[R] CSV file and date. Dates are read as factors!

[Don MacQueen]

It's really pretty simple.

First, if you supply as.is=TRUE to read.csv() [or read.table()] then 
your dates will be read as character strings, not factors. That saves 
the step of converting them from factor to character.

Then, use as.Date() to convert the date columns to objects of class 
"Date". You will have to specify the format, if your dates are not in 
the default format.

>  tmp <- as.Date('2002-5-1')
>  as.Date(Sys.time())-tmp
Time difference of 1184 days

If your dates include times, then use as.POSIXct() instead of as.Date().

>  tmp <- as.POSIXct('2002-5-1 13:21')
>  Sys.time()-tmp
Time difference of 1183.746 days

If you don't want to use as.is, perhaps because you have other 
columns that you *want* to have as factors, then either supply 
colClasses to read.csv, or else just use format() to convert the 
factors to character.

as.Date(format(your_date_column))

As an aside, you might save yourself some time by using read.xls() 
from the gdata package.

And of course, there's always the ugly work-around. In your Excel, 
create new columns in which the dates are formatted as numbers, 
presumably as the number of days since whatever Excel uses for its 
origin. Then, in R, you can simply subtract the numbers. If you have 
date-time values in Excel, this might be a little trickier.

-Don

At 9:28 PM -0400 7/27/05, 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 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
>
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-- 
--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA