[R] sorting by date

Jeff Jorgensen jcjorgensen at wisc.edu
Mon Feb 2 23:16:13 CET 2004


I have set up a data.frame and one of the columns contains a date of the 
form (with slashes as separators):


I would like to use formulas on other columns in the data.frame organized 
by date, for example:

tapply(var1, sort(date), mean)

However, when I try sort(date) it sorts based on the first two entries in 
the date field:

9/1/2001	9/1/2002	9/1/2003	9/2/2001 ...
5.6		7.5		6.4		7.0 ...

Instead of:

9/1/2001	9/2/2001	9/3/2001	9/4/2001 ...
5.6		6.1		7.2		6.8 ...

I would greatly appreciate any help in sorting chronologically.  Do I need 
to create separate columns for month, day, and year, and then use order() 
and then stipulate the hierarchy for which to sort the output?  Or, is 
there some other more efficient way?



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