how to insert data in a map container

Hello,

I have this.

teamstats.team_name = msk
teamstats.played_games = 1
teamstats.points_made = 80
teamstats.points_against = 20

the map looks like this :

Code:

---

struct teamstats {
    string team_name;
    int played_games, point_made, points_against, woord;
};

 map<string, teamstats> allteams;

---

I know I have to use teamstat.insert but what arguments does this must have ?

Roelof

The simplest way to insert items into a map involves using make_pair. In your case assuming that team_name is the key:

Code:

---

allteams.insert(make_pair(item.team_name, item));

---

Code:

---

#include <iostream>
#include <string>
#include <map>

using namespace std;

struct teamstats {
        int points_for,
                points_against,
                games;
};


int main() {
        int i;
        map<string,struct teamstats> eg;
        struct teamstats tmp;
        string names[3] = { "one", "two", "three" };

/* add to map */
        for (i=0;i<3;i++) {
                tmp.points_for = i;
                tmp.points_against = i*2;
                tmp.games = 5;
                eg.insert(pair<string,struct teamstats>(names[i],tmp));
        }

/* iterate thru map and verify */
        map<string,struct teamstats>::iterator it = eg.begin();
        while(it != eg.end()) {
                cout << it->first << endl;
                cout << "\tfor: " << it->second.points_for
                        << " against: " << it->second.points_against
                        << " (" << it->second.games << " games)\n";
                it++;
        }

        return 0;
}

---

Notice I left "team name" out of the struct since that is the first element of the map. The output should be:

one
for: 0 against: 0 (5 games)
three
for: 2 against: 4 (5 games)
two
for: 1 against: 2 (5 games)


Notice they are not in order.

Oke,

I think I understand the trick.
Can I do :

Code:

---

map<string, teamstats>::iterator it = allteams.find(one);
played_games +=1 ;
eg.insert(pair<string,struct teamstats>(names[i],tmp));

---

If one has played one more game.

Roelof

Quote:

---

Originally Posted by roelof

Can I do :

Code:

---

map<string, teamstats>::iterator it = allteams.find(one);
played_games +=1 ;
eg.insert(pair<string,struct teamstats>(names[i],tmp));

---

If one has played one more game.

---

Nope, again: "played_games" does not belong to the map. You had this misunderstanding before -- that somehow you could give a variable the same name as a struct member and use it as some kind of shorthand. No. If you have an independed "int played_games" this has nothing to do with the member of struct teamstats (that has the same name, but they have nothing else to do with one another).

However, the "second" of the map pair is a struct with a member played_games, so:

Code:

---


map<string, teamstats>::iterator it = allteams.find("one");
it->second.played_games++;

---

will work. Notice the iterator uses indirect notation (it->second) whereas the members of the struct use direct notation (second.played_games) since it is a map of structs, not struct pointers.

oke,

So if I want to change the value of points_made I have to do :

it.second.points_made= it.second.points_made + 10 ;
And don't I have to do something like insert to put the changesd value of points_made in the map ?

Roelof

Edit : I know there are not in order. Someone give me the tip to copy everything to a vector map and then sort it like I want it.

Quote:

---

Originally Posted by roelof

it.second.points_made= it.second.points_made + 10 ;
And don't I have to do something like insert to put the changesd value of points_made in the map ?

---

it->second.points_made (also you can use "it->second.points_made += 10")

No, you don't need to insert anything new. You are changing the value of an existing variable. The iterator is like a pointer to a member of the map, which can be identified by their "first" part, the string team name (find() etc use the first part, and it must be unique, ie, you cannot have two members with the same string identifier first part). The "second" part is the struct.

Oke,

I think I understand it.
Thank you for the help and explanation.

Roelof

I don't understand why you're trying to complicate things so much, MK27.
Instead of:

Code:

---

eg.insert(pair<string,struct teamstats>(item.team_name, item));

---

It's better to use pianorain's suggestion, since you don't have to specify type:

Code:

---

allteams.insert(make_pair(item.team_name, item));

---

Also, drop the "struct"! It's absolutely not necessary in C++. It only makes the code longer and harder to understand.
Now, while make_pair is good and all, there's an even better way to do it that is even shorter and more understandable:

Code:

---

allteams[item.team_name] = item;

---

Done. Simple, wasn't it?

Hello Elysia,

So if I understand it right.
the only thing I have to do is allteams[teamstats.team_name] = teamstats to put everything in the map of structs ?

Roelof

To insert your struct into the map, yes.

oke,

and the explanation of change a value is right from MK27 ?

Roelof

Using the [] operator for insert should only be done if you don't mind overwriting values. For example, let's say you've already got a team_name of "one" in your map. If you're expecting to insert a new team with the same name of "one", the [] operator will do this without any sort of warning. Using insert will protect you from accidently overwriting values already in your map. In the case above, using insert to insert your new team will return a pair of which the second value will tell you that you tried to insert a team with an existing team name.

Quote:

---

Originally Posted by roelof

and the explanation of change a value is right from MK27 ?

---

It works -- but there is a short sweet (should have been obvious...) option there too (mebbe I just forgot it):

Code:

---

allteams["one"].points_against += 10;

---

Sorry roelof, I actually don't use C++ that much.

Quote:

---

Originally Posted by Elysia

I don't understand why you're trying to complicate things so much, MK27.
Instead of:
It's better to use pianorain's suggestion, since you don't have to specify type:

---

I see like zero (or very trivial) difference there, but alternatives are nice.

Quote:

---

Originally Posted by Elysia

Now, while make_pair is good and all, there's an even better way to do it that is even shorter and more understandable:

---

That is much better. C++ just got a little less clunky. :p

Quote:

---

Originally Posted by MK27

It works -- but it looks like there is a short sweet (should have been obvious...) option there too (mebbe I just forgot it):

Code:

---

allteams["one"].points_against += 10;

---

---

Careful with that. Just remember that the index operator will insert the value if it doesn't exist and return a reference to the inserted item.
If this is what you want, then this is fine. But don't go looking up stuff in the map with it.

Hoi Elysia.

I don't want to looking up stuff , just update it with a new value according to the data - file.

Roelof

Can I use this allteams["one"].points_against += 10;

Points_against exist because I made it when I insert a record of a team the first time a team is being found in the text file.

Roelof

You can use it so long as you know that the struct is inside the map. If it isn't, it will be added.

I know that.

See this :

Code:

---

struct teamstats {
    string team_name;
    int played_games, point_made, points_against, woord;
};

 map<string, teamstats> allteams;

---

Roelof

That declaration shows nothing.
IF you have already added a teamstats to the map using the key "one", then yes, you can it.
I am simply restating the condition. If you meet this, then go ahead.

I came across this thread in my search to understand the map container and how to use it. I'm barely getting by with C (I just started programming a month ago), and C++ is an entirely different ball of wax. My application is much different, but the code in this thread has helped me immensely. Thank you so much.

Hello Elysia,

Is this better :

Code:

---

map<string, teamstats>::iterator it = allteams.find(home_team);
    if (it == allteams.end())
    {
        teamstats.home_team = home_team ;
        teamstats.played_games = 1 ;
        if home_score > away_score then teamstats.game_points = 3 ;
        teamstats.points_made = home_score ;
        teamstats.points_against = away_score;
        allteams[teamstats.team_name] = teamstats ;
    }
    else
    {
      allteams[home_team].played_games += 1;
      if home_score > away_score then allteams[home_team].game_points+=3;
      allteams[home_team].points_made += home_score ;
      allteams[home_team].points_against += away_score;
    };

---

Roelof

Quote:

---

Originally Posted by roelof

Hello Elysia,

Is this better :

Code:

---

map<string, teamstats>::iterator it = allteams.find(home_team);
    if (it == allteams.end())
    {
        teamstats.home_team = home_team ;
        teamstats.played_games = 1 ;
        if home_score > away_score then teamstats.game_points = 3 ;
        teamstats.points_made = home_score ;
        teamstats.points_against = away_score;
        allteams[teamstats.team_name] = teamstats ;
    }
    else
    {
      allteams[home_team].played_games += 1;
      if home_score > away_score then allteams[home_team].game_points+=3;
      allteams[home_team].points_made += home_score ;
      allteams[home_team].points_against += away_score;
    };

---

Roelof

---

That's very inefficient. You're looking up the same item 5 times. Here's a much clearer and more efficient way to do it:

Code:

---

map<string, teamstats>::iterator it = allteams.find(home_team);
if (it == allteams.end())
{
    teamstats &team = allteams[home_team];
    team.home_team = home_team;
    team.played_games = 1;
    if (home_score > away_score)
        team.game_points = 3;
    team.points_made = home_score;
    team.points_against = away_score;
}
else
{
    teamstats &team = it->second;
    team.played_games += 1;
    if (home_score > away_score)
        team.game_points +=3;
    team.points_made += home_score;
    team.points_against += away_score;
};

---

Horray for references!

However, if this were my own code I would consider that it is not necessary to store the team name both in the map's key and inside the struct that is the map's mapped value. I would also have a constructor and useful function called AddGame which all allows me to do the whole thing like this:

Code:

---

map<string, teamstats>::iterator it = allteams.find(home_team);
if (it == allteams.end())
    allteams[home_team] = teamstats(home_score, away_score);
else
    it->second.AddGame(home_score, away_score);

---

Wait, scratch that, what was I thinking! I would actually just write and use the default constructor, and replace all of that with just this:

Code:

---

allteams[home_team].AddGame(home_score, away_score);

---

That's right, it's now a one-liner! (excluding the default constructor and AddGame method)
Working out the constructor and contents of the AddGame member function I leave as an exercize for the reader. They're actually pretty simple though.

Oke,

I see your point.
But the meaning was if a team don't exist in the map then insert it into the map.
If a team does exist (else part) then update all the data.

If I understand your code right, then it seems that on else it will be inserted.

Roelof

The point iMalc was trying to get across is that index operator performs a lookup. Lookups take time which isn't constant. You're wasting time, so to speak.
Instead, if you were to remember where the struct is inserted, you can update it without a lookup, saving time instead.

Even in the single line code sample I posted if a team doesn't exist in the map then it insert its into the map, otherwise it updates it.

oke

Then I will study it and read a lot about references before I can write the addgame and constructor part.

This is very heavy stuff for a person who trying to learn c++ and write's his first programm.

Roelof

Quote:

---

Originally Posted by roelof

This is very heavy stuff for a person who trying to learn c++ and write's his first programm.

---

Yep, I imagine it is!
Give it a go and feel free to ask more questions about it if you get stuck. (In this same thread is probably fine too)

Quote:

---

Originally Posted by Elysia

The point iMalc was trying to get across is that index operator performs a lookup. Lookups take time which isn't constant. You're wasting time, so to speak.
Instead, if you were to remember where the struct is inserted, you can update it without a lookup, saving time instead.

---

Great point. So use a reference or an iterator.

Code:

---

//reference:
teamstats &ref = allteams["name"];
//iterator:
map<string, teamstats>::iterator it = allteams.find("name");

---

Oke,

Im know trying to understand how I can make a constructor.

I found this example

Code:

---

// constructing maps
#include <iostream>
#include <map>
using namespace std;

bool fncomp (char lhs, char rhs) {return lhs<rhs;}

struct classcomp {
  bool operator() (const char& lhs, const char& rhs) const
  {return lhs<rhs;}
};

int main ()
{
  map<char,int> first;

  first['a']=10;
  first['b']=30;
  first['c']=50;
  first['d']=70;

  map<char,int> second (first.begin(),first.end());

  map<char,int> third (second);

  map<char,int,classcomp> fourth;                // class as Compare

  bool(*fn_pt)(char,char) = fncomp;
  map<char,int,bool(*)(char,char)> fifth (fn_pt); // function pointer as Compare

  return 0;
}

---

I see that classcomp for me is teamstats.
and the map for me is map<string, teamstats> allteams;

But what does fncomp do here.
And why first , second and fourth ?

Roelof

This example is not about a constructor. I would not try to understand it at this point.
But, you don't have a book? You should first read up some chapter on constructors. Then you could make better progress.

Hello Elysia,

I don't have a book.
All the information I have can be found at this site en other sites.

Roelof

Edit 1 :
Is this good information : http://www.fredosaurus.com/notes-cpp...structors.html
Or is this better : http://cplus.about.com/od/learning1/ss/constructors.htm

I suggest you get one. You cannot learn the language on tutorials and webpages alone.
See the books thread. I can recommend Accelerated C++ myself.
Your local library might have some programming book available, too, if you're feeling cheap.

Quote:

---

Originally Posted by roelof

I see that classcomp for me is teamstats.

---

Nope. In fact, classcomp is a primitive class (C++ classes are really structs with a different label on them). It could have been written like this:

Code:

---

class classcomp {
    public:
        bool operator() (const char& lhs, const char& rhs) const {
            return lhs<rhs;
        }
};

---

This is then used as the comparison function for the map:

Code:

---

map<char,int,classcomp> fourth;

---

This determines how the elements are sorted internally. Try this:

Code:

---

#include <iostream>
#include <map>

using namespace std;

class classcomp {
        public:
                bool operator() (const char& lhs, const char& rhs) const {
                        return lhs>rhs;
                }
};

int main() {
        map<char,int,classcomp> test;
        map<char,int>::iterator it;

        test['a'] = 50;  // the int values are irrelevant in this program
        test['b'] = 50;
        test['c'] = 200;

        it = test.begin();
        while (it != test.end()) {
                cout << it->first << endl;
                it++;
        }

        return 0;
}

---

Notice I switched < to > in the compare function, so this prints out:

c
b
a

Quote:

---

Originally Posted by roelof

All the information I have can be found at this site en other sites.

---

I also very strongly recommend you get a beginner C++ book roelof. Your life will be much easier. You can probably find something used on Amazon for <$10 US.

Hello,

I have asked the library for the Accelared C++ book.
So i have to wait if they find it.

Roelof

Hello,

I have found a pf version of this book and have read chapter 0.
Two questions.

1) Is it still necessarly to put std:: before cout ?
2) How can I see if I did the exercises the right way ?

Roelof

You must do either of three things:
1) Use std::cout.
2) Put using std::cout; at the top of your file and then use cout.
3) Put using namespace std; at the top of your file and then use cout.
(But do NOT use a using directive in a header file.)

oke,

And do you have a answer to 2).

Roelof

Hmmm. I don't have that book lying around, so unfortunately, I do not know.
But there ought to some kind of answer section.

Hello Elysia.

The only thing I can find it this site : Accelerated C++
But there is only the source code and not the answers to the other questions.

Roelof

Quote:

---

Originally Posted by roelof

... chapter 0 ...

---

I really got a kick out of that... only a C book...