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copied!<p>A trivial one is to ask them to code up a breadth-first search of a tree from scratch. Yeah, if you know what you're doing it is trivial. But a lot of programmers don't know how to tackle it.</p> <p>One that I find more useful still is as follows. I've given this in a number of languages, here is a Perl version. First I give them the following code sample:</p> <pre><code># @a and @b are two arrays which are already populated. my @int; OUTER: for my $x (@a) { for my $y (@b) { if ($x eq $y) { push @int, $x; next OUTER; } } } </code></pre> <p>Then I ask them the following questions. I ask them slowly, give people time to think, and am willing to give them nudges:</p> <ol> <li>What is in @int when this code is done?</li> <li>This code is put into production and there is a performance problem that is tracked back to this code. Explain the potential performance problem. (If they are struggling I'll ask how many comparisons it takes if @a and @b each have 100,000 elements. I am <em>not</em> looking for specific terminology, just a back of the envelope estimate.)</li> <li>Without code, suggest to make this faster. (If they propose a direction that is easy to code, I'll ask them to code it. If they think of a solution that will result in @int being changed in any way (eg commonly order), I'll push to see whether they realize that they shouldn't code the fix before checking whether that matters.)</li> </ol> <p>If they come up with a slightly (or very) wrong solution, the following silly data set will find most mistakes you run across:</p> <pre><code>@a = qw( hello world hello goodbye earthlings ); @b = qw( earthlings say hello earthlings ); </code></pre> <p>I'd guess that about 2/3 of candidates fail this question. I have yet to encounter a competent programmer who had trouble with it. I've found that people with good common sense and very little programming background do better on this than average programmers with a few years of experience.</p> <p>I would suggest using these questions as filters. Don't hire someone because they can answer these. But if they can't answer these, then don't hire them.</p>

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