cppLogic errorMedium

Iterator invalidation in vector erase loop

Time: 0:00

BugFind the bug and fix the code

What it should do

Remove all even numbers from a vector of integers. Example: filterEvenNumbers([1, 2, 3, 4]) -> [1, 3]

What happens instead

Segmentation fault or skipped elements during vector iteration after erase

#include <iostream>
#include <vector>

std::vector<int> filterEvenNumbers(std::vector<int> nums) {
    for (auto it = nums.begin(); it != nums.end(); ++it) {
        if (*it % 2 == 0) {
            // erase invalidates it; incrementing ++it in loop causes UB
            nums.erase(it);
        }
    }
    return nums;
}

int main() {
    std::vector<int> res = filterEvenNumbers({1, 2, 3, 4});
    return 0;
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

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This bug belongs to the logic errors pattern

The wrong operator, an inverted condition, a formula the right way round but backwards.

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#include <iostream> #include <vector> std::vector<int> filterEvenNumbers(std::vector<int> nums) { for (auto it = nums.begin(); it != nums.end(); ++it) { if (*it % 2 == 0) { // erase invalidates it; incrementing ++it in loop causes UB nums.erase(it); } } return nums; } int main() { std::vector<int> res = filterEvenNumbers({1, 2, 3, 4}); return 0; }

Iterator invalidation in vector erase loop

What it should do

What happens instead

More logic errors to practise

Iterator invalidation in vector erase loop

What it should do

What happens instead

More logic errors to practise