我遇到的主要问题是第一次执行while循环，当条件满足时我无法退出程序。
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
final double milkPrice = 2;
final double sodaPrice = 2.25;
final double cbarPrice = 1.25;
final double gbearsPrice = 1.50;
final double chipsPrice = 1;
double usersMoney = 0;
double moneyLeft = 0;
double tempMoneyLeft = 0;
int milkQnty = 5;
int sodaQnty = 4;
int cbarQnty = 5;
int gbearsQnty = 6;
int chipsQnty = 6;
char usersSelection;
do{
System.out.println("Welcome to the Vending Machine!");
System.out.println("I sense that your are hungry... This is what I have to offer:");
System.out.println();
System.out.println("\tItem" + "\t\tPrice" + "\t\tQuantity");
System.out.println("\tMilk" + "\t\t$2.00" + "\t\t" + milkQnty);
System.out.println("\tSoda" + "\t\t$2.25" + "\t\t" + sodaQnty);
System.out.println("\tCandy Bar" + "\t$1.25" + "\t\t" + cbarQnty);
System.out.println("\tGummy Bears" + "\t$1.50" + "\t\t" + gbearsQnty);
System.out.println("\tChips" + "\t\t$1.00" + "\t\t" + chipsQnty);
System.out.println();
System.out.print("Please enter how much money you have to spend (enter -1 to shut down): ");
usersMoney = scan.nextDouble();
具体而言，我认为这就是我遇到问题的地方。它只是继续进行下一个恰好是嵌套的do-while循环的语句。
do {
System.out.print("Please make a selection:" +
"\nA-Milk, B-Soda, C-Candy Bar, D-Gummy Bears, E-Chips, X-Exit: ");
usersSelection = scan.next().toUpperCase().charAt(0);
switch (usersSelection) {
case 'A':
tempMoneyLeft = usersMoney - milkPrice;
moneyLeft = tempMoneyLeft - moneyLeft;
System.out.println("You have bought Milk for " + milkPrice + ". You still have " +
moneyLeft + " left.");
System.out.println();
break;
case 'B':
tempMoneyLeft = usersMoney - sodaPrice;
moneyLeft = tempMoneyLeft - moneyLeft;
System.out.println("You have bought Soda for " + sodaPrice + ". You still have " +
moneyLeft + " left.");
System.out.println();
break;
case 'C':
tempMoneyLeft = usersMoney - cbarPrice;
moneyLeft = tempMoneyLeft - moneyLeft;
System.out.println("You have bought Candy Bar for " + cbarPrice + ". You still have " +
moneyLeft + " left.");
System.out.println();
break;
case 'D':
tempMoneyLeft = usersMoney - gbearsPrice;
moneyLeft = tempMoneyLeft - moneyLeft;
System.out.println("You have bought Gummy Bears for " + gbearsPrice + ". You still have " +
moneyLeft + " left.");
System.out.println();
break;
case 'E':
tempMoneyLeft = usersMoney - chipsPrice;
moneyLeft = tempMoneyLeft - moneyLeft;
System.out.println("You have bought Chips for " + chipsPrice + ". You still have " +
moneyLeft + " left.");
System.out.println();
break;
case 'X':
System.out.println("Thank you for your purchases: Your change is " + moneyLeft);
break;
}
if(!(usersSelection == 'A' || usersSelection == 'B' || usersSelection == 'C' || usersSelection == 'D'
|| usersSelection == 'E' || usersSelection == 'X')) {
System.out.print("Invalid Entry!\t");
}
} while (usersSelection != 'X');
} while (usersMoney != -1);
System.out.println();
System.out.print("Thank you for your business!");
}
}
答案 0 :(得分：1)
重新阅读你的问题之后，我终于理解了你真正想要的东西。
实际上，如果你想在不满足第一个循环的条件时退出，那么由于你有嵌套循环，你必须改变内部的条件。
此外，您需要将内部循环从do..while更改为简单的while。
所以你的代码看起来像这样：
char usersSelection = ' ';
do {
usersMoney = scan.nextDouble();
while(usersMoney != -1 && usersSelection != 'X')//will be automatically skipped if -1 is entered
{
System.out.print("Please make a selection:"
+ "\nA-Milk, B-Soda, C-Candy Bar, D-Gummy Bears, E-Chips, X-Exit: ");
usersSelection = scan.next().toUpperCase().charAt(0);
//switch statement here
}
} while (usersMoney != -1);
如果输入-1，则使用此算法将直接退出。
答案 1 :(得分：0)
如果您将while条件更改为usersMoney == -1，它应该可以正常工作。正如你所写的，你要求用户输入-1退出，尽管在你的情况下你只是告诉“只有当usersMoney不同于-1”时。
我还建议在default内使用switch而不是冗长的if。只需将其替换为：
default: System.out.print("Invalid Entry!\t");
答案 2 :(得分：0)
不需要嵌套循环。当满足内循环的条件时，第一个循环的条件可能没有，因此它将继续运行，并且当用户具有-1时它将永远不会退出，除非他也选择＆＃39; X＆＃39;。
试试这个
do {
System.out.print("Please make a selection:" +
"\nA-Milk, B-Soda, C-Candy Bar, D-Gummy Bears, E-Chips, X-Exit: ");
usersSelection = scan.next().toUpperCase().charAt(0);
switch (usersSelection) {
case 'A':
tempMoneyLeft = usersMoney - milkPrice;
moneyLeft = tempMoneyLeft - moneyLeft;
System.out.println("You have bought Milk for " + milkPrice + ". You still have " +
moneyLeft + " left.");
System.out.println();
break;
case 'B':
tempMoneyLeft = usersMoney - sodaPrice;
moneyLeft = tempMoneyLeft - moneyLeft;
System.out.println("You have bought Soda for " + sodaPrice + ". You still have " +
moneyLeft + " left.");
System.out.println();
break;
case 'C':
tempMoneyLeft = usersMoney - cbarPrice;
moneyLeft = tempMoneyLeft - moneyLeft;
System.out.println("You have bought Candy Bar for " + cbarPrice + ". You still have " +
moneyLeft + " left.");
System.out.println();
break;
case 'D':
tempMoneyLeft = usersMoney - gbearsPrice;
moneyLeft = tempMoneyLeft - moneyLeft;
System.out.println("You have bought Gummy Bears for " + gbearsPrice + ". You still have " +
moneyLeft + " left.");
System.out.println();
break;
case 'E':
tempMoneyLeft = usersMoney - chipsPrice;
moneyLeft = tempMoneyLeft - moneyLeft;
System.out.println("You have bought Chips for " + chipsPrice + ". You still have " +
moneyLeft + " left.");
System.out.println();
break;
case 'X':
System.out.println("Thank you for your purchases: Your change is " + moneyLeft);
break;
}
if(!(usersSelection == 'A' || usersSelection == 'B' || usersSelection == 'C' || usersSelection == 'D'
|| usersSelection == 'E' || usersSelection == 'X')) {
System.out.print("Invalid Entry!\t");
}
if(usersMoney < 0) break;
} while (usersSelection != 'X');
System.out.println();
System.out.print("Thank you for your business!");
}