💻 CSC1003 LEC10-12 Midterm Revision

October 19, 2024

复习

.split("") 以引号内的字符为介分开字符串
a.equals(b) 比较 a 与 b 是否相等，常用于字符串，因为 “==” 不能比较字符串
int[] arr = new int[n] 将类实例化的语法
String sc = new Scanner(System.in) 同上，可用于实例化 Scanner
.nextLine() 用 Scanner 的实例化对象读取一个键盘输入的字符串

常见问题：如果在使用 nextInt()、nextDouble() 等方法后立刻调用 nextLine()，可能会遇到问题：nextInt() 只读取了数字部分，但换行符仍然留在输入缓冲区中，因此 nextLine() 会读取到这个残留的换行符，返回一个空字符串。解决方案：在读取整数后，消耗掉换行符，添加一个额外的 nextLine()。
```
int age = scanner.nextInt();
scanner.nextLine();  // 消耗掉换行符
String name = scanner.nextLine();
```
Integer.parseInt(x) 用于转换字符串的数据类型至整型或浮点型，注意不可以用于小数和整数相互转换。
int a = (int) 4.2 强行转换数字的数据类型，注意不能用于数字与字符串相互转换。
Math.round() 四舍五入

模拟题

    int[] binary = new int[1000];
    int id = 0;

    while (num > 0) {
        binary[id++] = num % 2;
        num = num / 2;
    }
    for (int i = id - 1; i >= 0; i--)
        System.out.print(binary[i] + "");

What is this clip of script most possibly used for? (To convert dec to bin number.)
What is the largest integer possible for variable “num” in decimal? (2¹⁰⁰⁰-1)

import java.util.Scanner;

public class TS {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double a = sc.nextDouble();
        double b = sc.nextDouble();
        double c = sc.nextDouble();

        if (isValid(a, b, c)) {
            double s = calc(a, b, c);
            System.out.println(s);
        } else {
            System.out.println("invalid");
        }

        scanner.close();
    }

    public static double calc(double a, double b, double c) {
        double s = (a + b + c) / 2;
        return Math.sqrt(s * (s - a) * (s - b) * (s - c));
    }

    public static boolean isValid(double a, double b, double c) {
        return (a + b > c) && (a + c > b) && (b + c > a);
    }
}

What is the output of the code if the keyboard input is “6 5 9”? (Nothing, because the scanner which is designed to take three double data cannot process a single String data.)
What is a possible input if the output is “6”? (e.g. “3” “4” “5”)

import java.util.*;

public class BirthdayParadox {

    public static int[] dayarray(int days) {
        int[] a = new int[days];
        Random random = new Random();
        for (int i = 0; i < days; i++) {
            a[i] = random.nextInt(365) + 1;
        }
        return a;
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int x = sc.nextInt();
        int count = 0;
        for (int n = 0; n < 1000000; n++) {
            int[] arr = dayarray(x);
            Set<Integer> set = new HashSet<>();
            // this transforms an array into a set
            for (int day : arr) {
                set.add(day);
            }
            if (set.size() != arr.length) {
                ++count;
            }
        }
        double result = (double) count * 100 / 1000000;
        System.out.println(result + "%");
    }
}

What is the intended purpose of the program? (Calculate the possibility of a duplicated date of birth amongst x people by testing with random dates 1000000 times.)
Can “1000000” be replaced by “1e7” in the program? (No. Because a loop cannot take a float value as part of its control statement, since they are very inaccurate.)

CSC1003