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Comment by lmilcin | Hacker News Reader

If you write a small piece of code and can't tell if it is going to work or not you don't really understand what it does.

We are not talking using external dependencies or frameworks. I am talking simple code that you just wrote in entirety, maybe couple of loops and conditionals.

If you can't predict what that small piece of code does, exactly, you are thoroughly unable to write reliable code, by definition.

Reliable code == code that does exactly what you intend it to do, nothing less, nothing more.

If you can't predict what your code does you are writing buggy code. Experimentation can uncover some of the bugs, but if you start with buggy code the experimentation will never remove all bugs -- only the ones that are easy to detect.

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I think different companies view this differently. I recently did an interview with a social media company (not Facebook, one of the other ones) where I wrote up a solution to the problem that "just worked" in one go, as did my extensions. After the interview, when I asked for feedback, the interviewer told me that I should have been running and testing my code as we went along. However, I've also heard in mentoring sessions from engineers at other companies that the goal in interviews is to write code that works the first time without having to test along the way.

In general, I prefer the approach of writing out the entire solution at one go, and trusting that bugs will be easy to fix at the end if the code is clean enough to understand what is happening at every step. This is in part because there usually isn't enough time in interviews for running tests at every crux moment. However, I think our current virtual environment may make it easier to test for (and to utilize) these types of programming best practices, since instead of writing code on the whiteboard, candidates and companies can use one of the interview interfaces that allows running code as you write it.

> After the interview, when I asked for feedback, the interviewer told me that I should have been running and testing my code as we went along.

Have they specified that you should be testing your code? If they haven't specified then it is wrong to expect some kind of arbitrary outcome.

People write code in many different ways, some do it very chaotically, some have very organized process.

I try to figure out which of these behaviors are fundamental to writing good code and which are not.

For example, I believe a habit of writing readable code IS fundamental to writing good code. If you don't have a habit of writing readable code, on average your code is going to be worse than if you had.

On the other hand, I believe writing unit tests IS NOT fundamental. Unit tests is one possible way of working with the code, but people were writing reliable code before unit testing became popular. Unit testing takes time and effort that can be allocated differently.

And so I try to look (I can't call it measuring because it is subjective) only at the behaviors that I believe are fundamental and dismiss others.

In the end, if you are smart person that can program well, you will be able to learn those other behaviors. But if you are not smart or you lack certain fundamental abilities, it is going to be much more difficult to fix it.

I do an interview in which we do TDD on a problem together. So we are frequently running tests during the interview. However, one of the things that's expected from the candidate is that they will be able to predict what happens when we run tests. Will the tests pass? Will an exception be thrown? How will it fail?

Maybe as do often the difference in this discussion is a quantitative one not one in principle.

Running tests frequently is not a problem. Not knowing what the code will do is.

Testing code as you go along is not experimentation. It saves time by allowing you to verify your assumptions are correct in a controlled way.

You can sit there and plan all you want until you think you understand it, but you never actually know until you run the code. Being a good programmer is all about simplifying complexity. Testing your code as you go along to verify your assumptions is one more way to do this.

It doesn't matter how you call it. If you know the code works then there is no benefit of running it. If you need to execute it it is only because you don't know if it works correctly.

Sometimes this is fine. I do this when I execute external services because I may not be sure how they work exactly and I want to correct mistakes before I do too much design resulting from my flawed knowledge.

But when it comes to your code you should be able to tell if it works correctly. If you can't there is couple of ways to deal with the problem that I adopted over the years:

-- techniques that make it easier to ensure code works correctly are frequently also improving it in other ways. For example, making code simpler is important not just for reliability.

-- specifying your internal module boundaries and APIs is critical for anything non trivial. Abstracting parts of the problem is enabling you to solve each part of the problem separately. You can't keep 10k lines of code in your mind and tell if it works correctly, but maybe you can split it into multiple modules and make sure each works correctly, separately.

-- when I have written a module that I am just not sure works correctly, I refactor it so that it is easier to tell if it does what it should be doing.

-- make it a habit to write code that is what I call "obviously correct". One way to do it is to make sure it can never get into wrong state.

> If you know the code works then there is no benefit of running it. If you need to execute it it is only because you don't know if it works correctly

People make mistakes. You have to run the code. Do you submit PR's without running code? If not, then we're agreeing. Making code simple enough to reason about without executing is a great goal, but you cannot replace running code.

> You can't keep 10k lines of code in your mind and tell if it works correctly, but maybe you can split it into multiple modules and make sure each works correctly, separately.

But ultimately they need to work together, and that's the most important part. Again, you need to run the code here.

> when I have written a module that I am just not sure works correctly, I refactor it so that it is easier to tell if it does what it should be doing.

This is a good goal, though again, I hope you run the code. You don't know what you don't know.

You have a good point overall. However, can you truly reduce the definition of reliable code to something that simple? I think there are a lot more factors that determine if code is reliable or not.

I actually think that this is good definition of reliable code -- product of an individual developer's process.

It does not say whether it was specified well, but I think an application can reliably execute wrong process. There is no contradiction.

I don't want to say that making sure that your code does exactly what you intend it to do is at all simple.

As your application grows, as you start incorporating dependencies, external systems, code that came from others, things that you do not know very well -- writing code that does exactly what you intend it to becomes very difficult or even impossible.

But on the lowest level, when we are talking couple of variables, conditionals, loops, maybe small data structure -- I would expect a developer to be able to write it to do what is exactly intended.

Because if you have that ability and use it for every piece of code that you write, the overall result (even if it does not guarantee reliability) will be better than a person that cannot do it.

In my experience (and it depends a lot on our HR and candidates that are being sent my way) only one in about 10 candidates (for senior Java dev position) can do that.

> If you write a small piece of code and can't tell if it is going to work or not you don't really understand what it does.

And if they don't understand it, they're going to B*&^% and Moan about having to fix the bugs once they come back.

The good news is their bitching and moaning won't last too terribly long. The bad news is that's because it will become clear they can't fix the bug, and someone else will end up doing it.

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