How to initialize a dictionary in C++?

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hello, I created a map in c++, but I do not know how to initialize it. the map is as follow:

std::map<int, list<std::tuple<int, int> >> groups;

actually I want to create a dictionary in C++ and I want to put the elements with the same attribute in one element of dictionary. for example I want to put the tuples {1,2},{2,2},{2,4} in groups[0] of dictionary. what is the solution?

What I have tried:

I tried the following line. but, it has error:

groups[0]=(make_tuple(4,4));

Posted 4-Apr-17 7:48am

Member 11807654

Updated 4-Apr-17 10:58am

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CPallini · Accepted Answer · 2017-04-04T10:58:00

Solution 1

You forgot the list. Try, for instance

C++

#include <iostream>
#include <map>
#include <tuple>
#include <list>
using namespace std;


int main()
{
  std::map<int, list<std::tuple<int, int> >> groups;

  groups[0] = list<std::tuple<int,int>>{make_tuple(3,4)};

  cout << get<1>(groups[0].front()) << endl;
}

Posted 4-Apr-17 10:58am

CPallini

Comments

Philippe Mori 4-Apr-17 18:56pm

There is a missing new before list constructor...

Member 11807654 5-Apr-17 3:10am

when I write " groups[0] = list<std::tuple<int,int>>{make_tuple(3,4)};" it gives the error:"type name is not allowed" and when I write "groups[0] = new list<std::tuple<int,int>>{make_tuple(3,4)};" it gives error:"error C2679: binary '=' : no operator found which takes a right-hand operand of type 'std::list<_Ty> *' (or there is no acceptable conversion)"

CPallini 5-Apr-17 3:32am

Which version of Visual Studio are you using?
Such piece of code compiles and runs fine both on Linux with g++ 5.4 and on Windows, with CL 18.00 (Visual Studio 2013).

Member 11807654 5-Apr-17 4:09am

visual studio 2012

Philippe Mori 5-Apr-17 7:31am

Well, I didn't pay attention it was a value. Usually ehen I have such embedded container, I would use push_back (or push_front if reversed order is desired) similar to CPallini comment below. Usually, it does not make much sense to replace the whole list as you typically want to update existing list.

Member 11807654 5-Apr-17 9:23am

thank you very much. according to your explanation I should use push_back :)

CPallini 5-Apr-17 3:22am

Why do we need to allocate the list on the heap?

Member 11807654 5-Apr-17 4:10am

I just want to create a dictionary and then use it. is there another way to do it?

CPallini 5-Apr-17 4:27am

Possibly such compiler version doesn't support uniform initialization. Try
#include <iostream>
#include
#include <tuple>
#include <list>
using namespace std;

int main()
{
std::map<int, list<std::tuple<int, int> >> groups;

list<std::tuple<int, int>> l = list<std::tuple<int, int>>();
l.push_front(make_tuple(3, 4));
groups[0] = l;

cout << get<1>(groups[0].front()) << endl;
}

Anyway I would use a more modern compiler.

Member 11807654 5-Apr-17 5:17am

thank alot. it works for me perfectly too :)

Member 11807654 5-Apr-17 11:43am

excuse me. do you know how can I find the maximum of second values in one element of dictionary? For example, if groups[0] of this dictionary has tuples {2,3},{4,2}, {5,7}, I want to find max{3,2,7}=7.