Re: map (associative array) loses values?

Tue, 11 Sep 2007 14:58:21 -0700
On Sep 11, 10:49 pm, "Jim Langston" <> wrote:

"Jim Langston" <> wrote in message


<> wrote in message

On 11 sep, 21:35, "Jim Langston" <> wrote:

"BobR" <> wrote in message


<> wrote in message...

On Sep 11, 5:59 pm, Kai-Uwe Bux <> wrote:

ah, ok. <string, int> works like a charm! :)

I'm using new, because I need the table somewhere else. The try/catch
is in my main, while the function resides in the SearchText class.
delete statements are there, I just didn't post them ;)

Ofcourse I'm just a c++ newbie (this is my first program actually),
what would you do different?

An alternative to Jim's suggestion:

// pass by non-const reference.
void SearchText::countWords( std::map<std::string, int> &map ){
    map["aap"] = 1;
    map["noot"] = 2;

    std::cout << map["aap"] << std::endl;
    std::cout << map["noot"] << std::endl;

// int main(){
  try {

     std::map<std::string, int> MyMap;

     SearchText text("test.txt");
     text.countWords( MyMap );
     std::cout << MyMap["aap"] << std::endl;
     std::cout << MyMap["noot"] << std::endl;
  catch( int ex) {
     cout << "Could not open file." << endl;
// return 0;
// } // main()

Sorry, I didn't test that. Post back if you have trouble ( I may have
something. <G>).

There are a lot of alternatives to mine and Bob's suggestion. In fact,
if I
was designing this class myself I would totally encapsulate the map
the class and main would only get to it via functions. Depending on
what it
would be for something like (untested code)

class SearchText
   SearchText( const std::string& FileName );
   int Value( const std::string& key ) const;
   int CountWords();
   std::map<std::string, int> Data_;


SearchText::SearchText( const std::string& FileName )
   // Open file and load Data_


int Value( const std::string& Key ) const
   if ( Data_.find( Key ) != Data_.end() )
      return Data_[Key];
      return 0;


int CountWords()
   // Return whatever it is this is trying to count


I am a strong believe in data encapsulation in classes, and try to
prevent giving points or references to my internal classes data.
you need the map for in mian you can encapsulate. Even encapsulate
operator[] if you wish.

int SearchText::operator[]( const std::string& Key )
   return Value( Key );

}- Tekst uit oorspronkelijk bericht niet weergeven -

- Tekst uit oorspronkelijk bericht weergeven -- Tekst uit oorspronkelijk
bericht niet weergeven -

- Tekst uit oorspronkelijk bericht weergeven -

Thanks a lot you guys for all the reactions!

The reason why I did not want to copy the entire map, was because I'm
going to use it for counting the words in

which is a 3,3 mb text file. ;)

However, as Jim Langston already mentioned (and maybe you too Kai-Uwe
Bux, I'm going to study your examples more closely), I can encapsulate
this map into the SearchText class, since the map and the file are
related and there is no need to seperate them. Any possible new file
will need a new instance of SearchText anyway.

Thanks a lot though. I never worked via this principle of newsgroups,
but I'm going to have a look here more often!

For those of you interested: I need this for the first assignment of a
Natural Language Processing course (at University of Amsterdam) and
next assignments will build on this one, so I wanted to make this
stuff so generic as possible. Some of you now might say: Use Python!
But I really want to learn C++ (and I would have to learn Python
too...) and the only way to do this is by programming :)

I can understand not wanting to copy the map being 3.3 megabytes. Be
aware, however, that if you copy your class, the map will get copied also.
So just make sure you pass your class by reference than by value. One way
to ensure this is to disable copying of your class. The normal way to do
this is to create a copy constructor ( and assignment operator) and make
them private to the class. That way if you accidently write some code
that would create a copy of the class, it won't compile and you will
realize it.

For the SearchText class that would be:

class SearchText
  // ...

   // Disable copy and assignment by making private.
   SearchText( SearchText const&) {}
   SearchText& operator=( SearchText const&) {}

Just empty copy constructor and assignment operator's private to the

Just a note. I noticed in that code that in operator= I don't have a return
statement. I looked in my own code where I'm doing this same thing and also
don't have a return statement. I don't know if that's a compiler flaw not
catching it, or the standard allows it. I think, however, that
SearchText& opeator=( SearchText const& ) { return this; }
would be more correct, even though it will never be called.

One more question though. Why are all you guys constantly typing
'std::something', instead of just typing 'using namespace std' one
time at the top of each file?

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