Hey all,
Started learning C++ a few days ago, and just finished my first half-useful program. It's basically a converter for weight, distance and currency.
My question is, is there any better way to do this? Any general tips?
And for some reason, when I did the do { code } while, I got an error code about the compiler not recognising the system and return commands. Then, suddenly, it worked. Why?

The code:

Code:
#include <iostream>
double pounds, kilograms, feet, metres, centimetres, inches, kilometres, miles, yards, EUR, USD, GBP, CNY;
using namespace std;
 
int choice;

int main()

{
	cout << "Weight: \n" << endl;
	cout << "Pounds      --> Kilograms:   type 1. " << endl;
	cout << "Kilograms   --> Pounds:      type 2. " << endl;
	cout << "\nDistance:\n" << endl;
	cout << "Feet        --> Metres:      type 3. " << endl;
	cout << "Metres      --> Feet:        type 4. " << endl;
	cout << "Inches      --> Centimetres: type 5. " << endl;
	cout << "Centimetres --> Inches:      type 6. " << endl;
	cout << "Miles       --> Kilometres:  type 7." << endl;
	cout << "Kilometres  --> Miles:       type 8." << endl;
	cout << "Yards       --> Metres:      type 9." << endl;
	cout << "Metres      --> Yards:       type 10." << endl;
	cout << "\nCurrency:\n" << endl;
	cout << "EUR --> USD: type 11." << endl;
	cout << "USD --> EUR: type 12." << endl;
	cout << "GBP --> USD: type 13." << endl;
	cout << "USD --> GBP: type 14." << endl;
	cout << "EUR --> GBP: type 15." << endl;
	cout << "GBP --> EUR: type 16." << endl;
	cout << "CNY --> EUR: type 17." << endl;
	cout << "EUR --> CNY: type 18." << endl;
	cout << "CNY --> USD: type 19." << endl;
	cout << "USD --> CNY: type 20.\n" << endl;

	do
	{
	
	cin >> choice;

    switch (choice)
	{
	case 1:
		cout << "\nEnter the number of pounds: ";
		cin >> pounds;
		cout << "\n" << pounds << " lbs is " << pounds * 0.45 << " kg." << endl;
		break;
	case 2:
		cout << "\nEnter the number of kilos: ";
		cin >> kilograms;
		cout << "\n" << kilograms << " kg is " << kilograms * 2.2 << " pounds." << endl;
		break;
	case 3:
		cout << "\nEnter the number of feet: ";
		cin >> feet;
		cout << "\n" << feet << " ft is " << feet * 0.3 << " m." << endl;
		break;
	case 4:
		cout << "\nEnter the number of meters: ";
		cin >> metres;
		cout << "\n" << metres << " m is " << metres * 3.2 << " ft." << endl;
		break;
	case 5:
		cout << "\nEnter the number of inches: ";
		cin >> inches;
		cout << "\n" << inches << " inches is " << inches * 2.55 << " ft." << endl;
		break;
	case 6:
		cout << "\nEnter the number of centimetres: ";
		cin >> centimetres;
		cout << "\n" << centimetres << " cm is " << centimetres * 0.4 << " inches." << endl;
		break;
	case 7:
		cout << "\nEnter the number of miles: ";
		cin >> miles;
		cout << "\n" << miles << " miles is " << miles * 1.6 << " km." << endl;
		break;
	case 8: 
		cout << "\nEnter the number of kilometers: ";
		cin >> kilometres;
		cout << "\n" << kilometres << " km is " << kilometres * 0.62 << " miles." << endl;
		break;
	case 9: 
		cout << "\nEnter the number of yards: ";
		cin >> yards;
		cout << "\n" << yards << " yards is " << yards * 0.91 << " meters." << endl;
		break;
	case 10:
		cout << "\nEnter the number of meters: ";
		cin >> metres;
		cout << "\n" << metres << " m is " << metres * 1.1 << " yards." << endl;
		break;
	case 11: 
		cout << "\nEnter the amount of EUR: ";
		cin >> EUR;
		cout << "\n" << EUR << " EUR is " << EUR * 1.26 << "USD. " << endl;
		break;
	case 12:
		cout << "\nEnter the amount of USD: ";
		cin >> USD;
		cout << "\n" << USD << " USD is " << USD * 0.79 << " EUR." << endl;
		break;
	case 13:
		cout << "\nEnter the amount of GBP: ";
		cin >> GBP;
		cout << "\n" << GBP << " GBP is " << GBP * 1.59 << " USD." << endl;
		break;
	case 14:
		cout << "\nEnter the amount of USD: ";
		cin >> USD;
		cout << "\n" << USD << " USD is " << USD * 0.63 << " GBP." << endl;
		break;
	case 15:
		cout << "\nEnter the amount of EUR: ";
		cin >> EUR;
		cout << "\n" << EUR << " EUR is " << EUR * 0.79 << " GBP." << endl;
		break;
	case 16:
		cout << "\nEnter the amount of GBP: ";
		cin >> GBP;
		cout << "\n" << GBP << " GBP is " << GBP * 1.26 << " EUR." << endl;
		break;
	case 17:
		cout << "\nEnter the amount of CNY: ";
		cin >> CNY;
		cout << "\n" << CNY << " CNY is " << CNY * 0.125 << " EUR." << endl;
		break;
	case 18:
		cout << "\nEnter the amount of EUR: ";
		cin >> EUR;
		cout << "\n" << EUR << " EUR is " << EUR * 7.99 << " CNY." << endl;
		break;
	case 19:
		cout << "\nEnter the amount of CNY: ";
		cin >> CNY;
		cout << "\n" << CNY << " CNY is " << CNY * 0.16 << " USD." << endl;
		break;
	case 20:
		cout << "\nEnter the amount of USD: ";
		cin >> USD;
		cout << "\n" << USD << " USD is " << USD * 6.35 << " CNY." << endl;
		break;
	default: 
		cout << "\nPlease enter a valid choice.\n " << endl;
		}
		}
		while ( choice !=1 && choice !=2 && choice !=3 && choice !=4 && choice !=5 && choice !=6 && choice !=7 && choice !=8 && choice !=9 && choice !=10 && choice !=11 && choice !=12
			    && choice !=13 && choice !=14 && choice !=15 && choice !=16 && choice !=17 && choice !=18 && choice !=19 && choice !=20);
			
	system("pause");
	return 0;
	}