Quantity reduction help

[Daniel13789]

Hello all! I've stumbled into a problem while trying to complete this code. I can't seem to figure out how to reduce the number of drinks when the program purchases one. Then I need to add a message when the user wants to purchase a soda when it's currently sold out.

For example: I need the program to reduce 1 soda can from the 'Cola' slot whenever the user purchases a can. Then when the amount of 'Cola' cans are sold out. The program will display a message saying it's sold out.

Any help would be greatly appreciated! Here's my code so far. If you have any questions, please let me know. Please help me out; I'm counting on you!

Please help, this is urgent!

//Drink Machine Simulator
#include<iostream>
#include<iomanip>

using namespace std;

int displayMenu(int);

float payment;
int choice;
float change;

int main()
{
	struct drinkInfo
	{
		char* drinkName;
		double cost;
		int quantity;
	};

	int x = 0;
	const int ARRAY_SIZE = 5;
	float drinkpayment = 0.0f;
	int selection = 0;
	float drinkchange = 0.0f;
	float earnings = 0.0f;
	float reduce = 0.0f;

	drinkInfo drinkList[ARRAY_SIZE];

	drinkList[0].drinkName = "Cola";
	drinkList[0].cost = .75;
	drinkList[0].quantity = 20;

	drinkList[1].drinkName = "Root Beer";
	drinkList[1].cost = .75;
	drinkList[1].quantity = 20;

	drinkList[2].drinkName = "Lemon-Lime";
	drinkList[2].cost = .75;
	drinkList[2].quantity = 20;

	drinkList[3].drinkName = "Grape Soda";
	drinkList[3].cost = .80;
	drinkList[3].quantity = 20;

	drinkList[4].drinkName = "Cream Soda";
	drinkList[4].cost = .80;
	drinkList[4].quantity = 20;



	do
	{
		//Totals the money earned in the machine.
		earnings += payment - change;
		cout << "Make a selection from the menu below!" << endl;
		//Shows the drink menu.
		selection = displayMenu(selection);

		if (selection == 1)
		{
			cout << "Enter the amount of money to be paid \n(won't accept anything greater than $1): $";
			cin >> payment;
			if ((payment < .75) ||(payment > 1))
			{
				cout << "That is an invalid amount of change. Please re-enter your amount.";
				cin >> payment;
			}
			else if ((payment >= .75) || (payment <= 1))
			{
			//Calculates the amount of change to give.
			change = payment - drinkList[0].cost;
			cout << "Your change is $" << change << "\n\n";
			}
		}
		else if (selection == 2)
		{
			cout << "Enter the amount of money to be paid \n(won't accept anything greater than $1): $";
			cin >> payment;
			if ((payment < .75) ||(payment > 1))
			{
				cout << "That is an invalid amount of change. Please re-enter your amount.";
				cin >> payment;
			}
			else if ((payment >= .75) || (payment <= 1));
			{
			change = payment - drinkList[1].cost;
			cout << "Your change is $" << change << "\n\n";
			}
		}
		else if (selection == 3)
		{
			cout << "Enter the amount of money to be paid \n(won't accept anything greater than $1): $";
			cin >> payment;
			if ((payment < .75) ||(payment > 1))
			{
				cout << "That is an invalid amount of change. Please re-enter your amount.";
				cin >> payment;
			}
			else if ((payment >= .75) || (payment <= 1));
			{
			change = payment - drinkList[2].cost;
			cout << "Your change is $" << setprecision(2)<<change << "\n\n";
			}
		}
		else if (selection == 4)
		{
			cout << "Enter the amount of money to be paid \n(won't accept anything greater than $1): $";
			cin >> payment;
			if ((payment < .80) ||(payment > 1))
			{
				cout << "That is an invalid amount of change. Please re-enter your amount.";
				cin >> payment;
			}
			else if ((payment >= .80) || (payment <= 1));
			{
			change = payment - drinkList[3].cost;
			cout << "Your change is $"<< setprecision(2) << fixed  << change << "\n\n";
			}
		}
		else if (selection == 5)
		{
			cout << "Enter the amount of money to be paid \n(won't accept anything greater than $1): $";
			cin >> payment;
			if ((payment < .80) ||(payment > 1))
			{
				cout << "That is an invalid amount of change. Please re-enter your amount.";
				cin >> payment;
			}
			else if ((payment >= .80) || (payment <= 1));
			{
			change = payment - drinkList[4].cost;
			cout << "Your change is $"<< setprecision(2) << fixed  << change << "\n\n";
			}
		}
		else if (selection == 6)
			//Displays the amount of money the machine earned.
			cout << "This machine has earned $" << earnings << "." << endl;
		//Returns you to the menu if invalid selection is made.
		else
		{
			cout << "Your selection is invalid. Please review the \nmenu and make another selection." << endl;
			displayMenu(selection);
		}


	}
	while (selection != 6);
	system("pause");
	return 0;
}
//Drink menu.
int displayMenu(int choice)
{
	cout << endl;
	cout << "1. Cola       $0.75" << endl;
	cout << "2. Root Beer  $0.75" << endl;
	cout << "3. Lemon-Lime $0.75" << endl;
	cout << "4. Grape Soda $0.80" << endl;
	cout << "5. Cream Soda $0.80" << endl;
	cout << "6. Quit" <<endl;
	cout << endl;
	cout << "Your selection: ";
	cin >> choice;

	return choice;
}