如何让这个while循环一次只提示用户一次

Question

//基本上我需要提示用户输入等级，并计算A的B的数量等等。我不知道如何让循环在每个用户输入中循环一次。它只是垃圾邮件输入考试成绩。编辑谢谢大家的帮助我想通了！

public static void main(String[] args) {
    Scanner input = new Scanner(System.in); 
    int numAs = 0;
    int numBs = 0;
    int numCs = 0;
    int numFs = 0;
    int grade = 0;
    while (grade >= 0 && grade <= 100)
    System.out.println("Enter an exam grade");
    grade = input.nextInt();
    if (grade >= 90) {numAs++;
    }else if (grade >=80 && grade<= 89 ) {numBs++;
    }else if (grade >= 70 && grade <= 79) {numCs++;
    }else if (grade <= 69) {numFs++;
    }else if (grade < 0 || grade > 100) {System.out.println("Error invalid grade entered.");
    }
    }

}

Answer 1

如果您正确地格式化了代码，您将看到While循环没有大括号，因此它只包含下面一行。

Scanner input = new Scanner(System.in);
int numAs = 0;
int numBs = 0;
int numCs = 0;
int numFs = 0;
int grade = 0;
while (grade >= 0 && grade <= 100) 
    System.out.println("Enter an exam grade");
    //WHILE ENDS HERE
grade = input.nextInt();
if (grade >= 90) {
    numAs++;
} else if (grade >= 80 && grade <= 89) {
    numBs++;
} else if (grade >= 70 && grade <= 79) {
    numCs++;
} else if (grade <= 69) {
    numFs++;
} else if (grade < 0 || grade > 100) {
    System.out.println("Error invalid grade entered.");
}

即使添加缺少的{，也无法解决多个问题——错误输入会发生什么；如何正确停止循环。因此，综合考虑所有因素，这可能会更好一些：

Scanner input = new Scanner(System.in);
int numAs = 0, numBs = 0, numCs = 0, numFs = 0;
int grade = 0;
String in = "";
while (true) {
    System.out.println("Enter an exam grade or Q to exit");
    //WHILE ENDS HERE
    in = input.nextLine();
    if ("Q".equals(in)) {
        System.out.println("That's it!");
        break;
    }
    try {
        grade = Integer.parseInt(in); //get caught if not a num
        if (grade < 0 || grade > 100) { //gets caught if bad num
            throw new Exception();
        }
    } catch (Exception e) {
        System.out.println("Error invalid grade entered.");
    }
    if (grade >= 90)
        numAs++;
    else if (grade >= 80 && grade <= 89)
        numBs++;
    else if (grade >= 70 && grade <= 79)
        numCs++;
    else if (grade <= 69)
        numFs++;
    System.out.println("Saved.");
}
//Continue after breaking the loop

Answer 2

在你的while声明之后，你缺少了你的花括号。如果没有指定大括号，while循环只包括以下语句，而不是整个块。

public static void main(String[] args) {
    Scanner input = new Scanner(System.in); 
    int numAs = 0;
    int numBs = 0;
    int numCs = 0;
    int numFs = 0;
    int grade = 0;
    while (grade >= 0 && grade <= 100){
    System.out.println("Enter an exam grade");
    grade = input.nextInt();
    if (grade >= 90) {numAs++;
    }
    else if (grade >=80 && grade<= 89 ) {numBs++;
    }
    else if (grade >= 70 && grade <= 79) {numCs++;
    }
    else if (grade <= 69) {numFs++;
    }
    else if (grade < 0 || grade > 100) {System.out.println("Error invalid grade entered.");
    }

    }
}