collapse mulitple columns into one column and generate an index variable

Question

I have three date columns as shown below

       Id Date1       Date2         Date3
       12 2005-12-22  NA            NA
       11 2009-10-11  NA            NA
         


        

Answer 1

This is a classic use of reshape to go from "wide" to "long" format. If d is your data.frame:

d2 <- reshape(d, idvar = "Id", v.names = "Date", timevar = "Index",
              varying = c("Date1", "Date2", "Date3"), direction = "long")

Result:

> d2
     Id Index       Date
12.1 12     1 2005-12-22
11.1 11     1 2009-10-11
29.1 29     1       
45.1 45     1       
39.1 39     1       
44.1 44     1       
12.2 12     2       
11.2 11     2       
29.2 29     2 2005-04-11
45.2 45     2       
39.2 39     2       
44.2 44     2 2005-04-16
12.3 12     3       
11.3 11     3       
29.3 29     3       
45.3 45     3 2008-11-06
39.3 39     3 2006-01-02
44.3 44     3       

If you don't want all the NA values (above) you can subset:

> d2[!is.na(d2$Date),]
     Id Index       Date
12.1 12     1 2005-12-22
11.1 11     1 2009-10-11
29.2 29     2 2005-04-11
44.2 44     2 2005-04-16
45.3 45     3 2008-11-06
39.3 39     3 2006-01-02