Refer to a value for a specific date

[Mike Russo]

Hi everyone,
I need to refer to a value at a specific date in a loop. For instance to calculate the constant instalment of a loan (Instalment variable) I need to refer in all periods (30 months) to the principal as of January 2022, set in the variable date. I am using a loop for 2 scenarios. How can I set in the loop the principal amount as of January 2022 for all periods?

Thank you all for the help

[Nick Cox]

https://www.stata-journal.com/articl...article=dm0055 covers related tricks. Something like

egen principal_jan_2022 = mean(cond(date == ym(2022, 1), principal, .))

or

egen principal_jan_2022 = mean(cond(date == ym(2022, 1), principal, .)) , by(id)

should get you closer.

Here's a reproducible example:

. webuse grunfeld , clear

. su year

    Variable |        Obs        Mean    Std. dev.       Min        Max
-------------+---------------------------------------------------------
        year |        200      1944.5    5.780751       1935       1954

. egen invest1939 = mean(cond(year == 1939, invest, .)) , by(company)

. l company year invest* in 1/30

     +------------------------------------+
     | company   year   invest   inv~1939 |
     |------------------------------------|
  1. |       1   1935    317.6      330.8 |
  2. |       1   1936    391.8      330.8 |
  3. |       1   1937    410.6      330.8 |
  4. |       1   1938    257.7      330.8 |
  5. |       1   1939    330.8      330.8 |
     |------------------------------------|
  6. |       1   1940    461.2      330.8 |
  7. |       1   1941      512      330.8 |
  8. |       1   1942      448      330.8 |
  9. |       1   1943    499.6      330.8 |
 10. |       1   1944    547.5      330.8 |
     |------------------------------------|
 11. |       1   1945    561.2      330.8 |
 12. |       1   1946    688.1      330.8 |
 13. |       1   1947    568.9      330.8 |
 14. |       1   1948    529.2      330.8 |
 15. |       1   1949    555.1      330.8 |
     |------------------------------------|
 16. |       1   1950    642.9      330.8 |
 17. |       1   1951    755.9      330.8 |
 18. |       1   1952    891.2      330.8 |
 19. |       1   1953   1304.4      330.8 |
 20. |       1   1954   1486.7      330.8 |
     |------------------------------------|
 21. |       2   1935    209.9      230.4 |
 22. |       2   1936    355.3      230.4 |
 23. |       2   1937    469.9      230.4 |
 24. |       2   1938    262.3      230.4 |
 25. |       2   1939    230.4      230.4 |
     |------------------------------------|
 26. |       2   1940    361.6      230.4 |
 27. |       2   1941    472.8      230.4 |
 28. |       2   1942    445.6      230.4 |
 29. |       2   1943    361.6      230.4 |
 30. |       2   1944    288.2      230.4 |
     +------------------------------------+

.