Bug in Mata's st_view() function with time-series operators?

[Sebastian Kripfganz]

I experienced an unexpected behavior of Mata's st_view() function. In the following, I want to obtain the first three observations of the variables w L.w k L.k in Mata. If I specify them as the varlist L(0/1).(w k), the function returns w w k k, without applying the lag operator:

. webuse abdata

. mata: st_view(X=., (1::3), "L(0/1).(w k)")

. mata: X
                  1              2              3              4
    +-------------------------------------------------------------+
  1 |   2.576543331    2.576543331   -.5286502242   -.5286502242  |
  2 |   2.509745598    2.509745598   -.4591824114   -.4591824114  |
  3 |   2.552526474    2.552526474   -.3899362981   -.3899362981  |
    +-------------------------------------------------------------+

If instead I specify the varlist as L(0/1).w L(0/1).k, with lag operators separately for the two variables, I obtain the desired result:

. mata: st_view(X=., (1::3), "L(0/1).w L(0/1).k")

. mata: X
                  1              2              3              4
    +-------------------------------------------------------------+
  1 |   2.576543331              .   -.5286502242              .  |
  2 |   2.509745598    2.576543331   -.4591824114   -.5286502242  |
  3 |   2.552526474    2.509745598   -.3899362981   -.4591824114  |
    +-------------------------------------------------------------+

When using the st_data() function instead of st_view(), it behaves nicely as expected in both cases:

. mata: st_data((1::3), "L(0/1).(w k)")
                  1              2              3              4
    +-------------------------------------------------------------+
  1 |   2.576543331              .   -.5286502242              .  |
  2 |   2.509745598    2.576543331   -.4591824114   -.5286502242  |
  3 |   2.552526474    2.509745598   -.3899362981   -.4591824114  |
    +-------------------------------------------------------------+

. mata: st_data((1::3), "L(0/1).w L(0/1).k")
                  1              2              3              4
    +-------------------------------------------------------------+
  1 |   2.576543331              .   -.5286502242              .  |
  2 |   2.509745598    2.576543331   -.4591824114   -.5286502242  |
  3 |   2.552526474    2.509745598   -.3899362981   -.4591824114  |
    +-------------------------------------------------------------+

Do you agree with me that the first example above is a bug?

[William Lisowski]

I agree that the documentation in the output of help mf_st_view says that the rowvector j (third argument of st_view()) may be specified in the same way as with st_data() (where it is the second argument). Having done that and gotten different results, I think this bears reporting to Stata Technical Services.

[Sebastian Kripfganz]

I have reported it to Stata Technical Support. The Stata developers are now looking into it.

[Sebastian Kripfganz]

To close this topic: The bug has been fixed with the 05aug2019 update in Stata 15.