Trick Questions

destraynor

Thursday, January 26. 2006

Trick Questions

Computer Science Education Programming
I said in a comment earlier that trick questions are only good for testing the anomalies of the language. This is another screwed up piece of code my system produced that (somewhat) verified it.
  1. class x{
  2. public static void main(String args[])
  3. {
  4. int x = 17;
  5. int y = 013;
  6. System.out.println("x+y = " + (x+y));
  7. }
  8. }
  9. What does this print?

I'm sure many of you have seen this style of question before, and you all probably know that the answer 28. If you're surprised by that, don't worry. Its supposed to be surprising. Its a trick question. What happens if we remove the brackets around the x+y?

  1. class x{
  2. public static void main(String args[])
  3. {
  4. int x = 17;
  5. int y = 013;
  6. System.out.println("x+y = " + x+y);
  7. }
  8. }
  9. What does this print?

What does it print now? Anyone? Okay, the answer is 1711. Did you get it? See I don't like these types of questions, because they only provide information under one circumstance.

If someone gets both of these questions right, I can assume that they have reasonable proficiency in tracing the execution of a small piece of java. If they get it wrong, I can't really say they are bad at tracing, nor can I say much about their skill at all. Its similar to the notion of "acceptance" in complexity theory, I can say if they're good, but I can't say if they're bad. Thats what trick questions are all about. They should only really be used as a last question on a homework sheet, or thrown out during a lecture to keep the A++ students awake. Everyone else just sees it as yet another question they got wrong.

Bad lecturers like to use them to show how great and cool they are at programming, the same way they do other ridiculous things like not bracketing loops, or starting loops with while (i-- <= str.length()). It is to give them a little shot of confidence in their own ability to write (bad) code, but as a side effect it also gives them another reason why they are shit at lecturing. So, long story short, be careful before using the tricks of a language. You're only really impressing yourself.

Posted by Des Traynor in Computer Science Education, Programming at 06:52 | Comments (10) | Trackbacks (0)

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Ah, thanks for the clarification Des. I had an inkling that the wisdom of using the questions in that other blog post as a peer guidance might have been a little questionable.
#1 Paul Mara (Homepage) on 2006-01-27 02:20 (Reply)
im suprised by the answer 31 for the top piece of code.
is that a double trick question or a typo?
#2 niall m on 2006-01-27 03:05 (Reply)
Sorry, yeah, its 28. I have edited the post.
#3 Des Traynor (Homepage) on 2006-01-27 03:48 (Reply)
A very good point Des. I'm a firm believer that, when used in a limmited way, trick questions can be usefull but like you said over-use in exams is a really bad idea. They can, however, serve a very usefull function if used AND EXPLAINED IN DETAIL in lectures. When students can work through a trick question and not be confused then you have really thought them the language. It's all about using them right and in moderation. Get it wrong and you will just confuse, depress and disshearten your students and that is absolutely not the point.
#4 Bart B (Homepage) on 2006-01-27 06:53 (Reply)
isn't it "30" and "1713"?
#5 leon (Homepage) on 2006-02-16 22:48 (Reply)
No, its 28, and thats the trick.

Any integer beginning with a zero in Java is treated as an octal (i.e. base 8), so 013 actuallys is (1x8) + (3x1) == 11.

As I said in the post, if someone gets this answer right, I can safely assume they know a bit of Java, but if someone gets it wrong, it's wrong to assume anything.

Thanks for reading
#6 Des (Homepage) on 2006-02-17 07:10 (Reply)
i just like to point out that the octal result you used to calculate 28 is wrong.

You wrote:(1x8) + (3x1) == 11

Actually the answer is 8+2+1 = 11

thats cos the octal for 013 is:
000001011
#7 Tony on 2006-12-14 20:16 (Reply)
Hi Tony,
I think you're confusing Octal and Binary.
Counting in Octal means that all numbers are represented using the digits 0-7, to represent varying powers of 8.

1011 is binary for 11, that is true but
13 is octal for eleven, and Octals is what I was talking about here.

Just so ya know.

Des
#8 Des Traynor (Homepage) on 2006-12-14 20:25 (Reply)
Google is the best search engine
#8.1 Stacy Harris (Homepage) on 2007-01-16 23:55 (Reply)
I realize this article is old but I felt compelled to comment on it. I have been breezing through college Computer Science classes that seem to test just these kinds of questions.

First it started in high school with the Computer Science AP exam. An exam meant for high school students to receive College level credit. It was basically a big exam with these exact questions. I was told up front that points would be taken off for using .length or .size() inappropriately. Not a problem for me, I liked syntax tricks, they are easy for me... but what does this have to do with Computer SCIENCE.

Seems like the same thing in college. A third year class, introducing C++ had us actually write some Object Oriented programming project (ohhhh) but what showed up on all of the test? Not an algorithm question, nothing about deciding which data structure to use and how to go about it, instead we have "find the error in this code" and "explain what this code means '**ptr++;'."

This actually leads to the other arguments I have seen on the Internet... the serious degradation of quality programmers. I haven't felt challenged, nor have I even been presented with a real challenge at college and I find it almost shameful. When did Computer Science go so wrong?

To maybe clear it up the math a little more, Octal is base 8. That means to convert to our usual base 10 you would perform the following.

The right most digit's decimal value is (digit x 8^0). That is 8 to the zeroth power. Followed by the next digit (digit x 8^1) meaning 8 to the first power. Continuing to increment the power of 8 for each digit. Just like binary, just like any "base" number. Therefore:

Octal 013 =
(1 x 8^1) + (3 x 8^0) =
(1 x 8 ) + (3 x 1) =
8 + 3 = 11 (Decimal)


----
1) Comments don't seem to like astericks.
2) An '8' followed by a ')' produces a smilie =)
#8.2 Joseph Pecoraro (Homepage) on 2007-08-26 20:28 (Reply)

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This website is the online diary of me, Des Traynor, a User Experience Researcher in Dublin, Ireland. I work with Contrast.

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