I'm solving 27E — Number With The Given Amount Of Divisors. My code involved a function mul(a, b) that will return 1018 + 7 if multiplying a and b will lead to overflow, and min(1018 + 7, a * b) otherwise. I checked for overflow using the condition a*b div a != b.
When I submit my solution using C++14, it seemed that the overflow check failed, which lead to WA on test 5.
However, when I uncomment the if statement in the mul function (which, theoretically don't print anything out), it get accepted.
I resubmitted the WA code with C++11 and C++17 Diagnostics, and it get accepted too.
I don't know what happened here. I suspected that some kind of optimization from compiler caused this strange behavior, but unfoturnately I know nothing about compiler optimization. Can anyone give me a proper explanation about this strange behavior? Thanks in advance.
You can check for overflows something like: a > inf / b. I think it's a better practice and it involves only one division instead of a multiplication and a division (so should be faster)
Thanks for the suggestion
Alternatively, you can just multiply them as floats, although I'm not sure about the performance.
Signed overflow is undefined behavior, read here about that.
I've never thought that overflow is undefined behavior, thanks for informing me this.
Also, I glad that this happened when I'm practicing, not in an actual contest.
Actually you can use -fsanitize=undefined to check for overflow as it will indicate you which line does the overflow happen. (Warning: About 2 to 3 times slower compile time, only recommend when you are trying to debug). Moreover, fsanitize also highlight any errors you might have in your code (such as out-of-index access or any undefined behaviors like that ...). Hope it helps!