An off-by-one error includes one element too many or too few in an iteration or index range. In a container loop, changing < to <= feels intuitive but tests one position beyond the last element. section 2.6
Reduce the opportunity for boundary mistakes by:
- Replacing index-based iteration with a range-based for loop when the task processes each element. section 2.6
- Using const references for class elements unless the loop must modify them; numeric values can usually be copied. section 2.6
Listing 2.11 Using index values to iterate over a container
template <typename T>
T sum(const std::vector<T>& values) {
T sum = (T)0;
for (int i = 0; i < values.size(); ++i) #1
sum += values[i];
return sum;
}
int main() {
std::vector<int> values;
for (int i = 0; i < 10; ++i)
values.push_back(10*i + i); #2
std::cout << sum(values) << '\n';
return 0;
}
The indexed sum loop makes the continuation test, index access, and boundary risk explicit. section 2.6
Listing 2.12 Using range-based for values when iterating over a container
template <typename T>
T sum(const std::vector<T>& values) {
T sum {}; #1
for (const T& value : values) #2
sum += value;
return sum;
}
int main() {
std::vector<int> values;
for (int i = 0; i < 10; ++i) #3
values.push_back(10*i + i);
std::cout << sum(values) << '\n';
return 0;
}
The range-based correction removes explicit continuation and update expressions while visiting every container element. section 2.6
