Set and subset.

[hahacprog]

Hello, I had problem on the set and subset. So let say i have:
A = {1, 2, 4 , 6}
B = {1, 7, 9}

is there any method to list out the elements in the B that are not belong to A.
So i expected the output list to be:
7, 9

i tried to understand unordered_set, but still no luck.

[Peter87]

You can use std::set_difference to do exactly this.

http://en.cppreference.com/w/cpp/algorithm/set_difference

#include <algorithm>
#include <iostream>
#include <iterator>
#include <set>

int main()
{
	std::set<int> A = {1, 2, 4 , 6};
	std::set<int> B = {1, 7, 9};
	std::set<int> C;
	
	std::set_difference(B.begin(), B.end(), 
	                    A.begin(), A.end(),
	                    std::inserter(C, C.begin()));

	for (int i : C)
	{
		std::cout << i << std::endl;
	}
}

[hahacprog]

thank for your reply. can this be aplied to string array too?
string A[4] = {"cat", "dog", "fish", "bird"};
string B[2] = {"cat", "snake"};

because i still didnt get how the declaration of "set<int>" work. I had run your code too, and it give this error:

set.cpp:13:26: error: in C++98 ‘A’ must be initialised by constructor, not by ‘{...}’
set.cpp:13:26: error: could not convert ‘{1, 2, 4, 6}’ from ‘<brace-enclosed initialiser list>’ to ‘std::set<int>’
set.cpp:14:22: error: in C++98 ‘B’ must be initialised by constructor, not by ‘{...}’
set.cpp:14:22: error: could not convert ‘{1, 7, 9}’ from ‘<brace-enclosed initialiser list>’ to ‘std::set<int>’

Thanks in advance

[shadowmouse]

As far as I can tell, Peter87's code is using C++11 uniform initialisation. Have a look at the std::set constructors (on this site's reference) to see how to construct them without curly braces.

[hahacprog]

i tried to read the documentation gave by Peter87. so i manage to get the result after i modified the code.

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

using namespace std;

int main()
{
	int a[4] = {1, 2, 4 , 6};
	int b[3] = {1, 7, 9};
	
	vector<int>A;
	vector<int>B;
	vector<int>C;
	vector<int>D;

	for (int i = 0; i<5; i++)
	{
		A.push_back(a[i]);
	}
	
	for (int i = 0; i<4; i++)
	{
		B.push_back(b[i]);
	}

	set_difference(B.begin(), B.end(), A.begin(), A.end(), inserter(C, C.begin()));
	cout<<C[0]<<endl;

}

cheers

[MiiNiPaa]

Or, figure out how to turn on C++1114 on your compiler. If you are not forced to use outdated compiler, chances that you simply did not enble all moder features of the language.