R help - Jul 2005 - Dates are read as factors!

I am using read.csv to read a CSV file (produced by saving an Excel file
as a CSV file). The columns containing dates are being read as factors.
Because of this, I can not compute follow-up time, i.e.
Followup<-postDate-preDate. I would appreciate any suggestion that would
help me read the dates as dates and thus allow me to calculate follow-up
time.
Thanks
John

John Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
Baltimore VA Medical Center GRECC and
University of Maryland School of Medicine Claude Pepper OAIC

University of Maryland School of Medicine
Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524

410-605-7119 
- NOTE NEW EMAIL ADDRESS:
jsorkin at grecc.umaryland.edu

On Wed, 2005-07-27 at 23:07 -0400, John Sorkin wrote:
> I am using read.csv to read a CSV file (produced by saving an Excel file
> as a CSV file). The columns containing dates are being read as factors.
> Because of this, I can not compute follow-up time, i.e.
> Followup<-postDate-preDate. I would appreciate any suggestion that would
> help me read the dates as dates and thus allow me to calculate follow-up
> time.
> Thanks
> John
John,

Depending upon how may columns you are reading, you could:

1. Use the 'colClasses' argument in read.csv() to specify the data types
for each incoming column, one of which is 'Date'

Or

2. Post process the data after importing by using the as.Date()
function. See ?as.Date for more information and pay particular attention
to the 'format' argument.

My own preference is number 2.

Once the requisite columns are converted to the Date class, you can
engage in date based arithmetic, etc.

HTH,

Marc Schwartz