hello friends i have created a calculator in c++ using the class concepts having 4 basic operation : 
1.  Addition upto n terms 
2. Subtraction upto n terms
3. multiplication upto n terms.
4.division

please give suggestion to improve the code....

here's the code:

  calculator.h


#ifndef CALCULATOR_H
#define CALCULATOR_H


class calculator
{
    public:
        int calc();
        void switchcase(int);
        void addition();
        void subtraction();
        void multiplication();
        void division();
    private:
        int a,choice,n,b;
        float res=0;
        char ch;
};

#endif // CALCULATOR_H

calculator.cpp


#include <iostream>
#include <cstdlib>
#include "calculator.h"
using namespace std;

void calculator::addition()
{
    cout << "\n\tEnter The numbers you want to add : ";
    cin >> n;
    res=0;
    for(int i=0; i<n; i++)
    {
        cout << "Enter the number "<< i+1 <<": ";
        cin >> a;
        res=res+a;
    }
    cout << "\nThe result is : " << res;
}

void calculator::subtraction()
{
    cout << "\n\tEnter the numbers you want to subtract : ";
    cin >> n;
    res=0;
    for(int i=0; i<n; i++)
    {
        cout << "Enter the number "<< i+1 <<": ";
        cin >> a;
        if(i==0)
        {
            res=a;
        }
        else
        {
            res=res-a;
        }
    }
    cout << "\nThe result is : " << res;
}

void calculator::multiplication()
{
    cout << "\n\tEnter the numbers you want to multiply : ";
    cin >> n;
    res=1;
    for(int i=0; i<n; i++)
    {
        cout << "Enter the number "<< i+1 <<": ";
        cin >> a;
        res=res*a;
    }
    cout << "\nThe result is : " << res;
}

void calculator::division()
{
    cout << "\nEnter the first number : ";
    cin >> a;
    cout << "Enter the second number : ";
    cin >> b;
    res=a/b;
    cout << "\nThe result is : " << a <<"/"<< b <<" = " <<res;
}



void calculator::switchcase(int choice)
{
    switch (choice)
    {
    case 1:
        addition();
        break;
    case 2:
        subtraction();
        break;
    case 3:
        multiplication();
        break;
    case 4:
        division();
        break;
    default:
        cout << "You have entered an wrong choice ....";
    }
}

int calculator::calc()
{
  do{
    cout << "\n\t\t\t##################";
    cout << "\n\t\t\t####CALCULATOR####";
    cout << "\n\t\t\t##################";

    cout << "\n\n\t1.Addition";
    cout << "\n\t2.Subtraction";
    cout << "\n\t3.Multiplication";
    cout << "\n\t4.Division";
    cout << "\n\nPlease Enter Choice (1/2/3/4) : ";
    cin >> choice;

    switchcase(choice);

    cout << "\n\n\tDo you want to continue? (Y/n) : ";
    cin >> ch;
    system("clear");
  }while(ch=='y' || ch=='Y');
}

Main.cpp


#include <iostream>
#include "calculator.h"
using namespace std;

int main()
{
    calculator obj;
    obj.calc();
    return 0;
}