如何在 C++ 中使用默认值将 std::set 转换为 std::map

Question

I have a Set of keys. I want to convert these keys into the key values of a Map. I conveniently want to set each of the matching values in the Map to the same value (1).

Here is a reproducible example.

set<string> keys;
keys.insert("key1");
keys.insert("key2");

map<string,int> keys_with_values;
// I want keys_with_values to equal 
     "key1": 1, "key2": 1 

Do I have to loop through the set and insert into the map? If so, what is the best way to do this?

Answer 1

Thank you, @Sam Varshavchik --here is how I implemented the loop

 set<string> keys;
 keys.insert("key1");
 keys.insert("key2");
 map<string,int> keys_with_values;
 for(auto key : keys) {
     keys_with_values[key] = 1;
 }
 cout << keys_with_values["key1"]; // 1

Answer 2

I want to add a further, very c++-y way for doing that:

#include <set>
#include <map>
#include <iterator>
#include <string>
#include <algorithm>

int main () {
    std::set<std::string> keys;
    keys.insert("key1");
    keys.insert("key2");

    std::map<std::string,int> keys_with_values;
    std::transform(keys.cbegin(), keys.cend(), std::inserter(keys_with_values, begin(keys_with_values)), [] (const std::string &arg) { return std::make_pair(arg, 1);});
}

Answer 3

Here's another way using lambdas:

for_each(keys.begin(), keys.end(), [&](auto key)
{
    keys_with_values.insert({ key, 1});
});

And for completeness sake, here's one using iterators:

set<string>::iterator it = keys.begin();
while (it != keys.end())
{
    keys_with_values.insert({ *it, 1});
    it++;
}