Create 2 new variables from existing variables: one date, and one with value equal to another variable's name

[Rachel Soifer]

* Example generated by -dataex-. For more info, type help dataex
clear
input float(study_id red yellow green)
 1 16044 16532     .
 2     .     . 21254
 3     . 19516 19213
 4 18359     . 18890
 5     .     . 18438
 6 22042 17578     .
 7     .     . 20036
 8     . 20179     .
 9     .     . 15718
10 18874 22474 19873
end
format %td red
format %td yellow
format %td green

Here's what I need to do: generate 6 new variables.
date1 and color1 will be the date and corresponding variable name (color) for one of the date values here. There should be no missing values for these 2 new variables because every row has at least 1 date in it.
date2 and color2 will be the date and corresponding variable name (color) for the second date value, for those who have one. Those individuals who only had 1 date value (n=4)will be have missing values for these 2 new variables.
date3 and color3 will be the date and corresponding variable name (color) for the third date value, for those who have it. Only 1 individual has 3 dates so this variable will have 9 missing values.

Chronological order doesn't really matter, because I'm going to eventually reshape it to long anyway. So I don't care if date1 is the earliest date or date3 is earliest or if it's in some random order.

I have no idea how to do this so any help would be much appreciated.

[Roman Mostazir]

Something like this? :

//Current date:

lis

     +----------------------------------------------+
     | study_id         red      yellow       green |
     |----------------------------------------------|
  1. |        1   05dec2003   06apr2005           . |
  2. |        2           .           .   11mar2018 |
  3. |        3           .   07jun2013   08aug2012 |
  4. |        4   07apr2010           .   20sep2011 |
  5. |        5           .           .   25jun2010 |
     |----------------------------------------------|
  6. |        6   07may2020   16feb2008           . |
  7. |        7           .           .   09nov2014 |
  8. |        8           .   01apr2015           . |
  9. |        9           .           .   13jan2003 |
 10. |       10   04sep2011   13jul2021   30may2014 |
     +----------------------------------------------+


//You wanted:

ren (red yellow green) (date1 date2 date3)

reshape long date, i(study_id) j(col)
clonevar color=col
lab define c 1 "Red" 2 "Yellow" 3 "Green"
lab val color c
replace color =. if date==.

reshape wide date color, i(study_id) j(col)

 lis, clean noobs

    study_id       date1   color1       date2   color2       date3   color3  
           1   05dec2003      Red   06apr2005   Yellow           .        .  
           2           .        .           .        .   11mar2018    Green  
           3           .        .   07jun2013   Yellow   08aug2012    Green  
           4   07apr2010      Red           .        .   20sep2011    Green  
           5           .        .           .        .   25jun2010    Green  
           6   07may2020      Red   16feb2008   Yellow           .        .  
           7           .        .           .        .   09nov2014    Green  
           8           .        .   01apr2015   Yellow           .        .  
           9           .        .           .        .   13jan2003    Green  
          10   04sep2011      Red   13jul2021   Yellow   30may2014    Green