initializing map<string, vector<string> >

Question

I am initializing map<string, vector<string> > as follows:

map <string, vector<string> > dict;

dict["USA"].push_back("NYC");
dict["USA"].push_back("LA");
dict["USA"].push_back("Chicago");
dict["USA"].push_back("Dallas");

dict["India"].push_back("Delhi");
dict["India"].push_back("Bombay");

dict["Australia"].push_back("Melbourne");
dict["Australia"].push_back("Sydney");
dict["Australia"].push_back("Adelaide");

I find this cumbersome. The same thing can be done in tcl as follows which is cleaner:

array set dict {
USA {NYC LA Chicago Dallas}
India {Delhi Bombay}
Australia {Melbourne Sydney Adelaide}
}

Is there a more cleaner way to initialize in C++? My compiler is gcc 3.4.6

GCC 3.4.6??? You should really consider upgrading to a newer compiler... There is a boost library to ease this type of initialization but I am not sure that it can be used with such an old compiler (6 years is almost an eternity) — David Rodríguez - dribeas
– David Rodríguez - dribeas, Commented Mar 30, 2012 at 22:01

Kerrek SB · Accepted Answer · 2012-03-30 21:43:12Z

43

Initialization had many limitations in the old C++. Your code is in fact not initializing anything at all; it's just calling a lot of member functions on an already initialized object.

In the current C++ (C++11) you can initialize your map properly:

std::map<std::string, std::vector<std::string>> const dict {
   { "USA", { "NYC", "LA", "Chicago" } },
   { "India", { "Delhi", "Bombay" }    }
};

answered Mar 30, 2012 at 21:43

Kerrek SB

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4 Comments

innochenti Over a year ago

oh god, this is beautiful!

emvee Over a year ago

It is really too bad it took them ~15 years to come up with it :P

emvee Over a year ago

Oh, and OP said his compiler is g++ 3.4.6, which is very very far from C++11 - so posting elegant solutions that the OP can't use might not be that helpfull ... alas.

Kerrek SB Over a year ago

@haavee: I'm aware of that. There wasn't much to add to the OP's predicament, though.

hatboyzero · Accepted Answer · 2012-03-30 21:57:13Z

9

If you're not opposed to using the Boost.Assign library and you are using C++ older than C++11, you can do it like this:

#include <boost/assign/list_of.hpp>
#include <boost/assign/std/vector.hpp>
#include <map>
#include <string>
#include <vector>

std::map<std::string, vector<std::string> > dict = boost::assign::map_list_of<std::string, std::vector<std::string> >
    ("USA",   boost::assign::list_of<std::string>("NYC")("LA")("Chicago")("Dallas"))
    ("India", boost::assign::list_of<std::string>("Delhi")("Bombay"))
;

edited Mar 30, 2012 at 21:57

answered Mar 30, 2012 at 21:45

hatboyzero

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Comments

emvee · Accepted Answer · 2012-03-30 22:02:41Z

8

If you're not afraid of using a bit of C-style macros and some helper constructs you might find this slightly less irritable; the initialization of the map is done in one line; you only need to fill in the data (which you must do anyway).

#include <iostream>
#include <map>
#include <vector>
#include <string>
#include <utility>

using namespace std;

struct entry {
    string  key;
    string* vals;
    size_t  nvals;
};
#define NVAL(x) (sizeof(x)/sizeof(x[0]))

int main( void ) {
    // Create your data 
    string      usa[]      = {"NYC", "LA"};
    string      india[]    = {"Delhi", "Mumbai"};
    entry       dd[] = {
                          {"USA", usa, NVAL(usa)},
                          {"India", india, NVAL(india)}
                        };
    map<string, vector<string> > dict;

    // initialize the map
    for(unsigned int i=0; i<NVAL(dd); i++)
        dict.insert( make_pair(dd[i].key, vector<string>(dd[i].vals, dd[i].vals+dd[i].nvals)) );

    // Verify
    for( map<string,vector<string> >::const_iterator ptr=dict.begin();
         ptr!=dict.end(); ptr++) {
        cout << ptr->first << ": ";
        for( vector<string>::const_iterator eptr=ptr->second.begin();
             eptr!=ptr->second.end(); eptr++)
                cout << *eptr << " ";
        cout << endl;
    }
    return 0;
}

answered Mar 30, 2012 at 22:02

emvee

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2 Comments

ildjarn Over a year ago

Your macro can be trivially replaced with a function template: template<typename T, std::size_t N> std::size_t NVAL(T (&)[N]) { return N; }.

emvee Over a year ago

That is indeed an elegant solution! Thx!

Luchian Grigore · Accepted Answer · 2012-03-30 21:44:19Z

8

You could do this, if C++11 is not available:

map <string, vector<string> > dict;

string usa[] = { "NYC" , "LA" , "Chicago" , "Dallas" };
dict["USA"] = std::vector<string>(usa,usa+4);

answered Mar 30, 2012 at 21:44

Luchian Grigore

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