How to insert and iterate elements to this kind of map. C++ - c++

Question

I tried on my own. But I was unable to do it. So, please help.
unordered_map<string, pair<string , vector<int>>> umap;
Or more precisely, how We can make pair of one string and one vector that can be used in map.

Answer 1

Well you can use insert function and insert them as a pair (Or precisely nested pairs).
For example Checkout this program :
#include <iostream>
#include <vector>
#include <string>
#include <unordered_map>
using namespace std;
int main()
{
unordered_map<string, pair<string , vector<int>>> umap;
//Insert like this
umap.insert(make_pair("first", make_pair("data1",vector<int>(3, 0))));
umap.insert(make_pair("second", make_pair("data2",vector<int>(3, 1))));
//or like this
string s = "new", t= "method";
vector <int> v = {1, 2, 3};
umap.insert(make_pair(s, make_pair(t, v)));
//This will print all elements of umap
for(auto p : umap)
{
cout << p.first << " -> " << p.second.first << " , VECTOR : " ;
for(auto x : p.second.second)
cout << x << ',';
cout << endl;
}
cout << endl;
//Let's see how to change vector already inside map
auto itr = umap.begin();
cout << "BEFORE : ";
for(auto x : (itr->second).second)
{
cout << x << ',';
}
cout << endl;
//We will push 42 here
(itr->second).second.push_back(42);
cout << "AFTER : ";
for(auto x : (itr->second).second)
{
cout << x << ',';
}
cout << endl;
}
Output Is :
new -> method , VECTOR : 1,2,3,
second -> data2 , VECTOR : 1,1,1,
first -> data1 , VECTOR : 0,0,0,
BEFORE : 1,2,3,
AFTER : 1,2,3,42,
I hope this helps.

Answer 2

This depends a lot on the complexity of what you are creating. For example, if you have some constants in your vector, you could make them in place:
umap.emplace("s", std::make_pair("s2", std::vector<int>{1, 2, 3, 4}));
It is more likely however that you will be making the internals in some complex way. In which case you could more easily do it as separate constructions.
std::vector<int> values;
values.push_back(1);
auto my_value = std::make_pair("s2", std::move(values));
umap.emplace("s2", std::move(my_value));
Using move to move the data around ensures minimal copying.
Finally, to iterate the items, it is normal to use range-based for loops:
for (const auto& [key, value]: umap) {
std::cout << key << ": ";
const auto& [name, values] = value;
std::cout << name << " {";
for (auto val : values) {
std::cout << val << " ";
}
std::cout << "}\n";
}
Here you can check out a live example.