Get difference between two array - c++

Question

I have 2 string tables and I would like to display the differences between the two tables.
Example :
Tab1 = "user1, user2, user3, user4"
Tab2 = "user3, user4, user2"
The difference in this case is that user 1 is missing.
I would like to know if there is a function that allows you to do this in the standard library or in Qt.
Thank you in advance.

Answer 1

What you need is std::set_difference which copies the elements from the sorted v1 which are not found in the sorted v2 to the diff. As mentioned, you should first sort your vectors and then search for difference.
#include <iostream>
#include <algorithm>
#include <vector>
#include <iterator>
int main() {
std::vector<std::string> v1 { "user1", "user2", "user3", "user4" };
std::vector<std::string> v2 { "user3", "user4", "user2" };
std::vector<std::string> diff;
std::sort(v1.begin(), v1.end());
std::sort(v2.begin(), v2.end());
std::set_difference(v1.begin(), v1.end(), v2.begin(), v2.end(),
std::inserter(diff, diff.begin()));
for (auto const& i : diff)
std::cout << i << std::endl;
}

Answer 2

If your string tables are stored as a string as you depicted, i.e,
Tab1 = "user1, user2, user3, user4"
Tab2 = "user3, user4, user2"
then you can use regular expression to first find out the users in Tab1 and store them in a set.
While searching for users in the Tab2, you can check if that user is already present in the set, if yes then remove that user from the set, if no then insert that user in the set.
Code:
#include <iostream>
#include <string>
#include <regex>
#include <set>
using namespace std;
ostream& operator<< (ostream& out, const set<string>& S)
{
for (const string& s : S)
out << s << endl;
return out;
}
int main()
{
string Tab1 = "user1, user2, user3, user4";
string Tab2 = "user3, user4, user2";
regex reg("user[0-9]{1,}");
set<string> S;
sregex_iterator pos1(Tab1.cbegin(), Tab1.cend(), reg);
sregex_iterator end1;
while (pos1 != end1)
{
S.insert(pos1->str());
++pos1;
}
sregex_iterator pos2(Tab2.cbegin(), Tab2.cend(), reg);
sregex_iterator end2;
while (pos2 != end2)
{
string temp = pos2->str();
if (S.find(temp) != S.cend())
S.erase(temp);
else
S.insert(temp);
++pos2;
}
cout << S << endl;
}

Answer 3

You can reduce computational complexity if you present you tables with std::set (or std::unordered_set). Then no need to sort tables (it's not cheap operation). I modify
NutCracker answer with usage of std::unordered_set:
#include <iostream>
#include <unordered_set>
#include <vector>
int main()
{
std::unordered_set<std::string> set1{ "user1", "user2", "user3", "user4" };
std::unordered_set<std::string> set2{ "user3", "user4", "user2", "user8" };
std::vector<std::string> diff;
for (const auto& s : set1)
{
auto it = set2.find(s);
if (it == set2.cend())
{
diff.emplace_back(s);
}
}
for (const auto& s : set2)
{
auto it = set1.find(s);
if (it == set1.cend())
{
diff.emplace_back(s);
}
}
for (const auto& s : diff)
std::cout << s.c_str() << std::endl;
}