help with classes

can someone give me an example of how to use classes? i read the tuorial and still dont get it, just a simple example of how to use functions in a class. this is what i got so far but im getting 3 errors on this.

Code:

#include	<iostream>   

using namespace std;

class Average

{
	public:
		float findaverage();
	
	private:
			int		score;
			float	sum;
			int     NumScores;
};


float Average::findaverage()

{
	float findaverage(int score, int NumScores);

	cout <<"enter a score" << endl;

	while ( score != 0 )
	{
		sum += score;
		NumScores++; 
        cout << "Enter a test score ( 0 to quit ): "; 
        cin >> score; 
	}


	return sum/NumScores;

}

int main()
{
	Average Numbers;
	Average.findaverage();

	cout <<"The average is" << Average <<endl;
	return 0;
}

what are your errors?

Code:

addit.cpp
C:\Program Files\Microsoft Visual Studio\MyProjects\addit\addit.cpp(44) : error C2143: syntax error : missing ';' before '.'
C:\Program Files\Microsoft Visual Studio\MyProjects\addit\addit.cpp(44) : error C2143: syntax error : missing ';' before '.'
C:\Program Files\Microsoft Visual Studio\MyProjects\addit\addit.cpp(46) : error C2275: 'Average' : illegal use of this type as an expression
        C:\Program Files\Microsoft Visual Studio\MyProjects\addit\addit.cpp(10) : see declaration of 'Average'
Error executing cl.exe.

addit.obj - 3 error(s), 0 warning(s)

Code:

#include <iostream>
using namespace std;

class Sprite{
 private:
   int HP;
   int MP;
   int Att;
   int Def;
   int Lvl;

 public:
   Sprite(int hp, int mp, int att, int def);
   ~Sprite();
   int getHP(){return HP;}
   int getMP(){return MP;}
   int getAtt(){return Att;}
   int getDef(){return Def;}
   int getLvl(){return Lvl;}
   void LvlUP();
};

Sprite::Sprite(int hp, int mp, int att, int def){
   Lvl=1; HP=hp; MP=mp; Att=att; Def=def;
}

void Sprite::LvlUP(){
   Lvl++;
   HP += Lvl*5;
   MP += Lvl;
   Att += Lvl;
   Def += Lvl;
}

int main(){
/*A pointer that points to the whole class, and can de-reference anything inside of it*/
   Sprite *Sebba = new Sprite(50,10,5,5);
   cout << "Sebba has "<<Sebba->getHP()<<" hit points." <<endl;
   Sebba->LvlUP();
   cout << "Sebba now has "<<Sebba->getHP()<<" hit points." <<endl;
   return 0;
}

thanks Krak ill check your example.

Originally Posted by InvariantLoop

thanks Krak ill check your example.

I just updated it real quick so be sure you're reading the updated version that works okay.

Code:

#include    <iostream>   

using namespace std;

class Average

{
    public:
        float findaverage();
            
    private:
            int score;
            float sum;
            int NumScores;
};


float Average::findaverage()
{
    //float findaverage(int score, int NumScores);
    //The function header is included at the beginning of the definition, not inside the definition.
    //You already have the function header in the previous line.  In addition, you declared the function
    //without any parameters, so your function defintion cannot suddenly have two parameters:


    cout <<"enter a score" << endl;

    while ( score != 0 )
    {
        sum += score;
        NumScores++; 
        cout << "Enter a test score ( 0 to quit ): "; 
        cin >> score; 
    }


    return sum/NumScores;

}

int main()
{
    Average Numbers;
    //Average.findaverage();
    //You declared a variable named Numbers of type Average, so Numbers is your variable name.  
    //It's similar to when you declare a variable like this: int num;
    //You wouldn't then do this: int = 10;

    //You can declare a variable to store the average.  The type of the variable has to be the
    //same type as the return type of the function.
    float avg = Numbers.findaverage();
    
    //cout <<"The average is" << Average <<endl;
    cout <<"The average is" << avg <<endl;
    return 0;
}

Now, you won't get any errors, however the result won't be correct. If you don't initialize your class member variables, they will contain junk values. So, when you do:

sum += score;

that's the same as:

sum = sum + score;

and sum on the right side of the equals sign does not equal zero the first time you do that--it contains some junk value. You have the same problem with:

NumScores++;

NumScores contains a junk value that you increment.

The way you initialize class members is with a constructor function. A constructor function has the same name as the class, and it has no return type:

Code:

Average()
{
    score = 0;
    sum = 0;
    NumScores = 0;

    cout<<"constructor was called"<<endl;
}

You don't have to pass member variables to a class function. Unlike regular variables, member variables are accessible directly from inside a class function. Notice in the constructor above, I did not pass score, sum, or NumScores to the function, yet I can still change their values inside the function. That is true with all class functions--you can use the member variables inside the function to change their values. The constructor will be called automatically whenever you create a class object, e.g Average Numbers.

You will have to change your while loop condition because score will start off equaling 0.

can you please explain more in detail? i see that it gives the wrong result but i dont undersatnd why? when i initialize sum =0; should i do this as private in the class?

when i initialize sum =0; should i do this as private in the class?

You can't do this:

private:
int sum = 0;
float sum = 0;
int NumScores = 0;

There are a couple of ways to assign values to members:
1) make them public and then just assign 0 to them anywhere in your code in main().
2) create a 'setter' function, which is a function that takes a parameter, and then sets the value of a private member to the value sent to the function
3) use a constructor

You should stick with 3).

i see that it gives the wrong result but i dont undersatnd why?

while ( score != 0 )

score starts off equaling zero, so the loop is skipped. You could ask the user to enter -100 if they want to quit the loop and check for that instead.

ok i used the initialization constructor but i still get the wrong result. 12/3 = 4 not 3.

Code:

#include    <iostream>   

using namespace std;

class Average

{
    public:
            float findaverage();
            Average();
            
    private:
            int score;
            float sum;
            int NumScores;
};
Average::Average()
{
    score = 0;
    sum = 0.0;
    NumScores = 0;

    cout<<"constructor was called"<<endl;
}


float Average::findaverage()
{
    //float findaverage(int score, int NumScores);
    //The function header is included at the beginning of the definition, not inside the definition.
    //You already have the function header in the previous line.  In addition, you declared the function
    //without any parameters, so your function defintion cannot suddenly have two parameters:


    cout <<"enter a score" << endl;

    while ( score != -100 )
    {
        sum += score;
        NumScores++; 
        cout << "Enter a test score ( 0 to quit ): "; 
        cin >> score; 
    }


    return sum/NumScores;

}

int main()
{
    Average Numbers;
    //Average.findaverage();
    //You declared a variable named Numbers of type Average, so Numbers is your variable name.  
    //It's similar to when you declare a variable like this: int num;
    //You wouldn't then do this: int = 10;

    //You can declare a variable to store the average.  The type of the variable has to be the
    //same type as the return type of the function.
    float avg = Numbers.findaverage();
    
    cout <<"The average is " << avg <<endl;
    return 0;
}

Your logic loop:

Code:

cout <<"enter a score" << endl;

while ( score != -100 )
{
        sum += score;
        NumScores++; 
        cout << "Enter a test score ( 0 to quit ): "; 
        cin >> score; 
}

A logic loop that works

Code:

 cout << "Enter a test score ( 0 to quit ): "; 
cin >> score;

while ( score != -100 )
{
     sum += score;
     NumScores++; 
     cout << "Enter a test score ( 0 to quit ): ";     
     cin >> score;
}

See the difference?

yep thanks for pointing that out.