Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Int a = 5; a = a++ + ++a; a =? (2011)

gynvael.coldwind.pl

Post reply on HN
Print view View original

51–60 of 246 posts

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#51

Post by fweimer »

Aardwolf wrote: post #7

What's the reason that C didn't define the order of this? The horrible undefined behavior of signed integer overflow at least can be explained by the fact that multiple CPU architectures handling those differently existed (though the fact that C even 'attracts' its ill-defined signed integers when you're using unsigned ones by returning a signed int when left shifting an uint16_t by an uint16_t for example is not as…

Sethi-Ullman register allocation reorders subexpression evaluation to achieve efficient register allocation: https://dl.acm.org/doi/10.1145/321607.321620

With modern register allocators and larger register sets, code generation impact from following source evaluation is of course lower than it used to be. Some CPUs can even involve stack slots in register renaming: https://www.agner.org/forum/viewtopic.php?t=41

On the other hand, even modern Scheme leaves evaluation order undefined. It's not just a C issue.

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#52

Post by tombert »

I have always hated this crap; the fact that I'm not 100% sure the result of this indicates that maybe the ++ operator (pre or postfix) is something that should be avoided?

I don't do a lot of C anymore, but even when I did, I always would do increments on separate lines, and I would do a +=1, or just a = a + 1. I never noticed a performance degradation, and I also don't think my code was harder to read. In fact I think it was easier since I think the semantics were less ambiguous.

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#53

Post by p0w3n3d »

On my CS lectures algorithms professor used this pseudo language when writing an algorithm on a whiteboard :

  I
On the next hour another professor was giving lecture on C++ programming. I asked him the question: what would happen if we compiled

  i = i++
He went into some deep elaboration on it, but reassumed that only idiot would write like this...

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#54

Post by syngrog66 »

The smart nerd will know precisely how to decode that line's results.

The wise nerd will not allow lines like it in their codebase, in the first place and, having seen one, will refactor it (probably involving more lines or parentheses) to make it more clear and easier to maintain.

The latter approach scales better, in long run.

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#55

Post by p0w3n3d »

susam wrote: post #5

The code in the post seems very similar to the one in my own post from 2010: https://susam.net/sequence-points.html int a = 5; a += a++ + a++; I do remember that this particular code snippet (with a = 5, even) used to be popular as an interview question. I found such questions quite annoying because most interviewers who posed them seemed to believe that whatever output they saw with their compiler version was the co…

How many tennis balls can fit in a bus?

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#56

Post by wat10000 »

LPisGood wrote: post #22
susam wrote: post #5

The code in the post seems very similar to the one in my own post from 2010: https://susam.net/sequence-points.html int a = 5; a += a++ + a++; I do remember that this particular code snippet (with a = 5, even) used to be popular as an interview question. I found such questions quite annoying because most interviewers who posed them seemed to believe that whatever output they saw with their compiler version was the co…

In some sense, and without the interviewer knowing, that is actually a great scenario for an interview. If you can convince someone in a position of authority that they’re wrong about something technical without upsetting them then you’re probably a good culture fit and someone who can raise the average effectiveness of your team.

The best interview questions spawn discussions. This is a pretty good one for that. We could dive into what makes it UB, why a particular compiler might do it a certain way, what results we'd likely see from other compilers, and why the standard might say that this sort of thing is UB.

"What does this produce?" and expecting an answer of "17" is a bad question even if UB didn't mean the expected answer is wrong.

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#57

Post by LPisGood »

wat10000 wrote: post #56
LPisGood wrote: post #22

Earlier quoted context omitted.

In some sense, and without the interviewer knowing, that is actually a great scenario for an interview. If you can convince someone in a position of authority that they’re wrong about something technical without upsetting them then you’re probably a good culture fit and someone who can raise the average effectiveness of your team.

The best interview questions spawn discussions. This is a pretty good one for that. We could dive into what makes it UB, why a particular compiler might do it a certain way, what results we'd likely see from other compilers, and why the standard might say that this sort of thing is UB. "What does this produce?" and expecting an answer of "17" is a bad question even if UB didn't mean the expected answer is wrong.

I don’t work a ton with C, but I wonder how C programmers keep track of what behavior is and is not defined. It seems like there are many possible edge cases.

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#58

Post by kibwen »

fc417fc802 wrote: post #47
kibwen wrote: post #38

Earlier quoted context omitted.

In any language where the practice of iteration isn't achieved via C-style for-loops, having an operator devoted to increment just doesn't make sense (let alone four operators, for each of pre/post-increment/decrement). This is one of those backwards things that just needs to be chucked in the bin for any language developed post-2010.

When used well it makes for compact readable code. I don't see what it has to do with for loops or operators specifically. For example you can do the same in scheme while iterating by means of tail recursion.

> I don't see what it has to do with for loops or operators specifically.

The reason that these operators pull their weight in C is because iteration over arrays is achieved by manual incrementation (usually via the leading clauses of the for-loop) followed by direct indexing. Languages with a first-class notion of iteration don't directly index in this way, which overwhelmingly eliminates not only the vast majority of array indexing operations in codebases but also the need to manually futz with the inductive loop variable. Case in point, Rust doesn't have `++` in any form, and it doesn't miss it, because Rust has first-class iteration; on the then relatively rare occasion where you want do want to increment, you can do `+=1`, which doesn't have the footguns of `++` due to assignment being a statement rather than expression, while leading to a simpler language due to leveraging the existing `+=` syntax rather than needing a whole new set of operators.

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#59

Post by SilasX »

susam wrote: post #5

The code in the post seems very similar to the one in my own post from 2010: https://susam.net/sequence-points.html int a = 5; a += a++ + a++; I do remember that this particular code snippet (with a = 5, even) used to be popular as an interview question. I found such questions quite annoying because most interviewers who posed them seemed to believe that whatever output they saw with their compiler version was the co…

Heh, one time when I got this style of question[1] (but for JavaScript), I took a glance at it and said "Um ... you really shouldn't write code like that." The interviewer replied, "Oh. Yeah. Fair point." And then went on to another question.

[1] By which I mean predicting the behavior of error-prone code that requires good knowledge of all the quirks of the language to correctly answer.

Post reply on HN