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# std::set_difference

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt1, class InputIt2, class OutputIt > OutputIt set_difference( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, OutputIt d_first ); | (1) |
template< class InputIt1, class InputIt2, class OutputIt, class Compare > OutputIt set_difference( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, OutputIt d_first, Compare comp ); | (2) |

Copies the elements from the sorted range `[first1, last1)` which are not found in the sorted range `[first2, last2)` to the range beginning at `d_first`.

The resulting range is also sorted. The first version expects both input ranges to be sorted with operator<, the second version expects them to be sorted with the given comparison function `comp`. Equivalent elements are treated individually, that is, if some element is found `m` times in `[first1, last1)` and `n` times in `[first2, last2)`, it will be copied to `d_first` exactly std::max(m-n, 0) times. The resulting range cannot overlap with either of the input ranges.

### [edit] Parameters

first1, last1 | - | the range of elements to examine
first2, last2 | - | the range of elements to search for
comp | - | comparison function object (i.e. an object that satisfies the requirements of `Compare`) which returns ​true if the first argument is less than (i.e. is ordered before) the second. The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The signature does not need to have const &, but the function object must not modify the objects passed to it. The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to both Type1 and Type2. ​
Type requirements
- `InputIt1` must meet the requirements of `InputIterator`.
- `InputIt2` must meet the requirements of `InputIterator`.
- `OutputIt` must meet the requirements of `OutputIterator`.

### [edit] Return value

Iterator past the end of the constructed range.

### [edit] Complexity

At most 2·(N1+N2-1) comparisons, where N1 = std::distance(first1, last1) and N2 = std::distance(first2, last2).

### [edit] Possible implementation

First version
    template<class InputIt1, class InputIt2, class OutputIt>
    OutputIt set_difference(InputIt1 first1, InputIt1 last1,
                            InputIt2 first2, InputIt2 last2,
                            OutputIt d_first)
    {
        while (first1 != last1) {
            if (first2 == last2) return std::copy(first1, last1, d_first);

            if (*first1 < *first2) {
                *d_first++ = *first1++;
            } else {
                if (! (*first2 < *first1)) {
                    ++first1;
                }
                ++first2;
            }
        }
        return d_first;
    }
Second version
    template<class InputIt1, class InputIt2,
             class OutputIt, class Compare>
    OutputIt set_difference( InputIt1 first1, InputIt1 last1,
                             InputIt2 first2, InputIt2 last2,
                             OutputIt d_first, Compare comp)
    {
        while (first1 != last1) {
            if (first2 == last2) return std::copy(first1, last1, d_first);

            if (comp(*first1, *first2)) {
                *d_first++ = *first1++;
            } else {
                if (!comp(*first2, *first1)) {
                    ++first1;
                }
                ++first2;
            }
        }
        return d_first;
    }

### [edit] Example

Run this code

    #include <iostream>
    #include <algorithm>
    #include <vector>
    #include <iterator>

    int main() {
        std::vector<int> v1 {1, 2, 5, 5, 5, 9};
        std::vector<int> v2 {2, 5, 7};
        std::vector<int> diff;

        std::set_difference(v1.begin(), v1.end(), v2.begin(), v2.end(),
                            std::inserter(diff, diff.begin()));

        for (auto i : v1) std::cout << i << ' ';
        std::cout << "minus ";
        for (auto i : v2) std::cout << i << ' ';
        std::cout << "is: ";

        for (auto i : diff) std::cout << i << ' ';
        std::cout << '\n';
    }

Output:

    1 2 5 5 5 9 minus 2 5 7 is: 1 5 5 9

### [edit] See also

includes | returns true if one set is a subset of another (function template) [edit]
set_symmetric_difference | computes the symmetric difference between two sets (function template) [edit]
std::experimental::parallel::set_difference (parallelism TS) | parallelized version of `std::set_difference` (function template) [edit]

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