Simple by() and 'if' question

[Noah Spencer]

I'm working with some historical data on NFL players. I want to create a dummy variable indicating whether a player is still active or not. Basically, I want Stata to produce active == 1 if a given player has an observation when year == 2019 and active == 0 otherwise.

I feel like this should be simple, but I'm struggling to code this. What I tried was:

by id: gen active = 1 if year == 2019
by id: replace active = 0 if year != 2019

, but this just put active == 1 for all 2019 observations and active == 0 for all other observations regardless of whether the player was active in 2019.

Does anyone have any advice here?

[Joseph Coveney]

Maybe something like

bysort id: egen byte active = max(year == 2019)

[Nick Cox]

bysort id (year) : gen active = year[_N] == 2019

would presumably work too. The problem behind #1 is that by id: makes no difference to what is done, but the above does make a difference, as is true also of #2.

[Karl Tjensvoll]

You could do it in two steps:

generate active = year==2019
bysort id: egen active_player = max(active)

Edit:
I didn't notice that Joseph Coveney posted something similar. Interesting to see if the egen function works that way, I didn't know that it did.

[Nick Cox]

The pertinent syntax for the command (not function) egen runs

max(exp) (allows by varlist)
creates a constant (within varlist) containing the maximum value of exp.

where the crucial detail is that you can feed max() an expression, which can be more complicated than a bare variable name. Thus

year == 2019

is such an expression. It evaluates to 1 when true and 0 when false, which is what is needed here.

[Noah Spencer]

Both Joseph and Nick's solutions worked for me. Thank you for your help!