如何使用以列表为值的无序映射

Question

I am trying to figure out how to maneuver around an unordered map with lists as a values. Is it possible to assign a list to a value by specified string key and then later add to that list some value or modify it or replace entire list? I also do not understand how you would print the specific list either.

Example:

std::unordered_map <string, list<int> > testmap;
list<int> templist;
templist.pushfront(10);
templist.pushfront(5);
testmap["First"] = templist;

How would I print the list templist from the unordered map and how can I add onto the list after its been assigned to "First"

Answer 1

You can do with an unordered map with a list as a value as you need.

For example for such definitions:

std::unordered_map <string, list<int> > testmap;
list<int> templist;

you can:

• replace or create entire list value:

testmap["First"] = templist;

• print some list item

std::cout << (*testmap["First"].begin()) << endl;

• or print all items

for (auto v : testmap["First"])
  std::cout << v << "\n";

• replace some list value

std::replace(testmap["First"].begin(), testmap["First"].end(), 20, 99)

Answer 2

How would I print the list templist from the unordered map and how can I add onto the list after its been assigned to "First"

Here is a demonstrative program

#include <iostream>
#include <string>
#include <list>
#include <unordered_map>

int main()
{
    std::unordered_map<std::string, std::list<int>> testmap;

    std::list<int> templist;

    templist.push_front( 10 );
    templist.push_front( 5 );

    testmap["First"] = templist;

    for ( const auto &item : testmap["First"] ) std::cout << item << ' ';
    std::cout << '\n';

    testmap["First"].push_front( 0 );

    for ( const auto &item : testmap["First"] ) std::cout << item << ' ';
    std::cout << '\n';
}

The program output is

5 10 
0 5 10 

Or you can even use for example an ordinary loop (or a standard algorithm like std::copy) to output the list.

#include <iostream>
#include <string>
#include <list>
#include <unordered_map>


int main()
{
    std::unordered_map<std::string, std::list<int>> testmap;

    std::list<int> templist;

    templist.push_front( 10 );
    templist.push_front( 5 );

    testmap["First"] = templist;

    for ( auto first = testmap["First"].begin(); first != testmap["First"].end(); ++first ) std::cout << *first << ' ';
    std::cout << '\n';

    testmap["First"].push_front( 0 );

    for ( auto first = testmap["First"].begin(); first != testmap["First"].end(); ++first ) std::cout << *first << ' ';
    std::cout << '\n';
}

The program output is

5 10 
0 5 10