我怎样才能循环这个决策结构？

Question

对不起，我很难得到这个循环，但我会停止调整它并询问 SO。我希望决策结构循环，我希望它通过输入“4”来停止循环，并且我希望每次都打印“进行选择”提示。谢谢！

public class HW1Geo {
    public static void main(String[] args) {

        // This program was created in order to allow the user to calculate the areas of rectangles, triangles, and circles.

        System.out.println("Thank you for using the MCCH GeoCal program. \nWith this program, you will be able to find the area of three kinds of shapes.\nThis program uses doubles.");
        System.out.println();

        Scanner keyboard = new Scanner(System.in);
        System.out.println("Please make a selection:");
        System.out.println("1 - Area of a rectangle");
        System.out.println("2 - Area of a triangle");
        System.out.println("3 - Area of a circle");
        System.out.println("4 - End program");
        int select = keyboard.nextInt();

        // This decision structure with a nested loop will allow the user to continue to make selections until they decide to quite the program.

        while(select != 4) {

            if(select == 1) {
                System.out.println("Enter the length of the rectangle.");
                double length = keyboard.nextDouble();
                System.out.println("Enter the width of the rectangle");
                double width = keyboard.nextDouble();
                System.out.println("The area of the rectangle is: " + rectArea(length, width));
                break;
            }
            else if(select == 2) {
                System.out.println("Enter the height of the triangle.");
                double height = keyboard.nextDouble();
                System.out.println("Enter the size of the base of the triangle.");
                double base = keyboard.nextDouble();
                System.out.println("The area of the triangle is: " + triArea(height, base));
                break;
            }
            else if(select == 3) {
                System.out.println("Enter the radius of the circle.");
                double radius = keyboard.nextDouble();
                System.out.println("The area of the circle is: " + cirArea(radius));
                break;
            }
            else if(select != 1 && select != 2 && select != 3 && select != 4) {
                System.out.println("Incorrect input.");
                break;
            }

        }
        System.out.println("Thank you for using this program.");
    }

    // This method is used to find the area of a rectangle.
    public static double rectArea(double length, double width) {
        double area = length * width;
        return area;    
    }
    public static double triArea(double height, double base) {
        double area = 0.5 * base * height;
        return area;
    }
    public static double cirArea(double radius) {
        double area = Math.PI * Math.pow(radius, 2);
        return area;
    }
}

Answer 1

你可以使用do-while 句法 做{在这里写代码} while（条件）

Answer 2

你的代码的问题是select的更新，我的意思是，它没有更新。

您可以使用do-while 代替while，这是您的代码的修改版本：

public class Main {
        public static void main(String... args) {

            System.out.println("Thank you for using the MCCH GeoCal program. \nWith this program, you will be able to find the area of three kinds of shapes.\nThis program uses doubles.");
            System.out.println();

            Scanner keyboard = new Scanner(System.in);
            int select = 0;

            do {
                System.out.println("Please make a selection:");
                System.out.println("1 - Area of a rectangle");
                System.out.println("2 - Area of a triangle");
                System.out.println("3 - Area of a circle");
                System.out.println("4 - End program");

                select = keyboard.nextInt();
                if (select == 1) {
                    System.out.println("Enter the length of the rectangle.");
                    double length = keyboard.nextDouble();
                    System.out.println("Enter the width of the rectangle");
                    double width = keyboard.nextDouble();
                    System.out.println("The area of the rectangle is: " + rectArea(length, width));
                } else if (select == 2) {
                    System.out.println("Enter the height of the triangle.");
                    double height = keyboard.nextDouble();
                    System.out.println("Enter the size of the base of the triangle.");
                    double base = keyboard.nextDouble();
                    System.out.println("The area of the triangle is: " + triArea(height, base));
                } else if (select == 3) {
                    System.out.println("Enter the radius of the circle.");
                    double radius = keyboard.nextDouble();
                    System.out.println("The area of the circle is: " + cirArea(radius));
                } else if (select < 1 || select > 4) {
                    System.out.println("Incorrect input.");
                }

            } while (select != 4);
            System.out.println("Thank you for using this program.");
        }

        // This method is used to find the area of a rectangle.
        private static double rectArea(double length, double width) {
            return length * width;
        }

        private static double triArea(double height, double base) {
            return 0.5 * base * height;
        }

        private static double cirArea(double radius) {
            return Math.PI * Math.pow(radius, 2);
        }

    }