Re: I need help with "Inheritance" and "Polymorphism"

"Fao" wrote:


<snip>

I think that assignment is seriously flawed. It is hard to get a simple
student assignment that uses polymorphism but how any one could do some
sensible polymorphism with that assignment is beyond me. I think you have
done about as good as anyone could do, under the circumstances.

Here is a pretty darn good definition of polymorphism from Wikipedia.
Wiki snip:
---------------------------
In object-oriented programming theory, polymorphism is the ability of
objects belonging to different types to respond to method calls of methods
of the same name, each one according to the right type-specific behaviour.
The programmer (and the program) does not have to know the exact type of the
object in advance, so this behavior can be implemented at run time (this is
called late binding or dynamic binding).

The different objects involved only need to present a compatible interface
to the clients (the calling routines). That is, there must be public methods
with the same name and the same parameter sets in all the objects. In
principle, the object types may be unrelated, but since they share a common
interface, they are often implemented as subclasses of the same parent.
Though it is not required, it is understood that the different methods will
also produce similar results (for example, returning values of the same
type).

-------------------- end snip -------------------

Thus really one wants three classes, a base class and at least two derived
classes. As I said, you have done good considering what you had to work
with. Here is the code you posted Friday patched up and tested. I didn't
test as seriously as if this were important but it passes some casual tests.
It does exhibit polymorphism , albeit in a kludgy form. Note also that
max2() sticks out like a goiter. Silk purses and pig's ears and all that.

As I scan this for a final time, I see things that should have been changed
for purity. For example, number should be demoted to a member function
rather than being part of the object. But I am nervous about changing
things after testing without a retest. The call on calculation() could be
embedded in MaxType, that might be neater. The indentation is a mix of my
style and your style.

If you feel a need to contact me, you can send me e-mail by subtracting 100
from my posted address.

-------------------------
/* cin.get() added here and there are a peculiarity needed by my compiler.
Feel free to remove.*/
#include <iostream>
#include <cfloat> // for DBL_MIN

using namespace std;

const int SENTINEL = -999;

class SumType
{
protected:
   double n[10]; // note changed type
   int sum;
   int counter;
   int number;
   // int *ptr0, *ptr1, *ptr; I never did figure out what you had in mind
here
public:
   SumType();
   void input();
   double output();
};
//------------------------
class MaxType: public SumType
{
protected:
   double max, max2;
public:
   MaxType() :max(0), max2(0) { }
   void calculation();
   double output();
   double get_max2() {return max2;} // new
private:
   void swap(int i, int j); // new code
   int c_max(void); // new code
};
///////////////////////////////////////////////////////////////////////////////////
SumType::SumType()
{
sum = 0;
counter = 0;
number = 0;
}
//............................
// get user input and save it for MaxType
void SumType::input()
{
cout << "Enter numbers : To stop program enter" << " " << SENTINEL <<endl;
cin >> number;
while (number != SENTINEL)
   {
   sum = sum + number;
   n[counter] = number; // this lack may have been your biggest problem.
   counter++;
   cin >> number;
   }
}
//.......................
double SumType::output()
{
return sum;
}
//..............
// swap n[i] nd n[j]
void MaxType::swap(int i, int j)
   {
   double temp = n[i];
   n[i] = n[j];
   n[j] = temp;
   }
//.....................
// find and return the index of the current maximum
int MaxType::c_max(void)
   {
   double maxx = n[0];
   int ix = 0;
   for(int i=1; i<counter; i++)
      if(n[i] > maxx)
         {
         maxx = n[i];
         ix = i;
         }
    return ix;
    }
//////////////////////////////////////////////////////////////////////////////////////
// compute and save max and max2
// technique: find and remove max from contention.
// This is inefficient but easier to write and debug

void MaxType::calculation()
   {
   if(counter<2)
      {
      cout << "Not enough input\n";
      cin.get();
      exit(1); // abort
      }
    int idx = c_max();
    max = n[idx];
    n[idx] = DBL_MIN; // won't find *him* again!
    idx = c_max();
    max2 = n[idx];
}
//.................
double MaxType::output()
{
return max;
}
//----------------------
void test_sum()
   {
   // test SumType
   SumType s1;
   s1.input();
   s1.output();
   cout << "The sum is "<< s1.output()<<endl;
   cout << "end sum test\n";
   }
//--------------
void test_max()
   {
   MaxType m1;
   m1.input();
   m1.calculation();
   m1.output();
   cout << "The largest number is "<< m1.output() << endl ;
   cout << "The second largest is " << m1.get_max2() << endl;
   cout << "End MaxTest\n";
   }

/////////////////////////////////////////////////////////////////
int main()
   {
   //test_sum();
   //test_max();

   // both types work by themselves. now try the polymorhic thing

   SumType ss;
   MaxType mm;
   SumType * psum1 = &ss;
   MaxType * pmax1 = &mm;
   psum1 -> input();
   cout << "The output of SumType is " << psum1->output() << endl;
   pmax1->input();
   pmax1->calculation();
   cout << "The output of MaxType is " << pmax1->output() << endl;

   cin.get();
   cin.get();
   return 0;
   }