Is there any other way to solve my problem?

[freiheit1991]

Hello everyone, I am very new to this forum and so in C++, I started just few weeks ago by reading a book I bought and programming small useless things just to practice the concepts but recently I completed my first useful program which works perfectly and does everything I want it to do. This is a program to convert units for example kilograms to pounds or Celsius to Fahrenheit, it has a main menu, and many sub-menus that allow the user to pick up any unit to convert. So my problem is the code, its super long, over 3500 lines, the reason is because I wrote every single possible combination the user might want to try, kilometer to mile, mile to kilometer, mile to mile and so on for all of them, hundreds of else if statements and double variables for each possibility. Below you can see part of my code and tell me if there is any other way to face this kind of issue.
Thanks in advance

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

void printIntro()
{
	cout << "\n What would you like to calculate?\n" << endl;
}

void printList()
{
	//Main Menu
	cout << "     1) Mass Units\n";
	cout << "     2) Distance Units\n";
	cout << "     3) Temperature Units\n";
	cout << "     4) Area Units\n";
	cout << "     5) Data Units\n";
	cout << "     6) Time Units\n";
	cout << "     7) Life Statistics\n";
	cout << "     8) Monthly Expenses\n";
	cout << "     9) Currency Conversion\n";
	cout << "     10) Exit\n\n";
}

/*Functions for each item in Main Menu*/
double startUM()// Start Unit Mass
{
	string unit1;
	string unit2;
	double amount;
	//All the posibilities the user might try (25 in total)
	const double tn_kg = 1000;  
	const double kg_tn = 0.001;  
	const double kg_g = 1000;   
	const double g_kg = 0.001;  
	const double tn_g = 1e+6;   
	const double g_tn = 1e-6;  //... 


	char answerExit = 'y';
	system("cls");
	cout << "                                  MASS UNITS                                  ";
	cout << " [tn]  Ton\n"
		<< " [kg]  Kilogram\n"
		<< " [g]   Gram\n"
		<< " [lb]  Pound\n"
		<< " [oz]  Ounce\n";
LOOP_massUnits:
	do {
		cout << "\n Insert unit to convert: ";
		cin >> unit1;
		cout << " Insert unit to obtain: ";
		cin >> unit2;
		if (unit1 == "tn" && unit2 == "kg")
		{
			cout << "\n How many " << unit1.c_str() << " do you want to convert? ";
			cin >> amount;
			cout << " " << amount << " " << unit1.c_str() << " are " << amount * tn_kg << " " << unit2.c_str() << ".\n";
		}
		else if (unit1 == "kg" && unit2 == "tn")
		{
			cout << "\n How many " << unit1.c_str() << " do you want to convert? ";
			cin >> amount;
			cout << " " << amount << " " << unit1.c_str() << " are " << amount * kg_tn << " " << unit2.c_str() << ".\n";
		}
		//...26 else if statements in total...
		else if (unit1 != "tn", "kg", "g", "lb", "oz" && unit2 != "tn", "kg", "g", "lb", "oz")
		{
			cout << "\nERROR: Invalid Unit\n";
			goto LOOP_massUnits;
		}
	} while (answerExit != 'y');
	return 0;
}

double startUL()//Start Distance Units
{
	//66 double variables representing all the posibilities... this program is getting long...
	//67 else if stataments (including for ERROR) for all the posibilities the user may want to try...
}

double startUT()//start Temperature Units
{
	//Code with a bunch of variables and else if statements
}

double startUA()//Start Area Units
{
	//Code 
}

double startUD()//Start Data Units
{
	//Code 
}

double startUTm()//Start Time Units
{
	//Code 
}

int startLF()//Start Life statistic
{
	//Code
}

double startES()//Start Monthly expenses
{
	//Code
}

double startCC()//Start Currency convertion
{
	//Code
}

int main()
{
	printPresentation();   
	printTitle();
	printIntro();
	printList();           //Main Menu
	int subj;              //Option to execute one function of the main menu items
	char posAnswer = 127;

LOOP: 
	do {
		cout << " Choose what you want to calculate inserting the number: ";
		cin >> subj;

		if (subj == 1) 
		{
		LOOP_UM:
			startUM();
			cout << "\n Go to Main Menu? (y/n) ";
			cin >> posAnswer;
			if (posAnswer == 'y')
			{
				system("cls");
				printMainMenu();
				printList();
				goto LOOP;
			}
			else
			{
				goto LOOP_UM;
			}
		}
		else
		{
			if (subj == 2)
			{
			LOOP_UL:
				startUL();
				cout << "\n Go to Main Menu? (y/n) ";
				cin >> posAnswer;
				if (posAnswer == 'y')
				{
					system("cls");
					printMainMenu();
					printList();
					goto LOOP;
				}
				else
				{
					goto LOOP_UL;
				}
			}
			else
			{
				if (subj == 3)
				{
				LOOP_UT:
					startUT();
					cout << "\n Go to Main Menu? (y/n) ";
					cin >> posAnswer;
					if (posAnswer == 'y')
					{
						system("cls");
						printMainMenu();
						printList();
						goto LOOP;
					}
					else
					{
						goto LOOP_UT;
					}
				}
				else
				{
					if (subj == 4)
					{
					LOOP_UA:
						startUA();
						cout << "\n Go to Main Menu? (y/n) ";
						cin >> posAnswer;
						if (posAnswer == 'y')
						{
							system("cls");
							printMainMenu();
							printList();
							goto LOOP;
						}
						else
						{
							goto LOOP_UA;
						}
					}
					else
					{
						if (subj == 5)
						{
						LOOP_UD:
							startUD();
							cout << "\n Go to Main Menu? (y/n) ";
							cin >> posAnswer;
							if (posAnswer == 'y')
							{
								system("cls");
								printMainMenu();
								printList();
								goto LOOP;
							}
							else
							{
								goto LOOP_UD;
							}
						}
						else
						{
							if (subj == 6)
							{
							LOOP_UTm:
								startUTm();
								cout << "\n Go to Main Menu? (y/n) ";
								cin >> posAnswer;
								if (posAnswer == 'y')
								{
									system("cls");
									printMainMenu();
									printList();
									goto LOOP;
								}
								else
								{
									goto LOOP_UTm;
								}
							}
							else
							{
								if (subj == 7)
								{
								LOOP_LF:
									startLF();
									cout << "\n Go to Main Menu? (y/n) ";
									cin >> posAnswer;
									if (posAnswer == 'y')
									{
										system("cls");
										printMainMenu();
										printList();
										goto LOOP;
									}
									else
									{
										goto LOOP_LF;
									}
								}
								else
								{
									if (subj == 8)
									{
									LOOP_ES:
										startES();
										cout << "\n Go to Main Menu? (y/n) ";
										cin >> posAnswer;
										if (posAnswer == 'y')
										{
											system("cls");
											printMainMenu();
											printList();
											goto LOOP;
										}
										else
										{
											goto LOOP_ES;
										}
									}
									else
									{
										if (subj == 9)
										{
										LOOP_CC:
											startCC();
											cout << "\n Go to Main Menu? (y/n) ";
											cin >> posAnswer;
											if (posAnswer == 'y')
											{
												system("cls");
												printMainMenu();
												printList();
												goto LOOP;
											}
											else
											{
												goto LOOP_CC;
											}
										}
										else
										{
											if (subj == 666)
											{
												secret666();
												cout << "\n Go to Main Menu? (y/n) ";
												cin >> posAnswer;
												if (posAnswer == 'y')
												{
													system("cls");
													printMainMenu();
													printList();
													goto LOOP;
												}
											}
											else
											{
												if (subj == 10)
												{
													char exiting;
													system("cls");
													cout << "  Thank you for using this program!\n"
													     << "             Press any key followed by ENTER to exit. ";
													cin >> exiting;
													exit(exiting);
												}
											}
										}
									}
								}
							}
						}
					}
				}
			}
		}
		if (subj != 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 666)
		{
			cout << "\n ERROR: Invalid number\n";
			goto LOOP;
		}

	} while (subj);

	return 0;
}

[Kevin C]

The solution would be, for each physical property, to store just a few values in vectors which would describe the necessary conversion factors.

For example:

vector<string> unitNames = { "centimeters", "meters", "kilometers", "feet", "yard"};
vector<double> factors = { 0.01, 1, 1000, 0.3048, 0.9144};
vector<double> offsets = { 0,0,0,0,0};

The offsets would play a role, for example, in conversions between Kelvins, Celsius Degrees and Fahrenheit degrees.