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

You can consider melting your data.

Here's an example:

library(data.table)
library(reshape2)
melt(as.data.table(mydf), id.vars = "Id", na.rm = TRUE)
#    Id variable      value
# 1: 12    Date1 2005-12-22
# 2: 11    Date1 2009-10-11
# 3: 29    Date2 2005-04-11
# 4: 44    Date2 2005-04-16
# 5: 45    Date3 2008-11-06
# 6: 39    Date3 2006-01-02

## More specific to what you want:
melt(as.data.table(mydf), id.vars = "Id", na.rm = TRUE)[, 
  variable := sub("Date", "", variable)][]
#    Id variable      value
# 1: 12        1 2005-12-22
# 2: 11        1 2009-10-11
# 3: 29        2 2005-04-11
# 4: 44        2 2005-04-16
# 5: 45        3 2008-11-06
# 6: 39        3 2006-01-02