How to write a unified filter iterator in c++?

by bobbie.kris , in category: General Help , 2 months ago

How to write a unified filter iterator in c++?

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1 answer

by sarai_bergstrom , 2 months ago

@bobbie.kris 

To write a unified filter iterator in C++, you can follow the steps below:

  1. Define an iterator class template that inherits from the std::iterator class and provides the necessary iterator traits. This class will be your unified filter iterator.
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template<class Iterator, class Predicate>
class UnifiedFilterIterator : public std::iterator<std::input_iterator_tag,
                                       typename std::iterator_traits<Iterator>::value_type>
{
    // Your implementation here
};


  1. Declare private member variables in the iterator class to store the underlying iterator, the end iterator, and the predicate function.
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private:
    Iterator current;
    Iterator end;
    Predicate predicate;


  1. Define the constructor of the iterator class to take the underlying iterator, the end iterator, and the predicate function as parameters. Initialize the member variables accordingly.
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public:
    UnifiedFilterIterator(Iterator begin, Iterator end, Predicate pred)
        : current(begin), end(end), predicate(pred) {}


  1. Implement the necessary operator overloads to make the iterator function correctly. a. Implement the dereference operator (operator*) to return the value of the underlying iterator when the predicate function returns true. reference operator*() const { return *current; } b. Implement the pre-increment operator (operator++) to advance the underlying iterator until it points to an element that satisfies the predicate function or reaches the end iterator. UnifiedFilterIterator& operator++() { while (current != end && !predicate(*current)) { ++current; } return *this; } c. Implement the post-increment operator (operator++) to call the pre-increment operator and return a copy of the iterator before the increment. UnifiedFilterIterator operator++(int) { UnifiedFilterIterator temp(*this); ++(*this); return temp; } d. Implement the equality operators (operator== and operator!=) to compare the underlying iterator and end iterator. bool operator==(const UnifiedFilterIterator& other) const { return current == other.current && end == other.end; } bool operator!=(const UnifiedFilterIterator& other) const { return !(*this == other); }
  2. Now that your iterator class is defined, you can use it in range-based for loops or with other algorithms that accept iterators.
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int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    
    UnifiedFilterIterator<decltype(numbers.begin()), bool(*)(int)> begin(numbers.begin(), numbers.end(), [](int num) { return num % 2 == 0; });
    UnifiedFilterIterator<decltype(numbers.end()), bool(*)(int)> end(numbers.end(), numbers.end(), [](int num) { return num % 2 == 0; });
    
    for (auto it = begin; it != end; ++it) {
        std::cout << *it << " ";
    }
    
    return 0;
}


In the example above, we define a unified filter iterator that filters out even numbers from a vector. The lambda function [](int num) { return num % 2 == 0; } acts as the predicate function to check if a number is even. The iterator is then used in a range-based for loop to print out the filtered numbers.