Job Interview Challenges

Question

The other day, in our PHP class, our teacher gave us a challenge used by a friend of his in job interviews. It works in every programming language, so it's not limited to PHP. He said that his friend uses this 'riddle' to weed out the people who can't think of a fast answer when it comes to logical challenges. The people that don't solve it won't get a job, of course.

The riddle is as follows:

    $a = 3;
    $b = 7;

    echo "a = $a";        // has to become 7
    echo "<br />";
    echo "b = $b";        // has to become 3

You basically have to switch the contents of both variables without doing lame things like $b = $a + 4. You cannot use a temporary variable either! I struggled with this, I have to admit that; I was like 'oooh yeah' when we finally got the answer.

I don't want to spoil this for anyone, so instead of posting the solution I'll just put a link.

Now, as for my question. I was wondering if there are more riddles like these out there, that people (that's you, SO) use in job interviews, etc. Perhaps even a bit harder than this one. My goal is to train my logical thinking a bit and get better in solving issues like this. Perhaps there are any books or websites out there devoted to stuff like this?

Answer 1

This isn't a "riddle", it's just an obsolete algorithm (the XOR swap algorithm).

The problem with the question (and others like it) is that the only thing it can tell an interviewer is whether or not the programmer has happened to memorize an odd bit of programming trivia. As an interviewer, I choose not to waste my or the interviewee's time with such nonsense. There are far more important things to do with that time.

Answer 2

First I totally agree with the comment from Will.

Second to answer your question look at this site for similar questions

Also the following two programmers questions can give you enough to study.

• Where can I find programming puzzles and challenges?
• https://softwareengineering.stackexchange.com/questions/12599/alternatives-to-project-euler

Answer 3

You could also do this with an XOR swap:

$a = $a ^ $b;
$b = $a ^ $b;
$a = $a ^ $b;

Technically it might be faster than the addition/subtraction, but really it's splitting hairs.

Answer 4

Actually this does NOT work on any langues. It works only for a very limited number of languages. So this is just a PHP related thing (can't actually think of any other language where this works).

To answer your questsion: There are many "riddles" like this, but the point about interviews is not always solving the riddle. If you can explain your thoughts right you could still get the job (also if your given answer isn't correct).

However I have never encoutered a riddle twice, so my guess is that every company creates their own puzzles that suits their needs (including language, etc).

Answer 5

I wonder what Joris' teacher's response was.

The biggest problem with this is that the trick is obscure and anyone who used it in a professional programming environment is really adding difficult to maintain code. This is a huge problem for a professional place, anywhere that has very large codebases needs easy-to-understand code so future workers can see what its doing without spending large amounts of time grokking code, or worse - not understanding it and making a mistake.

If you need an example, look at the Story of Mel. Very fancy code, neat trick, useless professional programmer. You might say the only reason this is acceptable is that it was a very tight environment where such tricks made a difference. In 99.999% of modern environments this would be marked as requiring rework.