how to find an element in a vector of vectors - c++

Question

I want to find string x in vector of vectors v:
std::vector<std::string> vA{"A", "B", "C"};
std::vector<std::string> vB{"D", "E", "F"};
std::string x = "E";
std::vector<std::vector<std::string>> v{vA, vB};
How to use std::find for my case ? Do you have some other ideas ?
Edit A result that bool value which indicates that at least one "E" has been found.
I wonder if it is possible to use somehow std::find* this way:
std::find*(v.begin(), v.end(), /*lambda with next std::find*/)

Answer 1

Keep it simple:
for (auto const &stringVector : v) {
auto it = std::find(std::begin(stringVector), std::end(stringVector), x);
if (it != std::end(stringVector)) {
// do what you want
break;
}
}
I'd like to make a comment, perhaps as a side-note. Your title asks "How to find an element in a vector of vectors". Your question, however, asks "How to use std::find for my case?"
You shouldn't use std::find because that's what you're "supposed to do". Use it when it's helpful, and avoid it when its harmful. Start from your data, not from the code.
EDIT
To answer your edit, you can use std::find_if.
using namespace std;
bool find_string(vector<vector<string>> const &v, string const &x) {
return find_if(begin(v), end(v), [&x] (vector<string> const &stringVector) {
return find(begin(stringVector), end(stringVector), x) == end(stringVector);
}) != end(v);
}
But this is less readable in my view.

Answer 2

With range-v3, it would be:
auto flat_view = v | ranges::view::join;
auto it = ranges::find(flat_view, x);
if (it != end(flat_view)) {
std::cout << "found\n";
}
Demo

Answer 3

If you're fine with just a pointer to the string:
std::string* foo(const std::vector<std::vector<std::string>>& v,
const std::string_view needle)
{
for(const auto& inner : v)
for(const auto& s : inner)
if(s == needle) return &s;
return nullptr;
}
If you want the iterators:
auto foo(const std::vector<std::vector<std::string>>& v,
const std::string_view needle)
{
for(auto oit = std::begin(v); oit != std::end(v); ++oit)
for(auto it = std::begin(inner); it != std::end(inner); ++it)
if(*it == needle) return std::make_pair(oit, it);
return std::make_pair(std::end(v), decltype(v[0].begin()){});
}

Answer 4

What about searching each vector in a loop and then using std::find?
#include <iostream>
#include <vector>
#include <algorithm>
//using namespace std;
int main(){
std::vector<std::string> vA{"A", "B", "C"};
std::vector<std::string> vB{"D", "E", "F"};
std::string x = "E";
std::vector<std::vector<std::string>> v{vA, vB};
for(auto inner_vec : v){
auto res = std::find(std::begin(inner_vec), std::end(inner_vec), x);
if(res != std::end(inner_vec))
std::cout << *res;
}
return 0;
}