cppOff-by-oneEasy

Raw array off-by-one out-of-bounds access

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BugFind the bug and fix the code

What it should do

Calculate sum of raw array elements. Example: sumArrayElements([10, 20, 30]) -> 60

What happens instead

Undefined behavior / garbage memory read in loop due to i <= size condition

#include <iostream>
#include <vector>

int sumArrayElements(std::vector<int> nums) {
    int total = 0;
    // Off-by-one: i <= nums.size() accesses out-of-bounds element
    for (size_t i = 0; i <= nums.size(); ++i) {
        if (i < nums.size()) {
            total += nums[i];
        } else {
            total += nums[i]; // UB read
        }
    }
    return total;
}

int main() {
    std::cout << sumArrayElements({10, 20, 30}) << std::endl;
    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 off-by-one errors pattern

The loop that runs one time too many, the slice that drops the last item, the index that starts in the wrong place.

Learn the off-by-one errors pattern Step through your own code

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#include <iostream> #include <vector> int sumArrayElements(std::vector<int> nums) { int total = 0; // Off-by-one: i <= nums.size() accesses out-of-bounds element for (size_t i = 0; i <= nums.size(); ++i) { if (i < nums.size()) { total += nums[i]; } else { total += nums[i]; // UB read } } return total; } int main() { std::cout << sumArrayElements({10, 20, 30}) << std::endl; return 0; }

Raw array off-by-one out-of-bounds access

What it should do

What happens instead

More off-by-one errors to practise

Raw array off-by-one out-of-bounds access

What it should do

What happens instead

More off-by-one errors to practise