How Can We Flatten Nested Iterators in C for Easier Sequential Iteration?

Flattening Iterators in C

Often in data science and data manipulation tasks, we encounter nested or hierarchical structures. Iterating over such structures in a single, sequential manner can be challenging. This brings us to the concept of flattening iterators, which provide a solution to this problem.

Flattening Iterator Implementation

Although there aren't any pre-defined implementations in major C libraries, we can create our own. Here's an example implementation that focuses on forward iteration:

template <typename OuterIterator>
class flattening_iterator {
    // ... (Code detailing the implementation of the flattening iterator)
};

Usage

To use the flattening iterator, we define auxiliary functions to ease its creation:

template <typename Iterator>
flattening_iterator<Iterator> flatten(Iterator it) {
    return flattening_iterator<Iterator>(it, it);
}

template <typename Iterator>
flattening_iterator<Iterator> flatten(Iterator first, Iterator last) {
    return flattening_iterator<Iterator>(first, last);
}

With these functions, we can flatten a nested structure and iterate over it as a single sequence:

std::vector<std::vector<int>> v = {
    {0, 1, 2, 3},
    {4, 5, 6, 7},
    {8, 9, 10, 11}
};

for (auto it(flatten(v.begin(), v.end())); it != flatten(v.end()); ++it) {
    std::cout << *it << ", ";
}
// Output: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11

This implementation handles cases where inner containers may be empty, ensuring seamless iteration across multiple levels.

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