Searching a vector inside another vector

Question

I am trying to see if vector v1 is inside vector v2.

For example, if v1= (b, a) and v2 = (g, e, f, a, b). I need to check both b and a present in v2.

The following code will help me only if order is same.

std::search(v2.begin(), v2.end(), v1.begin(), v1.end());

i.e., if v2 = (g, e, f, b, a)

Currently I am achieving through following way

for (std::vector<std::string>::iterator it = v1.begin(); it != v1.end(); ++it)
{
    if (std::find(v2.begin(), v2.end(), *it) != v2.end())
        std::cout << "found\n";
    else
        std::cout << "not found\n"; 
}

Is there a way to achieve using the above using std::search?

Answer 1

You could use std::set_intersection:

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
int main()
{
    std::vector<char> v1{'a','b','e','f','g'};
    std::vector<char> v2{'a','b'};
    std::sort(v1.begin(), v1.end());
    std::sort(v2.begin(), v2.end());

    std::vector<char> v_intersection;

    std::set_intersection(v1.begin(), v1.end(),
                          v2.begin(), v2.end(),
                          std::back_inserter(v_intersection));
    for(int n : v_intersection)
        std::cout << n << ' ';
}

See the reference

Please note that it is required that the two vectors are sorted using the same sort function prior to using std::set_intersection because it relies on comparing elements using operator<

Additionally you could use std::includes

#include <iostream>
#include <algorithm>
#include <cctype>
#include <vector>

int main()
{
  std::vector<char> v1 {'a', 'b', 'c', 'f', 'h', 'x'};
  std::vector<char> v2 {'a', 'b', 'c'};
  std::vector<char> v3 {'a', 'c'};
  std::vector<char> v4 {'g'};
  std::vector<char> v5 {'a', 'c', 'g'};

  for (auto i : v1) std::cout << i << ' ';
  std::cout << "\nincludes:\n" << std::boolalpha;

  for (auto i : v2) std::cout << i << ' ';
  std::cout << ": " << std::includes(v1.begin(), v1.end(), v2.begin(), v2.end()) << '\n';
  for (auto i : v3) std::cout << i << ' ';
  std::cout << ": " << std::includes(v1.begin(), v1.end(), v3.begin(), v3.end()) << '\n';
  for (auto i : v4) std::cout << i << ' ';
  std::cout << ": " << std::includes(v1.begin(), v1.end(), v4.begin(), v4.end()) << '\n';
  for (auto i : v5) std::cout << i << ' ';
  std::cout << ": " << std::includes(v1.begin(), v1.end(), v5.begin(), v5.end()) << '\n';

  auto cmp_nocase = [](char a, char b) {
    return std::tolower(a) < std::tolower(b);
  };

  std::vector<char> v6 {'A', 'B', 'C'};
  for (auto i : v6) std::cout << i << ' ';
  std::cout << ": (case-insensitive) "
            << std::includes(v1.begin(), v1.end(), v6.begin(), v6.end(), cmp_nocase)
            << '\n';
}

OUTPUT:

a b c f h x
includes:
a b c : true
a c : true
g : false
a c g : false
A B C : (case-insensitive) true

Here is the reference page (the example above is direct from the reference)

Either one could do the job depending on what you are trying to do.