* Construct lags of an existing variable
* Sean Becketti. 9-89
capture program drop lag
program define lag
version 2.1
qui {
  if "%_*"=="" {
    di in bl "-> lag varname [in range][if exp], Nlags(#) [Suffix(s)]
    exit 198
  }
  mac def _varlist "req ex min(1) max(1)"
  mac def _if "opt pre"
  mac def _in "opt pre"
  mac def _options "Nlags(int 1) Suffix(str)"
  parse "%_*"
  if "%_suffix"=="" {
    mac def _suffix = "%_varlist"
  }
  mac def _i = 1
  mac def _compare = "<="
  mac def _plus ="+"
  mac def _L = "_L"
  if %_nlags<0 {
    mac def _i = "-1"
    mac def _compare = ">="
    mac def _plus = "-"
    mac def _L = "_F"
  }
  if %_nlags==0 {
    di in re "mlags cannot be zero"
    exit 198
  }
  while (%_i %_compare %_nlags) {
    mac def _j=abs(%_i)
    mac def _k=%_j
    if %_j > 9 {
       mac def _s = substr("%_suffix",1,4)
    }
    if %_j > 1 & %_j <=9 {
       mac def _s = substr("%_suffix",1,5)
    }
    if %_j==1 {
       mac def _s = substr("%_suffix",1,6)
       mac def _k
    }
    mac def _name "%_L`%_k`%_s"
    g %_name = %_varlist[_n-%_i]
    lab var %_name "%_L(%_j) of %_varlist"
    mac def _i = %_i %_plus 1
 }
}
end