Why is my program not working ?

Code:

here my program:

#include<iostream>

using namespace std;

int main()
{
	//Declare Variables 
	char operation = ' ';
	int num1 = 0;
	int num2 = 0;
	int answer = 0;

	//Input Items
	cout<<"Enter A(Add),S(Subtract),M(Multiply) or D(Divide):";
	cin>>operation;
	operation = toupper(operation);
	cout<<endl;
	cout<<"Enter the first number:";
	cin>>num1;
	cout<<"Enter the second number:";
	cin>>num2;
	cout<<endl;

	//Calculate and Display Output

	switch(operation)
	{
	case 'A':
		answer=num1+num2;
		cout<<"Answer:"<<answer<<endl;
		break;
	case 'S':
		answer=num1-num2;
		cout<<"Answer:"<<answer<<endl;
		break;
	case 'M':
		answer=num1*num2;
		cout<<"Answer:"<<answer<<endl;
		break;
	case 'D':
		answer=num1/num2;
		cout<<"Answer:"<<answer<<endl;
		break;
	default:
		cout<<"Error"<<endl;
	}	//end switch

	system("PAUSE");
	return 0;
}	//End of Program

1. i want it to display an error message if any letter other than a,s,d,m is entered( it shouldnt ask for two numbers)

2. for my case 's' , i want it to swap the number(subtracting second number from first number) but only if the first number is larger or equal to the second number.

3. for my case 'd'. i want it to divide larger number by smaller number always.

*** i have read my text book over and over again and i cant seem to figure it out** please help me =]

seems like you want stuff to be done only IF certain conditions are met.

Cprogramming.com Tutorial: If Statements

1.

Code:

if(operation == 'a' || operation == 's' ......................)
{
//do this
}
else
//do that

2.

Code:

answer = num1-num2;
if(answer < 0)
   answer = answer*-1;

3.

Code:

if(num2 != 0 && num1 > num2)
{
answer = num1/num2;
}
else if(num1 != 0 && num1 < num2)
{
answer = num2/num1;
}
else
//assign w/e value to answer...since division by 0 is not allowed

btw i suggest you ask for the numbers first, then ask operation...that you can evade the first question all together and have the switch decide if the operation is legal... ie:

Code:

#include<iostream>

using namespace std;

int main()
{
	//Declare Variables 
	char operation = ' ';
	int num1 = 0;
	int num2 = 0;
	int answer = 0;

	//Input Items

	cout<<"Enter the first number:";
	cin>>num1;
	cout<<"Enter the second number:";
	cin>>num2;
	cout<<endl;

	cout<<"Enter A(Add),S(Subtract),M(Multiply) or D(Divide):";
	cin>>operation;
	operation = toupper(operation);

	cout<<endl;
	//Calculate and Display Output

	switch(operation)
	{
	case 'A':
		answer=num1+num2;
		cout<<"Answer:"<<answer<<endl;
		break;
	case 'S':
		answer=num1-num2;
                 if(answer < 0)
                         answer = answer*-1;
		cout<<"Answer:"<<answer<<endl;
		break;
	case 'M':
		answer=num1*num2;
		cout<<"Answer:"<<answer<<endl;
		break;
	case 'D':
                        if(num2 != 0 && num1 > num2)
                            {
                                    answer = num1/num2;
                                   
		cout<<"Answer:"<<answer<<endl;
                              }
                            else if(num1 != 0 && num1 < num2)
                           {
                               answer = num2/num1;
                 
		cout<<"Answer:"<<answer<<endl;
                            }
                  else
		cout<<"Divide by 0"<<answer<<endl;
		break;
	default:
		cout<<"Error"<<endl;
	}	//end switch

	system("PAUSE");
	return 0;
}	//End of Program

how would you make ask for the a , s , m , d first before it goes to ask for the num1 and num2 ?