varlist[#] - what do the brackets do?

[Marco Gallo]

Hi all,

as my title suggests, I can't figure out what square brackets behind varlist does. For example

 
 bysort id: drop if year[1] < 10

Does this drop all observations with a certain id if the first observation with that id fulfils the condition year<10?

Thank you in advance,
Marco

[Carlo Lazzaro]

Marco:
after having sorted for -id-, Stata will drop all the observations for which the first value of the variable -year- is lower than 10.

[Nick Cox]

What's the intent here? This might make more sense

bysort id (year) : drop if year[1] < 10

as otherwise you are at the mercy of year being unsorted for each identifier (with nothing else said).