I am having trouble getting my program to store values and to be changed throughout the functions. Any help would be much appreciated. Here is the code.

#include <iostream>
using namespace std;
void DisplayMenu(void);
void Enter_Inventory();
int Display_Inventory(int inventory_number_coke, int inventory_number_pepsi, int inventory_number_cd, int inventory_number_hires);
void Purchase_Soda();

int main()
{
	const char SENTINEL = 'Q';
	char Choice;

	do
	{
		DisplayMenu();
	
		cin >> Choice;
		cout << " " << endl;

	switch (Choice)

	{
		case 'E': case 'e':
			void Enter_Inventory();
			break;
		case 'P': case 'p':
			void Purchase_Soda();
			break;
		case 'S': case 's':
			cout << "This is sell" << endl;
			break;
		case 'D': case 'd':
			cout << "This is display" << endl;
			break;
		default:
			cout << "You typed in your entry wrong! Please choose again." << endl;
	}
	
	} while (Choice != SENTINEL);

	return 0;

}

void DisplayMenu(void)
{

	cout << "(E)nter inventory" << endl;
	cout << "(P)urchase soda" << endl;
	cout << "(S)ell soda" << endl;
	cout << "(D)isplay inventory" << endl;
	cout << "(Q)uit program" << endl;
	cout << " " << endl;
	cout << "Please choose by typing the first letter of your choice below." << endl;
	cout << " " << endl;
	cout << "Choice? ";
}

void Enter_Inventory()
{
	int product_value;
	int INC, INP, INCD, INH;
	
	int inventory_number_coke=0;
	int inventory_number_pepsi=0; 
	int inventory_number_cd=0; 
	int inventory_number_hires=0;
	
	cout << "Enter the product number of the inventory you want to submit!" << endl;
	cin >> product_value;
	void Display_Inventory(void);

	cout << "Please enter the number of cases" << endl;
	cout << " " << endl;

	if (product_value == 1)
		{
			INC = Display_Inventory(inventory_number_coke);
			cin >> INC;
		}
		
		else if (product_value == 2)
		{	
			INP = Display_Inventory(inventory_number_pepsi);
			cin >> INP;
		}
		
		else if (product_value == 3)
		{	
			INCD = Display_Inventory(inventory_number_cd);
			cin >> INCD;
		}

		else if	(product_value == 4)
		{
			INH = Display_Inventory(inventory_number_hires);
			cin >> INH;
		}

		else
		{
			cout << "Please re-type selection" << endl;
		}
}
	
	
	
int Display_Inventory(int inventory_number_coke, int inventory_number_pepsi, int inventory_number_cd, int inventory_number_hires)
{	

	
	cout << "|----------------------------------------|" << endl;
	cout << "| Product----    Name    ----# of cases--|" << endl;
	cout << "| Number ----            ----          --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   1    ---- Coca-Cola  ----     " << inventory_number_coke << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   2    ----   Pepsi    ----     " << inventory_number_pepsi << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   3    ---- Canada Dry ----     " << inventory_number_cd << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   4    ----   Hires    ----     " << inventory_number_hires << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	return inventory_number_coke, inventory_number_pepsi, inventory_number_cd, inventory_number_hires;
}
	
void Purchase_Soda()
{
	cout << "|----------------------------------------|" << endl;
	cout << "| Product----    Name    ----# of cases--|" << endl;
	cout << "| Number ----            ----          --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   1    ---- Coca-Cola  ----     " << inventory_number_coke << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   2    ----   Pepsi    ----     " << inventory_number_pepsi << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   3    ---- Canada Dry ----     " << inventory_number_cd << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   4    ----   Hires    ----     " << inventory_number_hires << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << " " << endl;
	cout << "How much do you want to purchase?" << endl;
}

<< moderator edit: added [co[u][/u]de][/co[u][/u]de] tags >>

make the variables global and don't pass any parameters. An alternative (and better) is to make a c++ class to hold data and associated methods.
[edit]Note: I did not run your program. Only made it compile without errors. :rolleyes: [/edit]

#include <iostream>
using namespace std;
void DisplayMenu(void);
void Enter_Inventory();

int Display_Inventory();
void Purchase_Soda();

int inventory_number_coke=0;
int inventory_number_pepsi=0; 
int inventory_number_cd=0; 
int inventory_number_hires=0;
	

int main()
{
	const char SENTINEL = 'Q';
	char Choice;

	do
	{
		DisplayMenu();
	
		cin >> Choice;
		cout << " " << endl;

	switch (Choice)

	{
		case 'E': case 'e':
			void Enter_Inventory();
			break;
		case 'P': case 'p':
			void Purchase_Soda();
			break;
		case 'S': case 's':
			cout << "This is sell" << endl;
			break;
		case 'D': case 'd':
			cout << "This is display" << endl;
			break;
		default:
			cout << "You typed in your entry wrong! Please choose again." << endl;
	}
	
	} while (Choice != SENTINEL);

	return 0;

}

void DisplayMenu(void)
{

	cout << "(E)nter inventory" << endl;
	cout << "(P)urchase soda" << endl;
	cout << "(S)ell soda" << endl;
	cout << "(D)isplay inventory" << endl;
	cout << "(Q)uit program" << endl;
	cout << " " << endl;
	cout << "Please choose by typing the first letter of your choice below." << endl;
	cout << " " << endl;
	cout << "Choice? ";
}

void Enter_Inventory()
{
	int product_value;
	int INC, INP, INCD, INH;
	
	
	cout << "Enter the product number of the inventory you want to submit!" << endl;
	cin >> product_value;

	cout << "Please enter the number of cases" << endl;
	cout << " " << endl;

	if (product_value == 1)
		{
			INC = Display_Inventory();
			cin >> INC;
		}
		
		else if (product_value == 2)
		{	
			INP = Display_Inventory();
			cin >> INP;
		}
		
		else if (product_value == 3)
		{	
			INCD = Display_Inventory();
			cin >> INCD;
		}

		else if	(product_value == 4)
		{
			INH = Display_Inventory();
			cin >> INH;
		}

		else
		{
			cout << "Please re-type selection" << endl;
		}
}
	
	
	
int Display_Inventory()
{	

	
	cout << "|----------------------------------------|" << endl;
	cout << "| Product----    Name    ----# of cases--|" << endl;
	cout << "| Number ----            ----          --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   1    ---- Coca-Cola  ----     " << inventory_number_coke << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   2    ----   Pepsi    ----     " << inventory_number_pepsi << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   3    ---- Canada Dry ----     " << inventory_number_cd << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   4    ----   Hires    ----     " << inventory_number_hires << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	return inventory_number_coke, inventory_number_pepsi, inventory_number_cd, inventory_number_hires;
}
	
void Purchase_Soda()
{
	cout << "|----------------------------------------|" << endl;
	cout << "| Product----    Name    ----# of cases--|" << endl;
	cout << "| Number ----            ----          --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   1    ---- Coca-Cola  ----     " << inventory_number_coke << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   2    ----   Pepsi    ----     " << inventory_number_pepsi << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   3    ---- Canada Dry ----     " << inventory_number_cd << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << "|   4    ----   Hires    ----     " << inventory_number_hires << "    --|" << endl;
	cout << "|----------------------------------------|" << endl;
	cout << " " << endl;
	cout << "How much do you want to purchase?" << endl;
}
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