > dd <- strptime(as.character(unique(cheese[,1])), format = '%m/%d/%y')
> > ceiling(dd$yday/7)
>  [1] 18 18 18 19 19 19 20 20 20 21 21 21 31 31 31 32 32 32 32 33 33 33 33 34 34
> [26] 34 35

As Andy and Pingping pointed out, this is more compactly written as

> format(dd, "%U")
 [1] "18" "18" "18" "19" "19" "19" "20" "20" "20" "21" "21" "21" "31" "31" "31"
[16] "32" "32" "32" "32" "33" "33" "33" "33" "34" "34" "34" "35"
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