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Stata Code Fragment: 
Working Across Variables

* say that you want to look across variables to see
* if any of the variables is zero (iszero) 
* when the variable was FIRST zero (whenzer)
* and the number of non missing variables (nonmiss)

* you can do this by loopoing across observations
* and then looping across variables

* 1. lets make the variables "iszero" "whenzer" and "nonmiss"
capture drop iszero whenzer nonmiss
gen iszero = .
gen whenzer = .
gen nonmiss = .

* 2. here is the list of variable we want to work across
local array "var1 var2 var3"  
local numvars : word count `array'

* 3. set up the loop for looping across observations
local obsno = 1
while `obsno' <= _N {
  * initialize variables
  local iszero = 0
  local whenzer = 0
  local nonmiss = 0

  * variable number
  local varno = 1

  * 4. loop across variables
  while `varno' <= `numvars' {
    local varname : word `varno' of `array'
    if (`varname'[`obsno'] != .) {
      local nonmiss = `nonmiss' + 1
    }
    if (`varname'[`obsno'] == 0)  {
      if (`iszero' == 0) {
        local whenzer = `varno'
      }
      local iszero = 1
    }

    local varno = `varno' + 1
  }

  * 5. make replacements based on looping across observations
  replace iszero  = `iszero'  in `obsno'
  replace whenzer = `whenzer' in `obsno'
  replace nonmiss = `nonmiss' in `obsno'
  local obsno = `obsno' + 1
}