Ah, Those Edge Cases! (11632)

The 27 March quiz tested your knowledge of collections, and the kinds of assumptions you might make when writing a program that accepts a collection as a parameter.

The way the quiz was formulated was different from the usual, and it definitely caused some frustration (one player wrote "Maybe it is because I was awake for about 2 hours last night doing some DBA work for a client, but I did not even understand the question that was being asked. Even after looking in the Library to see the Topic/Summary, my co-worker and myself are still confused on what exactly was being asked." and another: "This question was confusing. I assumed that the requirements were the requirements of the code that was being called. In other words, if all the conditions are met the code will work properly, not does this code enforce all the conditions.").

Our very smart and dedicated reviewers caught many issues with this quiz, and we spent a fair amount of time discussing ways to tighten up the quiz (and I followed almost all of their advice). Yet, in the end, we were "defeated" (that is, an issue was identified in the way the quiz was scored) by an edge case regarding string-indexed arrays - which was covered in the 22 March quiz.

Several players pointed out that choice 9991 (the choice that was scored as correct and stated that no requirements applied) would require an integer-indexed array, since the algorithm would not correctly display all elements in the collection if one of the index values was the empty string.

How right they are, as one player demonstrated with a script:

SQL: CREATE OR REPLACE PACKAGE plch_pkg AS
  2     TYPE my_collection IS TABLE OF VARCHAR2(10) INDEX BY VARCHAR2(20);
  3     index_type VARCHAR2(20);
  4  END plch_pkg;
  5  /
 
Package created

SQL: CREATE OR REPLACE PROCEDURE plch_proc(numbers_in IN plch_pkg.my_collection) IS
  2     l_index plch_pkg.index_type%TYPE := numbers_in.FIRST;
  3  BEGIN
  4     WHILE (l_index IS NOT NULL) LOOP
  5        DBMS_OUTPUT.put_line(numbers_in(l_index));
  6        l_index := numbers_in.NEXT(l_index);
  7     END LOOP;
  8  END;
  9  /
 
Procedure created

SQL: DECLARE
  2     l plch_pkg.my_collection;
  3  BEGIN
  4     l('b') := 'b';
  5     dbms_output.put_line('without null index');
  6     plch_proc(l);
  7     l('') := 'a';
  8     dbms_output.put_line('with null index');
  9     plch_proc(l);
 10  END;
 11  /
 
without null index
b
with null index
 
PL/SQL procedure successfully completed

Another players also wrote to say that "There is a requirement that the collection be non-empty, but no requirement that the collection is initialized (relevant for nested tables and varrays)." This is true. We did not include as a requirement in the list the need for a collection to be initialized. And we did not specify that you should only consider the requirements in the list.

What's a quiz administrator to do?

1. My biggest problem with this quiz is the possible complications for future quizzes. It is one (interesting) thing to know that an empty string as index value can cause odd behavior. It is quite another thing to have to account for this scenario in every quiz on string-indexed arrays. Seems like an unnecessary complication. So I plan to add to the assumptions for the daily PL/SQL quiz that unless otherwise mentioned, all index values of string-indexed collections have a non-NULL length. Any objections to my doing this?

2. Add to this quiz a statement to the effect that only those requirements listed in the question are to be considered. That is, there might be other requirements (such as "The nested table or varray must be initialized.") that could apply, but are not to be considered in this quiz.

3. Give credit for a correct answer on 9991 to all players who submit feedback indicating they marked this choice as incorrect because of the possibility of an empty string index value. I very much doubt that many players recalled this edge case and applied it to this quiz, but if you did, let us know and we will give you credit.

4. Give credit to any player who marked all choices as incorrect and submit feedback indicating that they did so because we did not include a requirement specifying initialization of nested tables and varrays. I say "all choices as incorrect" because if you thought of this additional requirement, then none of the choices could be correct (only the first even includes the "must be associative array" requirement).

Did I miss anything?