Print float as decimal and fix wrong output - c++

Question

I was experimenting with classes and I wrote this example code. The user enters their register number and two marks. The output should be their register number and the average of the two input marks. I have two questions:
How do I use the float type to display the output average marks in decimal form if I am using constructors?
Why is the output of the register number not correct? The code is given below.
#include<iostream.h>
#include<conio.h>
class abc
{
int reg, mark1, mark2;
public:
int avg;
abc(int reg, int mark1, int mark2)
{
avg = (mark1 + mark2) / 2;
}
void display()
{
cout<<"Your average mark is:\n"<<avg<<"\n";
cout<<"Your Register Number is:\n"<<reg<<"\n";
}
};
void main()
{
clrscr();
int num, m1, m2;
cout << "Enter your register number\n";
cin >> num;
cout << "Enter your Mark 1 and Mark 2:\n";
cin >> m1 >> m2;
abc s1(num,m1,m2);
s1.display();
getch();
}
I am getting the average (without the decimal) and the register number output is 11196.

Answer 1

In C++ the constructor arguments are not automatically stored in the class members. I would change your class declaration to the following, note that I have used different names for the members and function arguments (I like to use m_ for private member variables).
class abc
{
int m_reg, m_mark1, m_mark2;
public:
int avg;
abc (int reg, int mark1, int mark2)
: m_reg(reg), m_mark1(mark1), m_mark2(mark2)
{
avg = (m_mark1+m_mark2)/2;
}
void display()
{
cout<<"Your average mark is:\n"<<avg<<"\n";
cout<<"Your Register Number is:\n"<<m_reg<<"\n";
}
};
If you expect your result to be a decimal (and not rounded or floored to the nearest integer) you need to change your average declaration and calculation to:
// The new declaration
double avg;
// In your constructor
avg = (double) (m_mark1 + m_mark2) / 2.0;

Answer 2

All you need to do is add value to reg property in your class constructor:
abc (int r, int mark1, int mark2)
{
reg = r;
avg = (mark1+mark2)/2;
}
In addition to that, why is avg an integer instead of double? Consider this:
class abc
{
int reg,mark1,mark2;
public:
double avg;
abc (int r, int mark1, int mark2)
{
reg = r;
avg = (double)(mark1+mark2)/2.00;
}
void display()
{
cout<<"Your average mark is:\n"<<(int)avg<<"\n";
cout<<"Your Register Number is:\n"<<reg<<"\n";
}
};
If you want 4.4 to be displayed as 4, and 4.5 as 5 use this hack:
avg = avg + 0.5;
cout << (int)avg << "\n";