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Difference between revisions of "cpp/algorithm/set difference"

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Revision as of 17:33, 2 November 2012

 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
set_difference

Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 

Template:ddcl list begin <tr class="t-dsc-header">

<td>
Defined in header <algorithm>
</td>

<td></td> <td></td> </tr> <tr class="t-dcl ">

<td >
template< class InputIt1, class InputIt2, class OutputIt >

OutputIt set_difference( InputIt1 first1, InputIt1 last1,
                         InputIt2 first2, InputIt2 last2,

                         OutputIt d_first );
</td>

<td > (1) </td> <td class="t-dcl-nopad"> </td> </tr> <tr class="t-dcl ">

<td >
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 );
</td>

<td > (2) </td> <td class="t-dcl-nopad"> </td> </tr> Template:ddcl list end

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.

Contents

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 the second.

The signature of the comparison function should be equivalent to the following:

bool cmp(const Type1& a, const Type2& b);

While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)).
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 LegacyInputIterator.
-
InputIt2 must meet the requirements of LegacyInputIterator.
-
OutputIt must meet the requirements of LegacyOutputIterator.

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

Template:cpp/algorithm/dcl list includesTemplate:cpp/algorithm/dcl list set symmetric difference
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