Question

We’re looking for someone to help us enhance & maintain our high-quality, php-based prototype of a transactional web app. Ideally, who can communicate well, and do both front- and back-end web development (as well as smart/gets things done, etc.). Among other more general things, I’ve been using this question:

Given this:

    $foo = array(1, 3, 7);

write a function (on this whiteboard) to sum the values of the array.

It seems trivially easy to me, but has caused a couple of deer-in-the-headlights situations, which always makes me feel like a villain.

I’ve read a number of posts here and there (including both Joel’s and Jeff’s) saying that how candidates think, design skills, passion, etc. are more important than any specific technical skill, and I agree with that. Also, I can see that programming at a whiteboard is a little unrealistic. OTOH, this seems so basic to me that I’m inclined to see it as a fine first-pass filter between front-end devs (who know their way around html, css, and how to copy-and-paste a js function or two), and people who can really code. Thoughts?

A touch more info: I’m open to all sorts of answers: array_sum, a for loop, a foreach loop. Heck, if they want to write an ArraySum class, that would be overkill, but just fine. Using javascript, or another language would be fine, if they’re more comfortable with that. Even attempts with minor errors would be ok, but I’ve had a couple of complete freezes, so I just wanted to sanity check myself.

Answer 1

in my opinion, the question is perfectly valid, and tells a bit (not a lot, but an important bit) about the candidate, depending on how much time you invest in it. just tell her that it’s not a trick question beforehand, that it’s really, really as simple as it appears, so she doesn’t spend to much time thinking about the pitfalls and to minimize the deer-situation. throw in you do that just as a measure to filter out the people who apply for a programming without actually knowing anything about it but hoping they get hired anyway by pure luck (if they know how to code but are just nervous, that should take a bit of pressure away). let them code, but don’t focus if there is a $ missing or if the <?php tags are present or not.

if she provides array_sum or sum_array as an answer almost doesn’t matter, especially if the language in question is php (but if two candidates are equal otherwise … i can’t even remember the last time i had to use this function). and the use of an auto-completion and syntax-coloring (with predefined keywords) IDE vs. a dumb text editor matters a lot in this hindsight. in this case ask for an alternate, handcrafted solution.

if i was in the position of asking that question i wouldn’t ask for the right solution, i’d ask for ways that come to her mind how this problem could be solved, what pitfalls could arise in special cases. try to find out what she knows about programming, not what she knows about php. try to find out about intelligence, problem solving and creativity. altought experience matters a lot even when it comes to bang out code fast, it’s not a constant.

the solutions i’d provide, the pros/cons and what it tells about me …

• built-in array_sum (very fast and definitley not buggy, but inflexible): i have a bit of experience with traditional php projects

• for/loop constructs (good enough, reinventing the wheel. but can be used if there are different objects than numbers. pros: everybody will understand it): i can solve simple problems if there are no predefined copy&pasteable solutions

• array_reduce (with an offering to implementat array_reduce, if the interviewer wants to see it): unusual for a php programmer, so it seems i have knowledge and experience outside of the php sandbox

• an ArraySum-Object (with an ArraySum::add($value) method that keeps all values stored but caches the sum): i’m used to at least some of the oop-principles

• function () { return 11; } (with the disclaimer that this is a joke solution, but valid): i have (albeit crude) programmer-specific humour – a sign i’m personally interested in programming outside of work … some interviewers who are programmers (but not hackers) might interpret this as a willingness to use dirty hacks as placeholders (aehm) if time constraints are too tight

• a recursive solution would be nice. i can probably solve a bit more complex, algorithily problems too and most likley know my way around simple trees and data structures

• recursive divide and conquer: bonus! i know even more about algorithms.

try to get as much as possible out of this question (if time permits). in the end you’ll know a little bit about programming capability and a lot about experience (altought not necessarily PHP specific).

i’d choose this question over letting the candidate write out quicksort – a very specific question about knowledge almost never needed in the web dev world – any time.

disclaimer

the question is useless when …

• the interviewer is not a programmer. forget it if a hr-guy is doing it
• there’s a very tight time constraint when interviewing. then the result is almost random anyway.

additionally, who are you looking for? if you need a cheap grunt coder, even a simple question like this should work. if you need quality and experience, don’t waste too much time on it (but do it anyway).