Generating lagged variable for the groups in panel data

[alper alper]

Hello, I am trying to generate a new variable "z" that is the previous year's value of the variable "y" for every group of "x" as in the table.
As you see in the table, it should be missing at years of 2000.
I tried this code: bysort YEAR (x): egen z = l.y
But it gave me "z" values for years of 2000 that is not sensible because we don't have info for the year 1999.
Thanks

YEAR	x	y	z
2000	1004	2.64
2001	1004	-8.299	2.64
2002	1004	-1.807	-8.299
2003	1004	0.494	-1.807
2004	1004	2.536	0.494
2005	1004	3.592	2.536
2000	1013	21.864
2001	1013	-51.514	21.864
2002	1013	-100.07	-51.514
2003	1013	-5.914	-100.07
2004	1013	2.192	-5.914
2005	1013	5.57	2.192

[Carlo Lazzaro]

Alper:
welcome to this forum.
As far as I can see your results (that you should have better reported via CODE delimiters, as per FAQ) the values for 2000 are missing.

[William Lisowski]

Building on Carlo's reply, if you don't think the lagged value of y in 2000 should be the missing value (represented in Stata by a numeric value of . as described in the output of help missing) then what would you expect it to be, since you do not have data on the value of y in 1999?

[William Lisowski]

I thought perhaps I had misunderstood the example data in post #1: maybe that displayed what is wanted rather than what the code produced. The example below shows three ways of getting what you want; I note that the code you show in post #1 produces an error message and does not create the variable z.

* Example generated by -dataex-. For more info, type help dataex
clear
input int(YEAR x) float y
2000 1004    2.64
2001 1004  -8.299
2002 1004  -1.807
2003 1004    .494
2004 1004   2.536
2005 1004   3.592
2000 1013  21.864
2001 1013 -51.514
2002 1013 -100.07
2003 1013  -5.914
2004 1013   2.192
2005 1013    5.57
end
xtset x YEAR
generate z1 = l.y
bysort x (YEAR): generate z2 = l.y
bysort x (YEAR): generate z3 = y[_n-1]
list, clean

. list, clean

       YEAR      x         y        z1        z2        z3  
  1.   2000   1004      2.64         .         .         .  
  2.   2001   1004    -8.299      2.64      2.64      2.64  
  3.   2002   1004    -1.807    -8.299    -8.299    -8.299  
  4.   2003   1004      .494    -1.807    -1.807    -1.807  
  5.   2004   1004     2.536      .494      .494      .494  
  6.   2005   1004     3.592     2.536     2.536     2.536  
  7.   2000   1013    21.864         .         .         .  
  8.   2001   1013   -51.514    21.864    21.864    21.864  
  9.   2002   1013   -100.07   -51.514   -51.514   -51.514  
 10.   2003   1013    -5.914   -100.07   -100.07   -100.07  
 11.   2004   1013     2.192    -5.914    -5.914    -5.914  
 12.   2005   1013      5.57     2.192     2.192     2.192

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