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Interviewing programmers: coding test example explained

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Re: Interviewing programmers: coding test example explained

#81

Post by sfphotoarts »

tptacek wrote: post #10
roel_v wrote: post #8

Am I OCD when I cringe when I see 'p_rite' where he (I think) means 'p_write'? That alone would knock a few points off a candidate doing this test when he's interviewing with me. 'rite' can mean 'write' or 'right' or even 'allright' - why the unnecessary confusion?

You're not alone in feeling the OCD twinge, but are almost certainly alone in being willing to demerit a candidate writing that.

I believe the intent was not to misspell (because clearly given one side of the coin being 'read' the word 'rite' is clearly meant to be its counterpart) but was to make for neat alignment of the variable declarations. char w, r might have been easier and less twinge inducing, but I think it important to correct the misgiving that the person writing 'rite' was not misspelling but making a deliberate choice to align their variables. His own form of OCD, a trait common in good programmers.

Re: Interviewing programmers: coding test example explained

#82

Post by infinite8s »

Xurinos wrote: post #40
pvdm wrote: post #27

'\0' is null-terminated NOT zero-terminated, no ? By definition, a string in C is always null-terminated.

As it happens, '\0' translates to 0. Since they are composed of characters, C programmers tend to do the c == '\0' check instead of the shorted c == 0 check, just to make sure everyone knows we are still treating the char like a character in a string; it is just style. char c = 65 is equivalent to and just as legal as char c = 'A' So yes, C strings ARE zero-terminated. In fact, it is a bit weird to say they are null-…

What people really mean is NUL terminated (NUL is the name of the '\0' character - ASCII value 0x0000). But it probably just mangled some time ago and people write it as NULL.

Re: Interviewing programmers: coding test example explained

#83

Post by amalcon »

leftnode wrote: post #16

I'm nowhere near an experience C coder, but doesn't the in place part mean that you don't create any additional variables or strings and just modify z_terminated? Is that even possible? This is an honest question, because the solution is obvious this way, but a bit harder if you can't create additional variables.

"In-place" usually means "using only a constant amount of additional storage." You're using a constant amount of additional storage if you even use a function call (unless you're invoking certain compiler directives and/or optimizations), because C function calls usually involve placing a return address on the stack.

This is also why quicksort is considered to take up log(n) space: it calls itself recursively, and thus uses up a bit of the stack for each recursive call.

Re: Interviewing programmers: coding test example explained

#84

Post by RiderOfGiraffes »

jemfinch wrote: post #62
RiderOfGiraffes wrote: post #60

Earlier quoted context omitted.

Actually, they are not the same. See the trivia question in the lunk article.

I saw the trivia question in the link article, but it doesn't provide an answer, so I ignored it. EDIT: K&R says (page 60 in my second edition): """ The for statement for (expr1; expr2; expr3) statement is equivalent to expr1; while (expr2) { statement expr3; } except for the behavior of continue, which is described in Section 3.7. """ So apparently the trivia is the behavior of `continue`, which I think most C progr…

And it's because the knowledge is implicit and used when appropriate without thinking about it, that the question is, in my opinion, pointless as an interview question, except perhaps to see how the candidate reacts to it.

Re: Interviewing programmers: coding test example explained

#85

Post by zb »

petercooper wrote: post #61

Turns out his solution is almost the same as mine except I skip the termination stage and instead read one character more in the loop which catches the \0 from the original end of the string automatically. As I'm not really a C programmer, was I doing something bad/unrecommended? It seemed to work.. :-)

What happens if you pass '\0' as the character to remove?

Re: Interviewing programmers: coding test example explained

#86

Post by Retric »

RiderOfGiraffes wrote: post #57
Retric wrote: post #45

Earlier quoted context omitted.

Your routine should modify the given zero-terminated string in place, removing all instances of the given char. The main point of the whole exercise is to see if the candidate can write any code - anything after that is a bonus. Now if the input and output of a function are correct I think you where simply less clear than you may have thought. If you had said "string in place (don't allocate any memory), " I suspect…

The main point is as you quote. Secondary points are to see if the candidate can understand common expressions and idioms, and if not, either to look them up, or to ask. As it says elsewhere, the purpose is to get some code, then use it as a start for the discussion. If someone allocates memory then that's where I start. In that case they clearly they don't understand the usual meaning of the expression "in-place." >…

Some people allocated memory, copied out the input, then put back the bits they wanted to keep. That is not "in place". "Some people" implies more than one.

I would have done it in place because that was the simplest approach and not doing so would have been silly. However, just because it seems that way to us does not mean everyone knew you required more than just output in that fashion.

Anyway, I was more commenting on the style where you mix the requirements for an interface with the requirements for the algorithm. IMO "I need a function / API that does X efficiently" is much better than saying "does X using hash tables". AKA, if you don’t' want someone to use malloc then you can say so, but giving the reasons why you don't want malloc is more useful.

PS: It’s really a minor point and I thank you for doing this bit of research. I was simply trying to help you communicate in a more clear fashion.

Re: Interviewing programmers: coding test example explained

#87

Post by nuxi »

djb_hackernews wrote: post #11

hmm, is it normal to assume all strings in C are \0 terminated? What are the memory usage implications for that? I won't pretend to know C but suppose you have a string that is 'ab{100}\0' and you wanted to remove all of the bs, you'd end up with 'a\0b{99}\0' in memory correct?

That's the very definition of a C string - if it isn't \0 terminated, it's not a string. See section 4.1.1. of http://flash-gordon.me.uk/ansi.c.txt.

Re: Interviewing programmers: coding test example explained

#88

Post by shabble »

nevinera wrote: post #59
sram wrote: post #47

I always end up using perl when I have to do string manipulations and have let my c skills suffer as a result. Sad. Using perl regexp kills brain cells. #!/usr/bin/perl $in = ; $remove = ; chomp ($remove); chomp ($in); $in =~ s/$remove//g;

Bad boy! >hello|goodbye >| >>Result is 'hello|goodbye' >That's a nice dog you have there. >. >>Result is '' I usually handle this with: my $pattern = '\\'.substr($in,0,1); $in =~ s/$pattern//g; but it still doesn't feel safe. String operations in perl usually do what you want, but be careful with them!

     perl -pe'BEGIN { $x = quotemeta(shift) } s/$x//g;'
should do what you want. `quotemeta' is the function that does the escaping for your properly.

I was hoping to be able to use tr///, since it seems like it was built for this, but it creates its conversion tables at compile time, so there's no chance to interpolate the argument without some nasty nasty eval'ing.

Re: Interviewing programmers: coding test example explained

#89

Post by nocman »

roel_v wrote: post #41
tptacek wrote: post #36

Earlier quoted context omitted.

If you truly think the market for C developers is so favorable to employers that someone who can write the in-place-whitespace-removal function off the top of their heads is easy to find, then by all means, ding candidates for their spelling during interviews. Thanks for making it that much easier for the rest of us to hire. In the industry I work in --- computer software --- it is ludicrously hard to find people com…

This is not a simple misspelling though. If someone would write 'int countar = 0;' because he doesn't speak English well or whatever, that's fine. But spelling 'write' as 'rite' is indicative of a certain mindset - of favoring a certain quirky sense of beauty or style over clarity. And I didn't say it'd be autoding, just that it lowers my opinion of the author a bit. If the rest was OK that would be noise in between…

Some would argue that if it lowers your opinion of the author it is an "autoding" already (whether you're willing to admit it or not).

Take this scenario: You have a single position you are interviewing people for. You've got two candidates with entirely even qualifications. The only perceivable difference between them is that one of them writes interview code with variable names spelled like in the original post. The other guy spells things the way you would expect. I'm willing to bet you'd end up hiring the second guy. Sounds like an "autoding" to me. Or would you argue that you would not do that?

Re: Interviewing programmers: coding test example explained

#90

Post by rlpb »

djb_hackernews wrote: post #11

hmm, is it normal to assume all strings in C are \0 terminated? What are the memory usage implications for that? I won't pretend to know C but suppose you have a string that is 'ab{100}\0' and you wanted to remove all of the bs, you'd end up with 'a\0b{99}\0' in memory correct?

It's convention for C strings to be \0 terminated, and the standard library expects this just about everywhere (along with the kernel and just about everything else you might want to interface with). If you wanted to store a string as a (length, data) pair you could do that too, but you'd have to write most of your own string manipulation functions.
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