The difference of two sets is formed by the elements that are present in the first set, but not in the second one. The elements copied by the function come always from the first range, in the same order.

For containers supporting multiple occurrences of a value, the difference includes as many occurrences of a given value as in the first range, minus the amount of matching elements in the second, preserving order.

Notice that this is a directional operation - for a symmetrical equivalent, see set_symmetric_difference.

The elements are compared using operator< for the first version, and comp for the second. Two elements, a and b are considered equivalent if (!(a<b) && !(b<a)) or if (!comp(a,b) && !comp(b,a)).

The elements in the ranges shall already be ordered according to this same criterion (operator< or comp). The resulting range is also sorted according to this.

The behavior of this function template is equivalent to:

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template <class InputIterator1, class InputIterator2, class OutputIterator>
  OutputIterator set_difference (InputIterator1 first1, InputIterator1 last1,
                                 InputIterator2 first2, InputIterator2 last2,
                                 OutputIterator result)
{
  while (first1!=last1 && first2!=last2)
  {
    if (*first1<*first2) { *result = *first1; ++result; ++first1; }
    else if (*first2<*first1) ++first2;
    else { ++first1; ++first2; }
  }
  return std::copy(first1,last1,result);
}

Parameters

first1, last1

Input iterators to the initial and final positions of the first sorted sequence. The range used is [first1,last1), which contains all the elements between first1 and last1, including the element pointed by first1 but not the element pointed by last1.

first2, last2

Input iterators to the initial and final positions of the second sorted sequence. The range used is [first2,last2).

result

Output iterator to the initial position of the range where the resulting sequence is stored.
The pointed type shall support being assigned the value of an element from the first range.

comp

Binary function that accepts two arguments of the types pointed by the input iterators, and returns a value convertible to bool. The value returned indicates whether the first argument is considered to go before the second in the specific strict weak ordering it defines.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.

Return value

Example

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// set_difference example
#include <iostream>     // std::cout
#include <algorithm>    // std::set_difference, std::sort
#include <vector>       // std::vector

int main () {
  int first[] = {5,10,15,20,25};
  int second[] = {50,40,30,20,10};
  std::vector<int> v(10);                      // 0  0  0  0  0  0  0  0  0  0
  std::vector<int>::iterator it;

  std::sort (first,first+5);     //  5 10 15 20 25
  std::sort (second,second+5);   // 10 20 30 40 50

  it=std::set_difference (first, first+5, second, second+5, v.begin());
                                               //  5 15 25  0  0  0  0  0  0  0
  v.resize(it-v.begin());                      //  5 15 25

  std::cout << "The difference has " << (v.size()) << " elements:\n";
  for (it=v.begin(); it!=v.end(); ++it)
    std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}

The difference has 3 elements:
 5 15 25

Complexity

Data races

Exceptions

See also

set_union

Union of two sorted ranges (function template)

set_intersection

Intersection of two sorted ranges (function template)

set_symmetric_difference

Symmetric difference of two sorted ranges (function template)

merge

Merge sorted ranges (function template)