std::set_difference

 

C++

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Algorithm library

Functions
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each countcount_if mismatch equal	findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) fill fill_n transform generate generate_n removeremove_if remove_copyremove_copy_if	replacereplace_if replace_copyreplace_copy_if swap swap_ranges iter_swap reverse reverse_copy rotate rotate_copy random_shuffleshuffle  (C++11) unique unique_copy
Partitioning operations
is_partitioned (C++11) partition partition_copy (C++11)	stable_partition partition_point (C++11)
Sorting operations (on sorted ranges)
is_sorted (C++11) is_sorted_until (C++11) sort	partial_sort partial_sort_copy stable_sort nth_element
Binary search operations (on sorted ranges)
lower_bound upper_bound	binary_search equal_range
Set operations (on sorted ranges)
merge inplace_merge includes	set_difference set_intersection set_symmetric_difference set_union
Heap operations
is_heap (C++11) is_heap_until (C++11) make_heap	push_heap pop_heap sort_heap
Minimum/maximum operations
max max_element min min_element minmax (C++11) minmax_element (C++11)	lexicographical_compare is_permutation (C++11) next_permutation prev_permutation
Numeric operations
iota (C++11) accumulate inner_product	adjacent_difference partial_sum
C library
qsort	bsearch

 

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.

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 Type1 and Type2 respectively. ​
Type requirements
- InputIt1 must meet the requirements of InputIterator.
- InputIt2 must meet the requirements of InputIterator.
- OutputIt must meet the requirements of OutputIterator.

Return value

Iterator past the end of the constructed range.

Complexity

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

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; }

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

See also

includes	returns true if one set is a subset of another (function template)
set_symmetric_difference	computes the symmetric difference between two sets (function template)