initialising an associative container

This is a discussion on initialising an associative container within the C++ Programming forums, part of the General Programming Boards category; I was trying to initialise an associative container (map) in a function, but couldn't. Seems like whatever is initialised in ...

  1. #1
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    initialising an associative container

    I was trying to initialise an associative container (map) in a function, but couldn't. Seems like whatever is initialised in the function stays in the function. That's natural for a function. So, how are we going to initialise the container in a function and use it in the main function. The initialising statement looks something like this:

    map<char, int>phone_book;

  2. #2
    geek SilentStrike's Avatar
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    Hmm.. your problem is very unclear. Are you trying to do something like this?

    Code:
    void someFunction() {
    	map<char,int> phone_book;
    	// do some stuff with phone_book
    }
    
    int main() {
    	someFunction();
    	// now try to use phone_book
    	return 0;
    }
    Prove you can code in C++ or C# at TopCoder, referrer rrenaud
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  3. #3
    Registered User kitten's Avatar
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    You could make the container global, but that is generally bad idea. Other option is to let the function return a pointer to the map it created.

    Code:
    map<char, int>* CreateMap(void)
    {
      map<char, int>* pMap = NULL;
      pMap = new map<char, int>;
    
      // do something here, like check pMap != NULL
    
      return pMap;
    }
    
    int main(void)
    {
      map<char, int>* MapPtr;
    
      MapPtr = CreateMap();
    
      // remember to destroy the allocated map
      delete MapPtr;
    }
    Making error is human, but for messing things thoroughly it takes a computer

  4. #4
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    By initializing pointer?
    typedef std::map<char, char> CharMap;
    CharMap* theMap;
    void Init()
    {
    theMap = new CharMap;
    }

    or

    CharMap* CreateMap()
    {
    return new CharMap;
    }

    Is this your problem?

  5. #5
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    sorry for the confusing question.

    The below program works:

    #include <stdio.h>
    #include <iostream>
    #include <map>
    using namespace std;

    int main() {
    int i;
    char ch;

    map<char, int>table; //look-up table
    map<char, int>::iterator p;

    for(i=0; i<26; i++) {
    table.insert(pair<char, int>('A'+i, 65+i));
    }

    cout << "Enter key: ";
    cin >> ch;

    p = table.find(ch);
    if(p != table.end())
    cout << "Its ASCII value is " << p->second;
    else
    cout << "Key not in map.\n";

    return 0;
    }

    But when I build a map in a function like this:

    #include <stdio.h>
    #include <iostream>
    #include <map>
    using namespace std;

    void BuildMap(void); // map building function

    int main() {
    int i;
    char ch;

    BuildMap(); // create table here

    for(i=0; i<26; i++) {
    table.insert(pair<char, int>('A'+i, 65+i));
    }

    cout << "Enter key: ";
    cin >> ch;

    p = table.find(ch);
    if(p != table.end())
    cout << "Its ASCII value is " << p->second;
    else
    cout << "Key not in map.\n";

    return 0;
    }

    void BuildMap(void) {

    map<char, int>table; //look-up table
    map<char, int>::iterator p;

    }

    The main program does not reconise 'table' and 'p'. I'm using the STL(Standard Template Library) in C++.

    hope this clarify the problem.
    thanks for the previous reply.

  6. #6
    Registered User kitten's Avatar
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    The main program does not reconise 'table' and 'p'
    That's obvious because map and its iterator are local variables in the function BuildMap(). You have few reasonable ways. The one I prefer is to return the pointer to newly created map for the calling function from BuildMap() and initialize the iterator locally when you need it. So you don't need any global variables.

    There is a code example for you in my previous reply. I hope this helps.
    Making error is human, but for messing things thoroughly it takes a computer

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