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I have an if or while command in my program that only seems to evaluate the first observation. What’s going on?

A common mistake is to use the if command as the argument. This is incorrect Stata syntax. The if command was designed to be used with a single expression (often a local macro) inside programs and do-files. Using this command incorrectly results in the evaluation of the expression using only the first observation of the variable. The same behavior applies for the while command.

 . list

      +---+
      | x |
      |---|
   1. | 1 |
   2. | 1 |
   3. | 2 |
   4. | 3 |
   5. | 2 |
      |---|
   6. | 3 |
   7. | 1 |
   8. | 2 |
   9. | 1 |
  10. | 3 |
      +---+

 if x == 3 {
 replace x = 4
 }

 . list

      +---+
      | x |
      |---|
   1. | 1 |
   2. | 1 |
   3. | 2 |
   4. | 3 |
   5. | 2 |
      |---|
   6. | 3 |
   7. | 1 |
   8. | 2 |
   9. | 1 |
  10. | 3 |
      +---+

At first glance, one would wonder why this did not work. From a logical viewpoint, there is nothing wrong with the command. As mentioned above, Stata was designed to evaluate only a single expression for the if command, so it is evaluated only for the first observation, x = 1.

  if x == 1 {
   replace x = 4
  }

  (10 real changes made)

 . list

      +---+
      | x |
      |---|
   1. | 4 |
   2. | 4 |
   3. | 4 |
   4. | 4 |
   5. | 4 |
      |---|
   6. | 4 |
   7. | 4 |
   8. | 4 |
   9. | 4 |
  10. | 4 |
      +---+

This is obviously not what the user intended. The solution to this problem is often to rewrite the command using if as a qualifier rather than as a command. As a qualifier, if will evaluate each observation as the command passes over the data.

 . replace x = 4 if x == 3
 . replace x = 4 if x == 1

As noted earlier, the same behavior will be seen when using the while command.

 . list

      +---+
      | x |
      |---|
   1. | 1 |
   2. | 1 |
   3. | 2 |
   4. | 3 |
   5. | 2 |
      |---|
   6. | 3 |
   7. | 1 |
   8. | 2 |
   9. | 1 |
  10. | 3 |
      +---+

 while x == 3 {
  replace x = 4
 }
 
 . list

      +---+
      | x |
      |---|
   1. | 1 |
   2. | 1 |
   3. | 2 |
   4. | 3 |
   5. | 2 |
      |---|
   6. | 3 |
   7. | 1 |
   8. | 2 |
   9. | 1 |
  10. | 3 |
      +---+

 while x == 1 {
  replace x = 4
 }
 (10 real changes made)

 . list

      +---+
      | x |
      |---|
   1. | 4 |
   2. | 4 |
   3. | 4 |
   4. | 4 |
   5. | 4 |
      |---|
   6. | 4 |
   7. | 4 |
   8. | 4 |
   9. | 4 |
  10. | 4 |
      +---+