st: How to check if observation of a particular variable value exists? If not, create that observation.

[Ran Gu <lionup@gmail.com>]

Dear Statalisters:

Good day. I hope you can help me with the following question.

Suppose I have a data with 2 variables, say A and B, both with values
1 and 2. The data are sorted by A and B. Then there should be 4
records: (A=1 B=1) (A=1 B=2) (A=2 B=1) (A=2 B=2)

Suppose I don't know whether the 4 observations are all exist or not.
How could I check that in a do file?
Then, If a record is missing, say (A=1 B=1), I would like to create it.

for example:
forvalues i = 1/2 {
  forvalues j = 1/2 {
    if obs with A == `i' and B == `j' is missing {
      local new = _N+1
      set obs `new'
      replace A == `i' if _n==`new'
      replace B == `j' if _n==`new'
    }
  }
}


-- 
All the best,
Ran
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