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If you can use open source, do it. The interviewer wants to assess how you work - if someone spends ages writing a library that already exists somewhere, they're actually spending time writing bugs someone else has already fixed, when they could be using that library to add features. That's not an effective use of time.

I interviewed for an insurance company a few years ago in London, and their programming task was clearly algorithmic. There was a problem description, inputs, expected outputs, and no library in the world existed that could do it for me. They clearly wanted to assess my algorithmic skills.

Around the same time I interviewed for a company in Manchester where their main product was dealing with phone numbers. Although not the main point of the exercise, I still had to use some utilities related to telephone numbers, so I started writing a parser for that. Then I looked online briefly, and replaced everything with something made by Google for Android, which did the same thing a lot better than I could. I mentioned this in the interview, and it was clear I made the right decision, because parsing phone numbers was not the main task.

So, go with your gut. If you can use open source and it helps you save time, do yourself a favour and go for it.

One thing that you must check before you use open source software is the license.

If the company normally releases closed-source software, they will never let you use a program or library that is released under the GPL for a real programming task, because if they did they'd be forced to open source their software too. Chances are that they'd forbid the use of such software even for an interview task.

The best approach is to ask a direct question. As Player One said, they will appreciate your asking instead of assuming. If they say OSS is fine, go for the second question. Something like:

Can I use open source software even if it's covered by a copyleft license, like the GPL, which implies that the end product must be released under the same terms? I realize this is just an interview question and maybe it doesn't matter in this context, but when doing real work I'd certainly have to check with you to avoid potential legal problems.

This will let them know that you understand licensing problems, and that you aren't one of those code monkeys that blindly copy and paste whatever they find on the web without evaluating the implications.

At that point, regardless of what they answer, you have scored a point.

Justify and attribute the use of any open source code that you use

What is the line between using open source code and writing the code by your own for completing an interview task?

There's no hard and fast line. It depends on what the interviewer is trying to assess.

A general advise in software development is to use tried and tested code. That's one of the crucial purposes of open sourcing code, i.e. not to reinvent the wheel every time.

However, here writing code is part of an interview task. There isn't a precise Yes/No answer to whether you should use open source code or not. But it tends towards Yes, if you can justify the use of it.

Think from the point of view of the interviewer. They are trying to assess your skills to get a task done. While one of the intent of getting a candidate to write code is to see their ability to write the solution for a problem, it's also an important skill to assess, how well they can identify and obtain pre written code to solve a problem.

There shouldn't be any problem in using an open source library or component, but it would be best to give proper attribution and reasoning why you chose to use it.

It is important for the interviewer to understand how well you understand the problem and the code that solves the problem, instead of writing the code yourself.

However, though process and intent can vary from interviewer to interviewer. You can propose the interviewer about using certain open source components along with the reasoning behind using them. Do it before starting the task. Some interviewers may be precisely looking for your skill to implement a certain functionality and may want you to write the code from scratch.

Depends:

• If it's more an algorithmic/data crunching task, then probably better show understanding of the algorithm (e.g. tweak it for the specific usecase)


• if it is an 'show that you can create an whole app' task, then show that you can use preexisting building blocks (open source, or the frameworks the company asked for)

I would specifically ask your new employer this question. In the end, 90% of those tasks are used, so your employer can form an opinion on your skills.

HOWEVER: Every employer might be looking for something different in your task. I know some departments (like core development) are looking for potential candidates to perform very well "from the ground up", meaning they want to see how you are working with an array "by hand" and implement even the most basic algorithms by yourself. They want to assess your knowledge of a programming language even at it's most basic level (which is often considered as the most important the more "techy" it gets)

Other departments want to see, that you just get the job done and are up to date with the latest frameworks, open source solutions and the standard library and would probably count some things, that aforementioned department would have counted as a plus, as a big minus.

Some companies are just pretty old school by nature, some companies are all about modern solutions. The problem for you is, you can never know and while you may deliver a solid, reasonable solution with, or without open source components, it might just be missing, what the company is looking for.

So from my experience, the best way to find out about this and maybe even to catch a few bonus points by delivering exactly what they are looking for is to ask upfront.

Good luck!

Adding to what has been said so far by others that I totally agree with, I would like to point out here that most open source licenses have an attribution clause and/or require you to keep the license information intact for any derived work. As well as you often aren't allowed to promote the software as your own work if it isn't.

So even if you go against any advice given by the other answers, if that open source code is under a license having any of the before mentioned clauses, not disclosing to the interviewer that it is open source in the required way, would be a license infringement and could possibly backfire depending on the employer's view towards compliance.

Ask the interviewer to be sure. But a general rule of thumb- depends on how much code you're taking from the open source library. If the task is algorithmic, don't take something that writes the main algorithm. If the task is to write an app, don't find a version of the app online and take it. If its a framework and you're going to write the actual code of your app (algorithms and display), that's usually fine.

For example, a workplace of mine did a test where they had a few hours to write Minesweeper. Someone once found an entire online version and made minor tweaks. We didn't offer them the job, because we didn't know how well they could program from that. On the other hand, someone else used a widget to display the timer in a pretty way. That was totally cool with us. There's one algorithmically difficult part to Minesweeper (the algorithm to open up adjacent squares efficiently when they click on a 0). They didn't need to get that to its most efficient version, but I don't think we'd have passed anyone who didn't write it themselves.