Find if vector contains pair with second element equal to X

Question

I have this vector:

using namespace std;

vector< pair<short, string> > vec = {};

And I want to find out if exists a pair <a, b> with b == X.

I know about std::find from <algorithm> but don't know how to apply it here.

Should I write my own function to do that?

bool is_in_vec(X)
{
    for (auto& e : vec)
        if (e.second == X)
            return true;
    return false;
}

Is that efficient?

Answer 1

Your solution looks fine if you only want to know if there is an element satisfying your criteria present. I would use const references in the loop, because the loop should not change the elements of the vector:

for (const auto& e : vec) ....

If you want to use a standard library algorithm, you can try std::find_if:

const std::string X{"foobar"};

auto it = std::find_if(vec.begin(), 
                       vec.end(), 
                      [&X](const pair<short, string>& p)
                      { return p.second == X; });

Here, it is an iterator to the first element satisfying the condition, or equal to vec.end() if no element is found.