alt Hacker NewsThe 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 remember one particular interviewer who, after I explained that this was undefined behaviour and why, listened patiently to me and then explained to me that the correct answer was 17, because the two post-increments leave the variable as 6, so adding 6 twice to the original 5 gives 17.
I am very glad these types of interview questions have become less prevalent these days. They have, right? Right?
Replies
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.
Genuinely curious, so this is undefined behavior and depends on the compiler. I get that. Java, and other languages, can do these same operations but their compilers produce bytecode that runs on a virtual machine (JVM) compiled to machine code just-in-time. Would this same code in Java possibly yield different results based on the platform the JVM was running on because of the platform specific JIT compiler? Maybe that's part of the origin of the phrase "write once, test everywhere".
Do you want a job at a place where someone who doesn't understand UB makes the hiring decisions?
> 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 correct answer.
Other than the job for most programmers having nothing to do with whether they know the outcome, because hopefully they'd never write something like it or clean it up. And IF they found it they'd hopefully test it - given that it appears to be compiler dependent anyways.
Both major compilers yell at you for this nowadays... it's pretty unforgivable IMHO for somebody to be asking it as an exam or interview question if the right answer isn't "undefined":
<source>:5:10: warning: multiple unsequenced modifications to 'a' [-Wunsequenced]
5 | a = a++ + ++a;
|
<source>:5:7: warning: operation on 'a' may be undefined [-Wsequence-point]
5 | a = a++ + ++a;
| ~~^~~~~~~~~~~How many tennis balls can fit in a bus?
The interviewer asking stuff like that is a good sign to leave immediately.
> I am very glad these types of interview questions have become less prevalent these days. They have, right? Right?
Are you referring to the type of interview questions where the question is ill-defined and no one should know the answer, or the type where the question is reasonable and well-defined, but the interviewer doesn't know the answer?
I had a phone screen with Google once where they asked how to determine the length of a stretch of contiguous 1s within an infinite array of 0s. I suggested that, given the starting index i, you can check the index i+2 and then repeatedly square it until you find yourself among the zeroes, after which you can do binary search to find the transition from ones to zeroes.
The interviewer objected that this will grow the candidate end index too quickly, and the correct thing to do is to check index i+1 and then successively double it until you find the zeroes. We moved on.
I passed that phone screen. But I still resent it, because I checked the math later and "successive squaring followed by binary search" and "successive doubling followed by binary search" take exactly the same amount of time.
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.
>I am very glad these types of interview questions have become less prevalent these days. They have, right? Right?
I just refuse to do interviews like that any more.
Well... tried it on macOS using vanilla gcc, the results surprised me:
$ /bin/cat x.c; gcc -w -o x x.c; ./x
#include <stdio.h>
int main()
{
int a = 5;
a += a++ + a++;
printf("a = %d\n", a);
}
a = 18
- The first a++ expression results in 5, after a = 6 - The second a++ expression results in 6, after a = 7 - Only then the LHS a is evaluated for the addition-assignment, so we get: a = 7 + 5 + 6 = 18
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.