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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)

#81

Post by binaryturtle »

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…

It's defined. And called "operator precedence", both post/pre-increment have a higher precedence than the single "+".

At least according to this: https://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B#Exp...

I think the main confusion here comes from the fact that "a" is just a value, not a pointer, where it matters when the value/address which the pointer points at is accessed (before of after the increment of the pointer's own 'value').

Anyway… my C skills are rusty. Maybe I get it wrong. :) In any case I always would use brackets to avoid any ambiguity in constructs like this.

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

#82

Post by amavect »

tombert wrote: post #52

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 th…

I also started doing this. I feel that "b = expr(a); a++;" expresses what I mean better than "b = expr(a++)": store expr(a) in b, then store a+1 in a. Any good compiler will optimize the same.

After separating a++ onto its own line, replacing a++ with a+=1 or a=a+1 comes down to personal taste in syntax sugar. I vote for a+=1.

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

#83

Post by amelius »

tzs wrote: post #74
kentm wrote: post #23

Earlier quoted context omitted.

IMO, The only reasonable answer if asked this in an interview is “I would not write code where I have to know the answer to this question” These sorts of things are neat trivia to learn about things like sequence points but 99.9% of the time if it matters in your codebase you're writing something unmaintainable.

> IMO, The only reasonable answer if asked this in an interview is “I would not write code where I have to know the answer to this question” That's half of a reasonable answer. The other half is "but I do know the answer so if I see it when reviewing or working on someone else's code I can flag it or rewrite it, and explain to them why it is bad".

You might still make a mistake, even if you think you know the answer. It's much better to instrument the code to figure it out, or write a short test program.

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

#84

Post by IshKebab »

tzs wrote: post #74
kentm wrote: post #23

Earlier quoted context omitted.

IMO, The only reasonable answer if asked this in an interview is “I would not write code where I have to know the answer to this question” These sorts of things are neat trivia to learn about things like sequence points but 99.9% of the time if it matters in your codebase you're writing something unmaintainable.

> IMO, The only reasonable answer if asked this in an interview is “I would not write code where I have to know the answer to this question” That's half of a reasonable answer. The other half is "but I do know the answer so if I see it when reviewing or working on someone else's code I can flag it or rewrite it, and explain to them why it is bad".

No it isn't. You don't need to know the answer to know that it is bad code. The very fact that it isn't clear shows that.

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

#85

Post by dhosek »

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.

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

#86

Post by [deleted] »

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.

[deleted]

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

#87

Post by mywittyname »

p0w3n3d wrote: post #53

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...

Out of curiosity, I checked if gcc would optimize i = i++ out, and it does!

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

#88

Post by IshKebab »

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 really. In fact there are many things that are technically UB but are so common that compilers can't really treat them as UB. E.g. type punning via unions.

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

#89

Post by [deleted] »

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.

[deleted]

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

#90

Post by compiler-guy »

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.

The trick here is that the original expression contains undefined behavior. Your example does not.
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