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I was working on a leetcode question in c++ you can see the question properly - https://leetcode.com/problems/plus-one/

I was doing it again with a second approach where I will manually increase the value through a recursive call. It was getting an error only on leetcode compiler that -

Line 1034: Char 34: runtime error: addition of unsigned offset to 0x602000000250 overflowed to 0x60200000024c (stl_vector.h)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:1043:34

It was when we passed a vector only with a single value i.e. 9 It was running in my computer but not on leet code and I can't understand the bug. the code is-

#include <iostream>
#include <vector>
using namespace std;


class Solution
{
public:
    vector<int> incre(vector<int> &digits, int len)
    {

        if (digits[len - 1] != 9 && len != 0)
        {
            // if there is number before it which is not 9
            digits[len] = 0;
            digits[len - 1]++;
        }
        else if (len == 0)
        {
            // cout << "in zero loop-" << len << endl;
            // if no nmber then we have to add before it
            digits[len] = 0;
            vector<int>::iterator iter = digits.begin();
            digits.insert(iter, 1, 1);
        }
        else if (digits[len - 1] == 9)
        {
            // if the last number is again 9
            // cout << "in recursive loop-" << len << endl;
            digits[len] = 0;
            digits = incre(digits, len - 1);
        }
        else
        {
            cout << "Something else occures" << endl;
        }
        return digits;
    }

    vector<int> plusOne(vector<int> &digits)
    {
        if (digits.back() != 9)
        {
            digits.back()++;
        }
        else
        {
            digits = incre(digits, digits.size() - 1);
        }
        return digits;
    }
};



int main()
{
    vector<int> digits{9};
    // vector<int> digits{2,4,3,1, 9, 9};
    // vector<int> digits{2,4,3,1, 5, 8};
    // vector<int> digits{9, 9, 9, 9, 9, 9, 9, 9, 9};
    vector<int> res;
    Solution s;
    res = s.plusOne(digits);
    for (auto i : res)
    {
        cout << i << " ";
    }
    return 0;
}

you can copy run the whole code as I have given the whole code. The error occurs when I use the first test case - vector digits{9};.I know there are better approaches but I wanted to do it manually for practicing recursion. It would be very helpful if someone could point out the bug. Thanks in advance and sorry for grammatical mistakes.

DHRUV KABRA
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  • It would be better to learn C++ using a [good c++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) instead of by solving random online puzzles. Use `std::vector::at`. – Jason Oct 13 '22 at 17:40
  • If `len == 0` then you will have already accessed `digits[-1]` which, as you surely understand, is invalid. Also, if `len` is supposed to be the actual size of the vector, then `digits[len]` will be out of bounds, no matter its value. – Some programmer dude Oct 13 '22 at 17:40
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    Change `digits[len - 1] != 9 && len != 0` to `len != 0 && digits[len - 1] != 9` for the reason explained in above comment. Also you can use `std::vector::at`. – Jason Oct 13 '22 at 17:42
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    Suggestion: compile the code on a computer under your control, turn on the address and Undefined Behaviour sanitizers, feed the same inputs into the program, and get diagnostics tuned to your code on your computer. You'll probably get the exact line number and more easily read diagnostics. – user4581301 Oct 13 '22 at 17:42
  • There's a popular myth about a secret shortcut to becoming an elite C++ guru: throw away your C++ textbooks; instead do random coding puzzles that, otherwise, have no inherent learning value, they don't teach anything except bad programming habits. This myth comes from a bunch of clickbait web sites, like LeetCode, that promise to turn anyone into an instant C++ uberhacker, just by doing their puzzles. Everyone eventually realizes how useless these coding puzzles are, but only after wasting a massive amount of time doing one coding puzzle after another. And they have nothing to show for it. – prapin Oct 13 '22 at 20:52
  • @prapin So how would I practice c++.I actually learner c++ from free online courses. – DHRUV KABRA Oct 14 '22 at 09:20

0 Answers0