Hi all,

I would like to share something which happened with me in today's contest. I solved the following problem: Strong Password. Here is my submission link: Submission.

I was not sure why it was giving runtime error, and after the contest, I tried submitting it in C++20. It passed without any problem! Here is the code snippet which was the reason of runtime error:

string s;
cin >> s;
...
 
vvll post(s.size(), vll(10, -1));
... 
for (ll i = s.size() - 2; i >= 0; --i) {
    for (int j = 0; j < 10; ++j)
        post[i][j] = post[i + 1][j];
    ...
}

vvll is vector<vector<long long>>, vll is vector<long long> and ll is long long.

Can you identify the source of runtime error here? Hint: what if input string is "0"?

Reason of failure in C++ 17 (32 bit compiler):

1. s.size() - 2 should semantically result in -1 and data type of size_t is unsigned integer of $$$4$$$ bytes.
   • This computation results in 0xFFFFFFFF ($$$4$$$ bytes, unsigned number).
2. LHS is of data type signed long long, which is $$$8$$$ bytes.
   • LHS can safely handle consecutive $$$32$$$ bits which are all set to $$$1$$$.
   • Thus, $$$i$$$ will be set to 0x00000000 FFFFFFFF which is $$$4294967295$$$.

Reason of above code passing in C++ 17 (64 bit compiler):

1. s.size() - 2 should semantically result in $$$-1$$$ and data type of size_t is unsigned integer of $$$8$$$ bytes.
   • This computation results in 0xFFFFFFFF FFFFFFFF ($$$8$$$ bytes, unsigned number).
2. LHS is of data type signed long long, which is $$$8$$$ bytes.
   • LHS can't handle consecutive $$$64$$$ bits which are all set to $$$1$$$.
   • That value of $$$i$$$ is implementation defined and we can all agree that most of the implementations will simply treat this number as signed one, without any change in data.
   • Thus, $$$i$$$ will be set to 0xFFFFFFFF FFFFFFFF which is $$$-1$$$ and the code works as expected.