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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)

#21

Post by xigoi »

AlotOfReading wrote: post #11
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 valuable for compilers to be able to choose the instruction scheduling order. Standards authors try not to unnecessarily bind implementors. If post increment happened after the full statement is finished, then the original value has to be maintained until the next sequence point. Maybe the compiler will be smart enough to elide that, maybe not, but it's a lot more difficult to fix those kinds of edge cases than…

It would only make a difference in cases that are currently UB, so there is no program valid under current C that would be pessimized by this change.

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

#22

Post by LPisGood »

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.

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

#23

Post by kentm »

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…

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.

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

#24

Post by rcxdude »

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.

Or, also, in the reverse direction, if the interviewer is wrong about it and can't be convinced otherwise, it's probably not a great place to work.

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

#25

Post by bombcar »

Boxxed wrote: post #14

> The interesting thing here is the Undefined Behavior (UB), well... actually two UBs, thanks to which there are three possible correct answers: 11, 12 and 13. No, if you invoke undefined behavior any result at all is possible.

I feel we need another category - unspecified behavior. I think everyone would agree the compiler should putout ONE of those answers and that nasal demons would be out of spec.

The problem is that it’s not specified which should be picked, but all pick something.

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

#26

Post by rcxdude »

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…

Probably because when C was standardised there were already multiple implementations, and this was an area where implementations differed but it wasn't viewed as important enough to bring them in line with one approach.

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

#27

Post by bluGill »

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.

I know I did recommend someone after the interview because I looked it up and they were right. Great person to work with. Though I fully understand why most would hesitate.

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

#28

Post by topspin »

phendrenad2 wrote: post #16

Tried it on https://www.onlinegdb.com/online_c_compiler Returns 12. If I were designing C, it would return 13. But then again, I'm an assembly programmer.

Godbolt: clang and gcc compilers give 12. msvc compilers yield 13.

    #include 

    int main() {
        int a = 5;
        a = a++ + ++a;
        printf("%d\n", a);
        return 0;
    }
x64 msvc v19.50 VS18.2 output:

    example.c
    ASM generation compiler returned: 0
    example.c
    Execution build compiler returned: 0
    Program returned: 0
    13
x86-64 gcc 16.1 output:

    ASM generation compiler returned: 0
    Execution build compiler returned: 0
    Program returned: 0
    12
armv8-a clang 22.1.0 output:

    :5:10: warning: multiple unsequenced modifications to 'a' [-Wunsequenced]
        5 |     a = a++ + ++a;
          |          ^    ~~
    1 warning generated.
    ASM generation compiler returned: 0
    :5:10: warning: multiple unsequenced modifications to 'a' [-Wunsequenced]
        5 |     a = a++ + ++a;
          |          ^    ~~
    1 warning generated.
    Execution build compiler returned: 0
    Program returned: 0
    12

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

#29

Post by bluGill »

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…

The only other reasonable option is to make such garbage a compile time error. There is no reasonable definition of what code like that should do and if you write it in the real world you need find better job fit. I'd normally say McDonald's is hiring, but they don't want people like that either

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

#30

Post by tardedmeme »

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…

The C standard doesn't define things where two or more historical compilers disagreed and there wasn't an obviously correct way. This is defined behavior (left to right, assignment last) in Java, which is a different language.
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