Ask HN: Do senior engineers have a mental model before coding?

[unhired]

I failed an onsite interview for a senior position because I didn’t have a mental model of the subtask before coding each function during a 90 minute pair programming test. I only used javascript in a text editor. I ran node in console after small changes and additions to print, test, debug.

The feedback was you can’t do this when compiling time is long and development will be slow. I’ve heard of developers that don’t use debuggers because they have a mental model in their head.

Do most senior software engineers take time to build a mental model before coding and testing?

I prefer to create a draft quickly to build a mental model. Is my way of logging to console frequently and coding before building a mental model a junior habit that I should work on?

Am I unlikely to pass a FAANG interview like this?

Edit: It wasn’t a LeetCode problem. I had to implement an API where each call would change the state of the application on certain conditions. I heard the problem and clarified it on the whiteboard. I coded each function while talking and ask for clarifications.

[jrockway]

Yes, I pretty much have a mental model of what I want to build before I build it. I am not sure that I build the mental model in 30 minutes, though; rather I get an idea, think about it over the course of many days, talk with people about it, write down some design details, etc. Only after all of that does any programming begin, and the actual coding is then the easy part. You know what data structures you need. You know what the external API will look like. You know how to deal with the tricky parts; be it integration or some sort of unusually complex algorithm.

Doing the design work is what separates senior and non-senior engineers. The more details you get worked out before they are solidified by having 10 other systems depending on them, the better.

If you just do stuff with a rough idea of the direction you want to go in, it's very easy to get sidetracked by something that doesn't matter. It's also very easy to build something nobody wants. By having people agree on some sort of semi-detailed design that describes assumptions, corner cases, the API, etc... you dramatically increase your chances of building the computer program that people want.

I am not sure how you test for this in an interview... but it is important. If you don't design things before building them, that is something you should start working on.

[karmakaze]

It's easy to test for in an interview. Just have them describe the solution to you.. at a high level. If it can only be described at the code level, or as a procedural description then there is no abstract model, only implementation.

Usually, I'll think about a problem, some of the inherent characteristics, or desired behaviours then choose some applicable techniques/technologies, languages, algorithms, libraries. Then think about how to compose the program from parts to be developed. This all happens naturally without conscious sequencing.

That's what's great about being current here, I find out about things I don't need to use yet but could be useful at a later time. Or discover something that would have made a past project better if I'd known of it.

[paladin314159]

FAANG interviews aside, I find the idea of having a mental model before you write code extremely helpful. It's a lot easier to understand the purpose of individual functions and how things should interface with each other if you do. It won't be perfect at first, but the more you can get right in the first stab the better.

Your ability to build that mental model depends on both your experience/skill and the complexity of the problem. For simple problems, a more senior engineer might be able to come up with a mental model instantly because it's similar to lots of things they've done before. For really complex problems, it probably takes everyone a long time to build the correct mental model and you'd want to balance that with what you can figure out by diving into the implementation.

My guess would be that the interviewers' line of thinking is that if you couldn't build a mental model for a problem quickly, that means you're not proficient with problems like that. Which is probably true, but opens up the question of whether the interview is testing the right type of problem, of course.

[bb88]

> ...it probably takes everyone a long time to build the correct mental model

To be fair, 90% of most business logic is simple and straightforward, and should be written that way.

The real test of a senior engineer is to quickly see the hard 10% that isn't straightforward, and to design the rest of the system to minimize the effects of the complexities introduced by the hard 10%.

[squirrelicus]

14 years in the industry, and I often describe typing the code as the easy part of software engineering. Once I'm typing the code, I already know in large part what code needs to be typed. The hard part was understanding the problem model enough to know what code to type.

That being said, executing the code while typing it is critical. You cannot build a mental model of all edge cases. At best, your personal discipline will allow you have habits around preventing and catching edge cases. But you won't find enough edge cases just by thinking about the code to type. You do have to discover them by running the code, and the sooner you discover them, the faster you will be at producing high quality code.

As far as how much time I spend building a mental model vs coding... It's at least half mental model time. If I happen to predict enough edge cases, then it might be 90% of the time. But the critical part is that I'm not done until I've exhausted the edge cases I can imagine. If I discover sufficiently big edge cases that brick my code, it's time to step away from the computer and fix my broken mental model.

Note: tests are a way to do this, but so is white box poking and prodding at your program in debug mode.

[mstade]

Are you me? I started coding professionally in 2004 or 2005 (if memory serves) and a few years of tinkering before then. Your process describes mine almost to a T – building the mental model is the hard part, writing it down is mechanics more than anything else. It used to be the other way around; I didn't know how the mechanics worked so I worried more about that than the ideas, they seemed the trivial bit but over time that shifted. Not quite sure when it tipped over, but it definitely did.

(The last nota bene of your comment is particularly striking to me. I love tests and I try to automate them as much as possible, but sometimes you just gotta prod and poke the thing to really get a feel for it.)

[einpoklum]

For me - it's sometimes like that, but sometimes it's the exact opposite: I understand the problem through starting to code. Depends on the coding task; also on the language.

[squirrelicus]

I'm sure everyone is different, but it's worth saying that I find myself spending more time in the writing code phase with dynamically typed languages than statically typed languages.

[nomel]

I don't think it's a good idea to start the implementation firmly planted in the trees of the forest you're trying to see.

I think it's best to start with something like an outline. This could be comments, tests, sparse pseudo-code, or some level of documentation that explains what you're trying to do.

Once you have a rough idea of the pieces that are required and will roughly fit together, understanding the consequences of the decisions you make while implementing those pieces will come more naturally, and can help prevent rework. Most architecture mistakes, and often bugs, are from not understanding the consequences, which isn't really possible without having the mental model.

[johnrob]

Well, in the real world I find that you don’t understand the problem you want to solve until you’ve applied a working solution to it. Practically speaking I tend to deliver quickly to accelerate the process, then refactor once all the requirements have been exposed. As mentioned here, this probably doesn’t project well in an interview. It’s a case where the most efficient practice is not the most presentable one. I’ve failed interviews for this reason too. Lessons learned!

[jconley]

Same experience here. Been writing code for over 20 years. There's a reason we don't do big Waterfall projects any more. No amount of research uncovers all requirements. In the real world, with the ease of refactoring in powerful IDE's these days, there is no reason to spend too much time thinking about a small problem like those presented for whiteboard coding.

In practice, in an interview, this is the wrong approach. I've failed interviews for this as well.

Most of what differentiates a Senior Engineer is communication, mentorship, ability to compromise on product requirements, set realistic expectations, design solutions to big problems, think in the large and hold the whole system in their head, design API's, things like that.

I no longer do interviews that require me to write code. They're just ridiculous for the type of skills I'd hope a company would value in a Senior Engineer.

[ohyes]

I do not know about success in FAANG interviews.

This is just something that happens while I'm "thinking about how to approach the problem." I have an idea of what a computer can do and (hopefully) what the steps might be to solve the problem I'm trying to solve. I'll test out some things in my head and then start typing.

I think different people are different, using a console that doesn't tell me that you didn't have a mental model of how the program would work, it just tells me that you like to test as you go. That's a good, careful, practice and not a red flag to me at all.

read-eval-print loops are a time honored tradition in a number of communities. I've written entire programs by typing into a REPL and dribbling the results as I go. Languages with long compile times are genuinely unfun to work in.

So i guess my question for you, if you didn't have a mental model, how did you know what to type into the console?

Also, if developers didn't need debuggers because of their mental model, why are there bugs in the code that they are working on?

Mental models are necessarily imperfect.

Personally, I prefer your approach to writing a big huge chunk of code, assuming it is perfect, and testing it at the very end only to find all of the bugs.

[Pfhreak]

I usually sketch out what I'm going to write with comments and/or function names. It gives me the sort of 'blocks' of how data is going to flow, where responsibility is divided, etc. Then I go back in and fill the details of the block. Five or six comments becomes five or six chunks of code.

I don't usually try to maintain an entire state in my head, but I do try to make sure each of those blocks is something that I can reason about and hold the entire state of. If I'm finding it hard to figure out what each block does with data, I subdivide and write two comments.

For example, I might write something like:

// Iterate over every transform and do the matrix math

and realize that there's too much for me to hold mentally, so I'll do this:

// Iterate over every transform

// Invert each matrix

// Get the position vector

// Multiply by the character's scalar

... etc.

After that all scans, then I'll start writing the actual code.

[ToFab123]

I do exactly the same and it gives you a the added benefits, that you get a bit of documentation of the code for free.

[antiuniverse]

>I’ve heard of developers that don’t use debuggers because they have a mental model in their head.

Having heard these claims myself, I firmly believe that developers who eschew debuggers are either severely handicapping themselves out of misguided bravado, or at best are too lazy to invest minimal learning time to establish basic proficiency for huge reward. There's no reason not to avail yourself of extremely powerful tools like that.

That said, maybe don't start with text-mode debuggers...

[vegannet]

I think it very much depends on the work you’re doing. My co-workers depend on debuggers which are a crutch that allow them to _feel_ like they have control over our spaghetti code — “it’s fine, I can step through!” — but the code I produce (without a debugger but through TDD) is much more resilient. There’s certainly engineering problems that benefit from a debugger but I think for most modern work they’re not essential — and often a crutch.

[james_s_tayler]

While I don't think they are only ever used as a crutch, I'm starting to see more and more how they definitely are an enabler of spaghetti code.

It's not even just spaghetti but things that are very obtuse too. I often need a debugger whenever I see things in the code like if(variant.sourceId == null) and then below is the large unnamed block of code that also doesn't give any indication of what it implies. Because I have no idea what it implies, I have no idea how the data got into that state, so it's like... What case triggers this code?

I often wind up refactoring that stuff after computing an understanding of it. It usually comes out significanty easier to read and not likely you require a debugger to understand it.

So I that regard I think debuggers enable unnecessarily difficult to understand code to hang around.

[morenoh149]

I respectfully disagree. Debuggers are essential while you are debugging. Say you inherit some code and you are not sure where to make the necessary changes. Nothing beats stepping through until you find the place that does the thing and add your change.

[decasteve]

I think this is the crux of it and there’s a balance.

Debugging by printing the state of variables too frequently and resorting to line-by-line debugging of every function can be inefficient if it becomes a crutch for the developer. This could be due to a lack of confidence or a lack of understanding — or just habitual.

[pmikesell]

Well, Linus is one of those people: https://lwn.net/2000/0914/a/lt-debugger.php3

>> I happen to believe that not having a kernel debugger forces people to think about their problem on a different level than with a debugger. I think that without a debugger, you don't get into that mindset where you know how it behaves, and then you fix it from there. Without a debugger, you tend to think about problems another way. You want to understand things on a different _level_.

[mrunkel]

I mostly agree (the “too lazy” bit seems a little harsh).

I liken developers who proudly say they don’t use debuggers to surgeons who don’t take X-rays.

Sure, you could, but why make your life harder?

[marshray]

One of the best moves I made the first 10 years of my career was stepping through every single line of C++ I ever wrote just to observe it working as I thought (and a lot of times it wasn't). MSVC++/Visual Studio made this a very natural thing to do without breaking the flow.

Today, I use a variety of tools and languages and ... hmmm... I don't actually remember the last time I used a debugger on my own code.

[professorTuring]

Well, when debugging in C/C++ (and I have used lot of debuggers) you may find they don't help you to find the hard to spot problems.

Sometimes a bug just doesn't pop when debugging due to differences in the memory management of the debug mode or race conditions in multi-threaded due to different timings...

So, "text-debugging" (like printf("I'm here") debugging) sometimes is the only way to find precious bugs.

Back in the day most of my colleagues only relied in printf debugging. But we were just a bunch of linux geeks developing kernel modules, linux apps and QT apps (QT brought a nice IDE for debugging, truth to be told)...

[fergyfresh]

In my experience its purely random if/when someone will care about that. You just have to do a lot of interviews and if you're still coming up with near optimal solutions in the allotted time you should be okay.

I also do not think Senior Engineers have better mental models of these code trivia questions than Junior Engineers do on average. Seniors spend more time on architecture and facilitating Juniors to be more productive so I dk.

I have never been in an interview where piecing out the problem solving and debugging outside of whatever the IDE/editor of choice was frowned upon.

Maybe they just wanted to see you solve the solution in the pair programming window and thought you were copy/pasting your solutions from someone that was helping you off screen.

[james_s_tayler]

This.

Don't try to get the right answer. Try to get the right interviewer.

[codingslave]

FAANG interview is all about leetcode problems. Where your competition has already extensively studied the problem sets. If you cant get it the first time, and spend 15 minutes on bad solutions as you work through it, its a rejection. FAANG wants you to show up with near optimal solutions.

[bm1362]

I went to a FAANG interview today without preparing and was asked 7 leetcoding problems across 5 interviews. Even the behavioral one included a leetcode problem.

Most interviewers seemed to give an easy problem followed by a medium/hard. I think I succeeded, purely based on years of doing leetcode in the past and having a job at another FAANG-like.

[dboreham]

Hmm. I think you did have a mental model. What they're really complaining about is that you ran your code before it was complete, or at least before it was more complete than they would prefer.

I'm going to say that the point about edit compile run cycle is valid, but only in the context of a programming environment that actually has a long cycle. They didn't ask you to use one of those. So ultimately they're wrong or at least so uptight and annoying that you shouldn't go work for them.

[mntmoss]

I think your practices are fine - coding even a limited prototype reveals a lot of unanswered questions in a mental model. Once you have the model, you can apply it in a theoretical sense, but in my experience it takes longer to build one by not coding than it does by doing some coding, then some thinking. The only limitation on prototyping is that it's difficult to communicate what is being accomplished in the middle of the process, because you'll make something that looks negligent or incomplete - it isn't trying to answer all the concerns yet(that's what you do in production code).

[iamwil]

Depends. If I'm just sketching something in code, or I don't know what I'm actually building, I'll write it out in code first.

However, if I'm designing a known thing, where I understand what it's suppose to do; I usually devise a mental model of the thing I'm trying to build before I start committing to code.

Also, having a mental model for the system that you're designing makes it apparent whether someone else can pick up and understand your system.

I also don't really use a debugger. Not sure why though.

[meowface]

I've also never used a debugger. Python libraries like icecream (https://github.com/gruns/icecream) make print debugging pretty nice. I wonder if I'm missing out, though. I use VS Code a lot, and have been intending to see if I can fit its debugger into my workflow.

[morenoh149]

I think you are missing out. Debuggers allow you to know the state of variables at every line, not just the ones where you print debug. Debuggers also allow you to set watch expressions and occasionally you step through library code which begins helping you understand the source of your dependencies.

[iamwil]

Usually debuggers take extra steps to set up, and I find it only helps for code that's really hard to reason about. But the most time consuming thing is just stepping through the code, and if it happen to go too far, I have to start over from the top again.

I do like time-traveling debuggers though. Those give me something print statements don't give, and it's really great to examine the state as the application is progressing.

[agustif]

Just got this on a newsletter https://github.com/oslabs-beta/reactime

[codingdave]

I tend to build out a mental model of the bigger blocks of a feature, but will iterate on details as I code, just like you described. And I rarely run my code through a debugger because with that mental model and some judicious logging, I get the job done. But there are people who think coding in that way is asinine, and shows I'm not experienced. Different folks work in different ways.

But because of those differences, it may have been the right call to not move forward with you. That is not a judgment against you. It is an acknowledgement that you don't work the way they like to work, and may have had a hard time melding into the existing team. Or it may have worked, but hiring managers try to avoid risk where possible.

That is fine. Getting turned down for a job that you were perfectly capable of doing is a reality in this industry. Find another place, try again.

[peterhi]

The problem with mental models is that they are domain specific. My domain is sports data, solving problems is easy because I have 10 years of experience and a cookbook of models (can we say patterns) to apply to any given problem

Good mental models == Lots of experience

I suspect that the senior that interviewed you would be out of his depth if he moved into another domain. I definitely would. But then again having lots of experience helps you identify the patterns - sorry models - quicker and recognise the problems that you will need to guard against

"Building mental models" sounds like a deliberate cognitive act. To be honest it is more like recognising which part of an existing system would best fit the problem and what changes are need to be made to get a better fit

Which kinda makes it sound a lot less glamorous :)

Anyhow let me repeat:

Good mental models == Lots of experience

[bradleyankrom]

Just curious: which sport/industry are you working in?

[peterhi]

Data aggregation. Scores, stats and tv listings from multiple suppliers covering almost all sports worldwide. As B2B for people who don't want the hassle of doing it themselves

[ebj73]

Yes, it's important to develop a habit to plan ahead before coding. For anything bigger than small scripts or utilities, you need to do this.

On the other hand, there's the eternal conflict between the waterfall model and the iterative development model. The waterfall model dictates planning (almost) everything ahead, while the iterative model says plan sufficiently far ahead, then start coding, then take a break and re-evaluate, do some more planning, do some more coding, rinse, repeat. I certainly prefer an iterative model, and building prototypes, as you do. But even then, I'd say professional development work is 90% planning ahead, and only 10% actual coding. Probably even less.

[nurettin]

Yesterday I decided to write a simple log file watcher because customers especially requested for a simple script with no dependencies on the target machine. It had to detect any new logs that match the given pattern, too.

My plan was to detect new files and file modifications using two async tasks. Whenever a new file arrives, open a stream and read. Whenever a file is modified, find the previously opened stream and read. Every time I read, I push it onto an async Queue which will be consumed by another async task. That would emulate something similar to go channels. Multiple IO tasks, that sounds like a job for async/await. This was my mental model before I coded a single line.

After a few dozen minutes and a single confusing error (I forgot to pass the asyncio loop into the queue and the sleep functions) it was done. I even made it so that it would output the newly appended logs to the stdin of another process or another file. That way I could decouple my log watcher from the program that uses it.

I also opted for detecting file changes by using an async timed loop instead of inotify which would both simplify coding and buffer the new data to reduce read/write operations.

4-5 years ago, I would have experimented with code and tested different ways of accomplishing the same thing. Rather than favoring code experimentation, I had a product idea and implemented a minimal but complete example by reading docs .

[usgroup]

If you’re saying you write code without knowing what you’re writing then yes you should have a mental model.

It’s a lot easier to build in your head than to write code. You might go through a dozen ways of doing something in your mind which would take an age to do in code.

Further, writing code to produce a mental model is limiting because every line you write naively commits you to fewer possibilities. You can be a lot more revolutionary in your head at no added cost and without throwing away hours of work.

[zmj]

Yes - it's essential for debugging.

Your mental model tells you that if the system is in a given state, and receives a given input, what "should" happen. You can then look for evidence whether those things happened and narrow down a bug.

It also works the other way. If, given this input, that happened, your mental model tells you what the system state must have been. That can help you work backwards to identify a bug that put the system into a bad state.

[blihp]

Building at least a minimal mental model takes me far less time than trying to jump in and code something without one. When writing a trivial method/function, it often takes only a fraction of a second[1], but having it tells me if I'm on track to accomplish what I set out to. The mental model helps you plan and course correct... without it, you're likely either learning/exploring/doodling (which should help you build a mental model) or flailing about. You can get by on smaller scale/solo projects without one but it becomes an issue if you are expected to work in/with one more teams and or larger scope/scale projects.

[1] It takes effort in the early years, but like anything else you work at it does become second nature. I often don't consciously decide to build a mental model for simple tasks... it just happens reflexively with minimal time and virtually no effort. Now the model may be crap/wrong... but that's the value if it: as soon as what you're doing/observing doesn't match the model, you know you have a problem and need to course correct.

[s_m_t]

I think you probably should build a mental model. The way I see it the code/compile/debug loop style of programming is a bit like a gradient descent. You are probably going to fall into whatever solution pops up rather than really thinking about what makes sense.

With that being said no one is smart enough to not use a debugger.

[rraghur]

For largish problems, a general idea of the key structures and abstractions.. what's desirable, what's not... It becomes hard to think of how certain parts of the system mesh together... At that point, to avoid analysis paralysis, I write the scaffolding and some tests to see how the API feels... Most often, doing this clarifies stone assumptions that I was perhaps inadvertently making and leaves me with a path forward (after moving things around a bit)

So definitely iterative..

Whenever I've tried to design completely, it's rarely worked well. What has happened as I've grown older is that most parts of the system are orthogonal to others and it's usually not very complicated to compose parts together. So that leaves me with focusing on the core problem more up front and then layering in the other needed (but known) parts

[ilaksh]

I think everyone has to have _some_ kind of "mental model" otherwise you wouldn't be able to code anything except random keystrokes.

But next time just write some comments outlining what the function is supposed to do.

Also take whatever rationalization people give for what they do with a grain of salt. There are lots of reasons they might have picked someone else -- maybe there was a cheaper hire, or a prettier one, or you had bad breathe that day.

But comments with an outline before you start typing real code should satisfy most people's idea of having a mental model.

If you want to pass a FAANG interview with algorithmic/puzzle questions then that's totally different than pair programming and you will probably want to get a book specifically related to that or something. Like a book titled "Cracking the Code Interview" or something.

[ozzmotik]

I'm not a senior engineer by any means, despite all the time I've devoted to learning about CS and development in general; my main problem is that I just don't code anymore. That being said, I always have found that it is better to approach a problem with a model/plan in your head for how to implement your solution.

It's fully possible to go in blind and just develop a solution iteratively, and that methodology can be useful sometimes, especially when working with something completely foreign. But if you can take the time to develop a model in your mind of how it works, then all that's left is translating the details of that model into code.

[ChrisMarshallNY]

I try to keep as much in my head as possible.

Stuff in my head is always easy to change, and I find I need to change CONSTANTLY.

I create what I term a "napkin sketch," and try not to write down too much, afterwards.

Documentation is a HEAVY "concrete galosh."

That said, it's often imperative to write stuff down; especially in a team. It's just that every single thing you write down becomes tech debt.

I talk about that here: https://medium.com/@ChrisMarshallNY/concrete-galoshes-a5798a...

[TheOtherHobbes]

Mental models are good. You should have them.

Don't expect them to be complete. You will not be able to solve any non-trivial problem without discovering edge cases, unexpected complications, and misunderstandings that will only be revealed when you start to code.

I don't understand why anyone would believe that a mental model makes a debugger unnecessary. An accurate mental model won't protect you from certain kinds of implementation bugs, or from sneaky hard-to-spot typos that compile but do the wrong thing - etc.

[grumpy8]

As an interviewer, I like when candidates explain how they're going to solve the problem first. Either on the whiteboard, with comments or pseudo code. It's easier for me to help and guide them when I know where they're going. Some people prefer to start coding right away and, while I won't penalize them for doing so, if they go in the wrong direction without explaining their reasoning there's not much I can do to bring them back.

[dbcfd]

I'm lucky enough to have a memory/mind's eye where I can see the problem in my head almost always. I also tend to take time to flush that out in my head before I start coding, attempting to work through issues.

I'd like to think without having that, I'd take the time to put it in a day book.

In interviews, I've typically tried to draw/write out the problem (especially in terms of requirements) to help work through what I'm thinking.

[paulddraper]

A. It's pretty dumb to fail an live coding interview assignment for that reason.

B. Yes, mental models are absolutely critical.

---

A better interview assignment to test mental models would be to architect a data model or system. No code, just concepts and relationships.

On meaningfully sized non-FizzBuzz problems, starting by banging out code is like starting a vacation by getting in the car and driving. A better use of time is upfront thinking about possible plans, pros, and cons.

[ryan_w]

I'm not even a senior engineer, but I always like to do planning and get an idea of how I should go about a project before doing it.

It might take a while for me to come up with a design I like and it could bug me for days, but once I come up with something, even if it isn't "proper", the project moves along fast.

I always like to write on a whiteboard and draw out visual representations of each component I would need

[forze]

I had the same problem when I began. To be fast you definitely shouldn't compile during a testable part. It also messes with your working memory. Type all the code of the task, then after it's 'complete', check everything to see if you didn't miss something. THEN you compile. If the compiler gives some errors that is the moment where you fix them one by one.

[nitely]

Well, I'm amazed they allowed you to run the code at all. In an interview you are expected to test the code by going through it and write the state of the program as you go (i.e comments about the state of some variables in each iteration of some loop and stuff like that). The rest of the feedback is just how they justify themselves.

[kissgyorgy]

I'm not sure how could you write anything if you don't think about the problem before? I didn't like my last whiteboard interview because I just stare at the screen for a long time before writing anything, and sometimes even the best design decisions happen outside when I'm walking or under the shower.

[arenaninja]

Over the past year I've been relying on mental/written models especially when writing something new. It's useful for communicating - in a team setting I can communicate APIs to other team members before it's written and in interview settings you can use to explain your trail of thought

[pythonbase]

While I don't consider myself as a senior engineer, I do try to create mental models before coding. Also, in the code, I create the flow using comment statements which can later be replaced with actual code.

I developed the habit of dry running my code when I started to learn code using GW-BASIC, way back in time.

[silverlake]

Lisp and Smalltalk programmers worked this same way. You develop bits of code incrementally to build up to a larger program. It’s a benefit of a REPL, which is what you’re doing with print stmts. keep in mind interviews are mostly random, most interviewers are terrible. Don’t let it get you down.

[southphillyman]

Maybe I misunderstand what you're talking about, but wouldn't this manifest when you ask questions and talk about your approach?

Comunication while coding seems to be a big deal during interviews now so if you just started coded after hearing the problem I could see that being a negative.

[unhired]

It wasn’t a LeetCode problem. I had to implement an API where each call would change the state of the application on certain conditions. I heard the problem and clarified it on the whiteboard. I coded each function while talking and ask for clarifications.

[ju-st]

The problem sounds like they wanted you to implement a state machine, and to draw a state diagram before that.

[jerzyt]

I have to have a mental model before I touch the keyboard. It's probably wrong on the first iteration, but without a mental model how would I know if the code is doing what it's supposed to?

[scelerat]

The most useful coding tools I have are a pen and a pad of paper

[codeprimate]

I don't see how it's possible to code without a mental model. It's like drawing something without "seeing" it.

That sounds really really hard.

[unhired]

In drawing, you see something and sketch before inking and coloring.

[hydandata]

Plans are worthless, but planning is essential.

[tobinfricke]

> Do most senior software engineers take time to build a mental model before coding and testing?

What's the alternative?

[gHosts]

I regard engineering as a daemon that will magically give me anything, from a nice cuppa tea to a nuclear holocaust.

The daemon has a strong preference for nuclear holocaust.

The trick is to give it explicitly, concisely and precisely supply all the information it needs, out of the infinity of possibilities, to make a nice cuppa tea (preferably without creating a toxic waste problem).

I tend to construct my code.

I have the signatures of the functions at the boundary of the code visible, and then I create the data structures to hold the information required.

Then I wired the boundaries to the data model and uncover... holes. Gaps.

So I assume then my daemon will magically grant me all I need to plug that gap, so long as I tell it precisely what the gap is... and then test that what I have so far is "tea-like" and not emitting gamma rays.

And then turn myself into the daemon to create those parts I needed to plug the gaps.

So what is my mental model? A daemon with a preference for nuclear holocaust.

[purple_ducks]

Sometimes yes.

Sometimes no. That's when I do TDD.

[kissgyorgy]

You can only solve a problem with TDD well if you know EXACTLY what do you want to implement before even starting with exact input-output.

[purple_ducks]

That's blatantly not true.

I use TDD to pick apart the problem bit by bit implementing the next (then unknown) step in a very piece meal fashion.

edit: also, what kind of gatekeeping is

"You can only solve a problem with TDD well if you know EXACTLY what do you want to implement".

The end result is the same: a set of covering tests & working code.

[kissgyorgy]

Kent Beck, Martin Fowler and David Heinemeier Hansson disagrees with you: https://martinfowler.com/articles/is-tdd-dead/

[purple_ducks]

Wow slam dunk.

You really defeated my actual experience and proved your point on this one /s

[fsloth]

"Do most senior software engineers take time to build a mental model before coding and testing?"

I don't know. It probably depends on the problem.

If I've written anything remotely similar before, I probably come in with a history with the problem at hand and can at least guess what sort of solution might work.

If I've never written anything remotely similar I just need to sketch a lot to a paper, and try out things in code before I implement anything.

I don't know what the interview was actually like. But no one has been able to verbalize exactly the specific behaviour of a good programmer. The best gauge of good programmer is that they identify each other.

Now, this key factor also has a hidden catch. People are different! Nobody has identified that there is only one true way of being a top performer.

So, when you are interviewing, the interviewer has a specific model in her head ... of what a top performer is like to her.

But as far as I know, no one has done any research how many "top performer archetypes" there are.

So, I would say it's intellectually faulty to say there is only one way of being a senior contributor. Senior contributors output is gauged in production.

On the other hand, the only way to find top performers is to ask for other top performers to find them. And they are not necessarily expert psychologists, they just know how they work, and maybe have some notion of what the organization supports.

The interviewer might have had some archetype in their head that you did not fit with, but could not exactly verbalize, so they tried to generate anything resembling a reasonable rationalization for their intuition. So the actual problem might not have been your "lack of mental model", but some other unspecified lack of affinity with the specific way of working the interviewer expected (reasonably or not).

So there might be ways to be awesome that are unfamiliar to the interviewer, that they just don't understand.

On the other hand, they've likely seen what works in their organization and what does not, and that feeds into their intuition as well.

So the best way to ace an interview at an organization, is to psychologically match with the other top performers in the organization.

And the best way to suck at an interview is to be technically incompetent for the seat they are hiring - in this case no psychological affinity is going to help in the short term. But how to think, or how exactly to solve problems... that's a stretch to say there is only one way to do it. It sounds to me at worst it will lead to groupthink and lack of innovation opportunities due to cognitive monoculture.

Sorry, I don't actually know how to pass a FAANG interview or what you need for it.

[t-writescode]

TL;DR: different people have different programming styles and you might not be programming the way they expect you to; but that might not be the problem. The problem might be that you're focusing on the part of the interview that the interviewer doesn't care about and that makes it so that the interviewer gets frustrated and doesn't see your skill.

--

When I was younger, I got a chance to work on a game with a friend of mine. My friend was an incredibly skilled programmer, probably one of the best programmers who built things people used or could use, that I knew at the time.

I was a competition programmer.

We were both learning HTML5 at the time, and we both took on the task in very different ways. I believe we were writing Nibbles (Snake) or something like it, as a learning exercise, each.

I learned the basics: arrays, function creation, etc, and then went really fast at building the whole program without testing it, connecting all the pieces together in ways that I assumed would work. In the programming competitions I took part in, 3 people shared one computer, all doing different problems at the same time, so most of us wrote our code on paper before we put it in to the computer.

After my whole code for snake was written or at least as much as I wanted to have for a prototype (it's been many years since we did this, so I don't remember a lot), I started trying to run it. It didn't work, of course, or did messy things, and so I spent a bunch of time going through and fixing the bugs that were in my code.

My friend went a completely different approach. Of course I don't remember specifics, but it was something like this:

* he got a pixel drawing in a canvas

* he got a pixel moving on the canvas

* he got apples appearing on a canvas

* he got a chain of pieces of the snake following

* he got collision working

Of course, this is probably not what actually happened; but, this is basically what I remember about how he coded. He was very slow, methodical, and had a step-by-step process of how the whole code would work.

In the end, I think he had his program working before I did and much better; but, if you looked at just the start, you might assume he's not skilled.

--

Let's fast forward to today and a different person.

I was talking directly to a senior engineer at my company the other day. Very skilled, very, very skilled. We were talking about how we do things. It came out that I design systems and draw pictures to get flow and layout down. I like to work on weird, pie-in-the-sky, robust systems that tend to take a drawing to figure out later, because they're powerful and do powerful things; and, the goal of those systems tend to be to pre-emptively be ready for v-next and handle all of the requirements as cleanly as possible, and future requirements as cleanly as possible.

He greatly prefers to start hacking on a solution quickly and get something building and then work in the nuance and ugly later, refactoring whole sections over time to get a better and better solution.

When I remark on a dangerous test case or scenario area that I'm worried about when it comes to the upcoming solution, he's always thought of that problem area ahead of time, too; but, from what he tells me about how he works, he starts hacking basically right away; and, I don't think he does the crazy designs as his first version, either.

--

I'm not sure where I'm going with this; but, I think I want to say that this:

> I ran node in console after small changes and additions to print, test, debug.

is not a sufficient reason to think you're not senior; but, this:

> I didn’t have a mental model of the subtask

might be. Please notice that I reduced what I said down to a very, very narrow statement. If you eventually have a mental model, then I think you're fine here.

--

I'm going to add some more. This might be an interviewing problem, and I think it may be important to get a feel for what sort of interview it is, even asking them. If you're adding a bit, printing, testing, adding a bit, printing, testing, you might be adding a bit to things that your testing partner doesn't think matters. Did the interviewer expect the code to compile and run proper tests? Or were they just trying to understand your thinking process? If the interview only cares about your thinking process, then they probably don't care if you don't write the code to get the substring you need exactly right, or they might not care if you get rounding wrong, or output formatting wrong, because that's not the part of the problem you're being asked to solve.

You get better at this by interviewing more and practicing asking questions.

--

You mention this:

> The feedback was you can’t do this when compiling time is long and development will be slow

While this is true, for most of the work I do, I could write a small program on the side that tests the design I'm going for that compiles quickly and then inject that code into the larger code base and test the rigging. Maybe that's not how it is for the job you're looking at.

I've said a lot, I hope this helps.

[meco]

>the goal of those systems tend to be to pre-emptively be ready for v-next and handle all of the requirements as cleanly as possible, and future requirements as cleanly as possible.

>He greatly prefers to start hacking on a solution quickly and get something building and then work in the nuance and ugly later, refactoring whole sections over time to get a better and better solution.

I think you've articulated a dimension of programming that I've increasingly been observing and discussing with other engineers.

Do you happen to know a name for (or any language to describe) this kind of spectrum of problem solving where one end tends to pre-emptively encompass as many edge cases and future versions as possible, while the other focuses solely on the most obvious use case initially, adding in each edge case in successive versions of a "working" product (where working is used very loosely (i.e. single pixel on a screen is a "working" initial version))?

I definitely fall into the latter category, and find it very frustrating to work with engineers who want to chat edge cases that are days, weeks, or even months away from being relevant in my eyes. I would like to understand their thinking and this space of meta-problem solving better, but I'm not really sure what to call what we're talking about.

[unhired]

You and everyone else have helped a lot. Thank you.

[crimsonalucard]

>I prefer to create a draft quickly to build a mental model. Is my way of logging to console frequently and coding before building a mental model a junior habit that I should work on?

There is absolutely nothing wrong with this. There is zero research that says developing a mental model before a prototype is better (in fact research says the opposite is true). If compile times are like an hour, probably don't want to do this, but there are many languages where code to executable time is seconds.

A lot of senior engineers may disagree well, that's unfortunate, because those are the people interviewing you. They think they know better but there's literally no evidence.