How do I prevent this infinite loop?

[Justin5978]

Hello all,

I am working on a project for class. Here is the code:

#include <stdio.h>
#include <ctype.h>

void DisplayApps(char *appPtr);
void SetCost(char app, double *appCostPtr);
void MoneyChoice(double *depositPtr, double appCost, double Account);
int Compare(double Account, double appCost); 
void GetChange(double Account, double appCost, double *acctPtr);
void AddDeposit(double Deposit, double Account, double *acctPtr);
void Again(char *quitPtr);

int main()
{
	char App='i'; char Quit='i';
	int YayOrNay=0; int Xfactor=0;
	double Account=0; double Deposit=0; double AppCost=0;

	printf("-----------Welcome to The App Store!-----------\n\n");

	do
	{
	DisplayApps(&App);

	SetCost(App, &AppCost);

	if(Account<AppCost)
	{
		do
		{
		MoneyChoice(&Deposit, AppCost, Account);
		AddDeposit(Deposit, Account, &Account);
		Compare(Account, AppCost);
		}
		while(Account<AppCost);
	}
	printf("Proceeding to check out...\n");

	GetChange(Account, AppCost, &Account);

	Again(&Quit);
	}
	while(Quit != 'n');

	printf("\n\nThank you for shopping at The App Store!\n\n\n");

	return 0;
}

void DisplayApps(char *appPtr)
{
	char Selection='i';
	do
	{
	printf("Here are the applications available.\n");
	printf("G: Graphing Calculator				$14.99\n");
	printf("M: Music Downloader				$1.99\n");
	printf("S: SAT Study Prep				$7.99\n");
	printf("T: TxtHelp					$4.99\n");
	printf("A: Astronomy Guide				$5.99\n\n");
	printf("Application to purchase: ");
	scanf(" %c", &Selection);
	Selection=tolower(Selection);
	}
	while(Selection != 'g' && Selection != 'm' && Selection != 's' && Selection != 't' && Selection != 'a' );
	*appPtr=Selection;
	return;
}

void SetCost(char app, double *appCostPtr)
{
	double Cost=0;

	switch (app)
	{
	case 'g':
		Cost=14.99;
		break;
	case 'm':
		Cost = 1.99;
		break;
	case 's':
		Cost=7.99;
		break;
	case 't':
		Cost= 4.99;
		break;
	case 'a':
		Cost= 5.99;
		break;
	default:
		printf("Error.");
		break;
	}

	*appCostPtr = Cost;
}

void MoneyChoice(double *depositPtr, double appCost, double Account)
{
	double DepOption=0; double Cost = appCost; int Selection=0;

	do
	{
	printf("\nYou currently have $%.2f in your account.", Account);
	printf("\nThe item you wish to purchase costs $%.2f.", appCost);
	printf("\n\nPlease select how much credit you wish to add towards your account.");
	printf("\n1>>		$15.00");
	printf("\n2>>		$10.00");
	printf("\n3>>		$5.00");
	printf("\n4>>		$2.00");
	printf("\n5>>		$1.00");
	printf("\n\nAmount to be added to account:  ");
	scanf(" %d", &Selection);
	}
	while (Selection != 1 && Selection != 2 && Selection != 3 && Selection != 4 && Selection != 5);
	
	switch (Selection)
	{
	case 1:
	DepOption = 15.00;
	break;
	case 2:
	DepOption=10.00;
	break;
	case 3:
	DepOption=5.00;
	break;
	case 4:
	DepOption=2.00;
	break;
	case 5:
	DepOption=1.00;
	break;
	default:
	printf("...Error...");
	break;
	}

	*depositPtr = DepOption;

	return;
}

void AddDeposit(double Deposit, double Account, double *acctPtr)
{
	double Combine=0;
	Combine = Deposit + Account;
	*acctPtr= Combine;
	return;
}

int Compare(double Account, double appCost)
{
	if (Account > appCost)
	{
		printf("\nYou now have enough money to purchase the app!\n");
		return 1;
	}
	else
	{
		printf("\nYou still do not have enough money for that purchase.\n");
		return 0;
	}
}

void GetChange(double Account, double appCost, double *acctPtr)
{
	double Change=0;
	Change = Account - appCost;
	*acctPtr = Change;
	printf("\nYour account now has $%.2f remaining after the purchase.", Change);
	return;
}

void Again (char *quitPtr)
{
	char QuitAnswer = 'i';
	do
	{
	printf("\n\nWould you like to continue shopping(y/n)? ");
	scanf(" %c", &QuitAnswer);
	}
	while(QuitAnswer != 'y' && QuitAnswer != 'n');
	*quitPtr = QuitAnswer;
	return;
}

As far as I can tell, the code only has one problem with it. When I call on MoneyChoice() and it displays the numerical options for the amount of money I wish to add to the account, if I enter a letter it goes into an endless loop. I have a similar safeguard in DisplayApps() that limits the user to only letter answers and if anything other than the letters sought after (including numbers) is entered, it simply reprints the options again without a problem and does not allow for any other input. I was wondering how I can also do this with MoneyChoice() as I only want the user to enter a #1-5 and anything else will not be accepted. Thanks for taking the time to look over my issue.

[TheIdeasMan]

Have a look at this :

http://www.cplusplus.com/forum/beginner/104553/2/#msg564228

This has a separate ShowMenu function, and the logic is in the switch.

It also has while loops, which I much prefer over do loops.


I personally hate constructs like what you have on line 115, IMO they are error prone & not scalable.

With answers to questions (like on line 180), make use of the toupper function, and compare with 'Y' or 'N'. This makes the logic easier, and your code does not cope with the user entering either case (lower or upper)

Hope all goes well

[Ispil]

Well, the issue here is that in DisplayApps(), the variable is a character. In MoneyChoice(), it is an int. When you input a number into a character variable, it simply puts the character of the number into the variable. Trying to stick a character into an int variable... isn't so clean. Rather, it causes an error with the input, then tries to read again. Thus trapping itself in an infinite loop.

http://www.gidnetwork.com/b-63.html provides great documentation on this exact issue, and the solution.