Why is my program not working ?

Code:

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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

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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:

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

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2.

Code:

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

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3.

Code:

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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

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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

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