cppLogic errorMedium

std::map operator[] creates unintended entries

Time: 0:00

BugFind the bug and fix the code

What it should do

Check if key exists in map and return final map size. Example: checkKeyCount("missing") -> 2

What happens instead

Map size increases from 2 to 3 after key presence check

#include <iostream>
#include <string>
#include <map>

int checkKeyCount(std::string searchKey) {
    std::map<std::string, int> cache;
    cache["alpha"] = 1;
    cache["beta"] = 2;

    // operator[] inserts default-initialized entry if key does not exist
    if (cache[searchKey] > 0) {
        // found
    }
    return cache.size();
}

int main() {
    std::cout << checkKeyCount("missing") << std::endl;
    return 0;
}

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

⏱️0:00

Hints

🏆 0 hints = Flawless Solve

Think & Test Cooldown

2:00 remaining

Take a moment to read the bug symptom and trace the code with Run & Check before unlocking Hint 1.

Hint 2

Unlocks after Hint 1

Hint 3

Unlocks after Hint 2

This bug belongs to the logic errors pattern

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

Learn the logic errors pattern Step through your own code

<iframe src="https://trybughunt.com/embed/afcb06a8-7a1a-4489-b264-116689e49bb7" title="Debugging challenge: std::map operator[] creates unintended entries" width="100%" height="580" loading="lazy" style="border:1px solid #d4d4d8;border-radius:12px;max-width:760px"></iframe> <p style="font-size:14px">Debugging challenge from <a href="https://trybughunt.com/challenges/afcb06a8-7a1a-4489-b264-116689e49bb7">TryBugHunt</a></p>

#include <iostream> #include <string> #include <map> int checkKeyCount(std::string searchKey) { std::map<std::string, int> cache; cache["alpha"] = 1; cache["beta"] = 2; // operator[] inserts default-initialized entry if key does not exist if (cache[searchKey] > 0) { // found } return cache.size(); } int main() { std::cout << checkKeyCount("missing") << std::endl; return 0; }

std::map operator[] creates unintended entries

What it should do

What happens instead

More logic errors to practise

std::map operator[] creates unintended entries

What it should do

What happens instead

More logic errors to practise