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

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

#151

Post by lmm »

LPisGood wrote: post #57
wat10000 wrote: post #56

Earlier quoted context omitted.

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.

They don't. In the culture some kinds of undefined behaviour are taken seriously and some aren't. If you want to write code that "works", you emulate what popular performance benchmarks do (whether their code is undefined according to the standard or not), since those are the thing that C compiler developers actually care about.

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

#152

Post by _kst_ »

vishnugupta wrote: post #10

I am, thankfully, out of this craziness now but it was fun solving ton of such puzzles from Yashavant Kanetkar books while preparing for campus hiring interviews back in 2000. "Test Your C Skills" in particular. Fun times. https://www.scribd.com/document/235004757/Test-Your-C-Skills...

"Test Your C Skills" is a published book by Yashavant Kanetkar, apparently published in 2005, and still available in paperback. The document you linked to appears to be a scan of a printed copy of that book, and is almost certainly in violation of copyright. The cover and the title and copyright pages are notably missing.

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

#153

Post by benmmurphy »

hackernews capitalising Int makes this question kind of confusing. Because the question is meant to be about c++ behaviour but `Int` is not a standard c++ type. But Int is not java because java uses Integer and `Int` is not c# because c# has uses the explicit IntBitsize types. I think maybe the only languages where the title makes sense in is Scala or Swift.

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

#154

Post by _kst_ »

dhosek wrote: post #85

I don’t have gcc available so I can’t test it, but I wonder what it does with int a = 5; int b = a++; if it gives b==5 in this circumstance (which I would say is the correct value), then it seems that giving 13 for a++ + ++a is a bug in the compiler. I kind of feel like giving 6 as an answer would also be a bug in the compiler since postfix-++ should return the old value and then increment.

Your code:

    int a = 5;
    int b = a++;
has well defined behavior. The first line initializes a to 5. The second initializes b to 5 and sets a to 6. (The language doesn't specify the order of the two operations of assigning a value to be and incrementing a, but in this case it doesn't matter.)

Giving 13 for a++ + ++a is not a bug in the compiler. It's a bug in the code.

The correct answer to "what does a++ + ++a do" is "it gets rejected in code review and replaced with code that expresses the actual intent.

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

#156

Post by _kst_ »

summarybot wrote: post #36

> If you would like to test your compiler (posting back the results in the comments is really appreciated, especially from strange/uncommon compilers and other languages which support pre- / post- increment .... Uh, 85% of them show the wrong result so 85% of them clearly do not support pre and post increment.

If the behavior is undefined, there is no wrong result.

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

#157

Post by HarHarVeryFunny »

_kst_ wrote: post #132
tengwar2 wrote: post #122

Earlier quoted context omitted.

Not nasal demons in this case ( https://groups.google.com/g/comp.std.c/c/ycpVKxTZkgw/m/S2hHd... ): thaumasiotes shows that we can expect a numeric answer.

I don't see the name "thaumasiotes" at that link, nor do I see anything relevant to the code in the title. The behavior of "int a = 5; a = a++ + ++a;" is undefined. There is no guarantee of a numeric result, because there is no guarantee of anything .

It's only the order of evaluation that is undefined.

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

#158

Post by rtpg »

saghm wrote: post #20
marcosdumay wrote: post #9

Earlier quoted context omitted.

Applying the increment or decrement operators over the same variable more than once on the same line should be a compile-time error. Anyway, yes, this one example has an obvious order it should be applied. But still, something like it shouldn't be allowed.

Honestly, having increment in expressions rather than a statement feels like more of a footgun than a benefit. Expressions shouldn't mutate things.

int d = foo ? bar() : baz();

I think if anything people have been leaning more and more into expressions over statements, because when everything is an expression you end up being able to walk the gradient of complexity a bit more nicely than when you end up with a thing that just has to be broken down to a bunch of statements.

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

#159

Post by thaumasiotes »

vilhelm_s wrote: post #140
thaumasiotes wrote: post #105

Earlier quoted context omitted.

> It's Undefined Behavior. Susam's post doesn't make this clear. The quotes from K&R say that the modifications to the variable may take place in any order, but they don't directly say that doing this is Undefined Behavior, which would make it permissible to do anything, including e.g. interpreting the increments as decrements. The C99 standard is quoted saying this: >> Between the previous and next sequence point an…

The "shall" in the standard means it's undefined behavior. This is explained in the "Conformance" section, > 2. If a ‘‘shall’’ or ‘‘shall not’’requirement that appears outside of a constraint is violated, the behavior is undefined. Undefined behavior is otherwise indicated in this International Standard by the words ‘‘undefined behavior’’ or by the omission of any explicit definition of behavior. There is no differen…

> Compilers will not refuse to compile the code, indeed the blog post we are all commenting on reports the results from a bunch of different compilers.

Yes, I see that. I just said they should refuse.

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

#160

Post by afdbcreid »

HarHarVeryFunny wrote: post #157
_kst_ wrote: post #132

Earlier quoted context omitted.

I don't see the name "thaumasiotes" at that link, nor do I see anything relevant to the code in the title. The behavior of "int a = 5; a = a++ + ++a;" is undefined. There is no guarantee of a numeric result, because there is no guarantee of anything .

It's only the order of evaluation that is undefined.

Nope, there is no sequence point in the middle and modifying an object more than once between sequence points is undefined behavior.
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