std::set_difference in C++

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.
The elements in the both the ranges shall already be ordered.

1. Comparing elements using “<":
Syntax :

Template :
OutputIterator set_difference (InputIterator1 first1, InputIterator1 last1,
                               InputIterator2 first2, InputIterator2 last2,
                               OutputIterator 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.

Return Type :
An iterator to the end of the constructed range.
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// CPP program to illustrate
// std :: set_difference
  
#include <bits/stdc++.h>
  
int main()
{
    int first[] = { 5, 10, 15, 20, 25 };
    int second[] = { 50, 40, 30, 20, 10 };
    int n = sizeof(first) / sizeof(first[0]);
  
    std::vector<int> v2(5);
    std::vector<int>::iterator it, ls;
  
    std::sort(first, first + 5);
    std::sort(second, second + 5);
  
    // Print elements
    std::cout << "First array :";
    for (int i = 0; i < n; i++)
        std::cout << " " << first[i];
    std::cout << "\n";
  
    // Print elements
    std::cout << "Second array :";
    for (int i = 0; i < n; i++)
        std::cout << " " << second[i];
    std::cout << "\n\n";
  
    // using default comparison
    /* first array intersection second array */
    ls = std::set_difference(first, first + 5, second, second + 5, v2.begin());
  
    std::cout << "Using default comparison, \n";
    std::cout << "The difference has " << (ls - v2.begin()) << " elements :";
    for (it = v2.begin(); it < ls; ++it)
        std::cout << " " << *it;
    std::cout << "\n";
  
    return 0;
}

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Output:



First array : 5 10 15 20 25
Second array : 10 20 30 40 50

Using default comparison,
The difference has 3 elements : 5 15 25

2. By comparing using a pre-defined function:

Template :
OutputIterator set_difference (InputIterator1 first1, InputIterator1 last1,
                               InputIterator2 first2, InputIterator2 last2,
                               OutputIterator result, Compare comp);

Parameters :

first1, last1, first2, last2, result are same as above.

comp
Binary function that accepts two arguments of the types pointed
by the input iterators, and returns a value convertible to bool.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.

Return Type :
An iterator to the end of the constructed range.
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// CPP program to illustrate
// std :: set_difference
  
#include <bits/stdc++.h>
  
bool comp(int i, int j)
{
    return (i < j);
}
  
int main()
{
    int first[] = { 50, 40, 30, 20, 10 };
    int second[] = { 5, 10, 15, 20, 25 };
    int n = sizeof(first) / sizeof(first[0]);
  
    std::vector<int> v1(5);
  
    std::sort(first, first + 5);
    std::sort(second, second + 5);
  
    // Print elements
    std::cout << "First array :";
    for (int i = 0; i < n; i++)
        std::cout << " " << first[i];
    std::cout << "\n";
  
    // Print elements
    std::cout << "Second array :";
    for (int i = 0; i < n; i++)
        std::cout << " " << second[i];
    std::cout << "\n\n";
  
    // using custom comparison, function as comp
    /* first array intersection second array */
    ls = std::set_difference(second, second + 5, first, first + 5, v1.begin(), comp);
  
    std::cout << "Using custom comparison, \n";
    std::cout << "The difference has " << (ls - v1.begin()) << " elements :";
    for (it = v1.begin(); it < ls; ++it)
        std::cout << " " << *it;
    std::cout << "\n";
  
    return 0;
}

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Output:

First array : 10 20 30 40 50
Second array : 5 10 15 20 25

Using custom comparison :
The difference has 3 elements : 30 40 50

Possible Application : It is used to find the elements that are present only in first list and not in the second list.

1. It can be used to find the list of students that are attending only first classes.

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// CPP program to demonstrate use of
// std :: set_difference
#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
  
using namespace std;
  
// Driver code
int main()
{
    string first[] = { "Sachin", "Rakesh", "Sandeep", "Serena" };
    string second[] = { "Vaibhav", "Sandeep", "Rakesh", "Neha" };
    int n = sizeof(first) / sizeof(first[0]);
  
    // Print students of first list
    cout << "Students in first class :";
    for (int i = 0; i < n; i++)
        cout << " " << first[i];
    cout << "\n";
  
    // Print students of second list
    cout << "Students in second class :";
    for (int i = 0; i < n; i++)
        cout << " " << second[i];
    cout << "\n\n";
  
    vector<string> v(10);
    vector<string>::iterator it, st;
  
    // Sorting both the list
    sort(first, first + n);
    sort(second, second + n);
  
    // Using default operator<
    it = set_difference(first, first + n, second, second + n, v.begin());
  
    cout << "Students attending first class only are :\n";
    for (st = v.begin(); st != it; ++st)
        cout << ' ' << *st;
    cout << '\n';
  
    return 0;
}

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OUTPUT :

Students in first class : Sachin Rakesh Sandeep Serena
Students in second class : Vaibhav Sandeep Rakesh Neha

Students attending first classe only are :
 Sachin Serena

2. It can also be use to find the elements that are not present in first list only.
Program is given above.

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