AcWing《Java语法基础课》第3章循环语句

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AcWing《Java语法基础课》第3章循环语句

作者：

姜萍Jumping , 2023-01-15 15:15:01 , 所有人可见 , 阅读 436

第3章循环语句

笔记

while循环

可以简单理解为循环版的if语句。if语句是判断一次，如果条件成立，则执行后面的语句；while是每次判断，如果成立，则执行循环体中的语句，否则停止。

int i = 0;
while (i < 10) {
    System.out.println(i);
    i ++;
}

死循环：循环永久执行，无法结束。我们要避免写出死循环。

int x = 1;
while (x == 1)
    System.out.println("!");

do while循环

do while循环不常用。
do while语句与while语句非常相似。唯一的区别是，do while语句限制性循环体后检查条件。不管条件的值如何，我们都要至少执行一次循环。

int y = 1;
do {
    System.out.println("y!");
} while (y < 1);

for循环

基本思想：把控制循环次数的变量从循环体中剥离。

for (init-statement; condition; expression) {
    statement
}

init-statement可以是声明语句、表达式、空语句，一般用来初始化循环变量；
condition是条件表达式，和while中的条件表达式作用一样；可以为空，空语句表示true；
expression一般负责修改循环变量，可以为空。

for (int i = 0; i < 10; i ++ ) {  // 循环体中只有一条语句时，可以不加大括号
    System.out.println(i);
}

init-statement可以定义多个变量，expression也可以修改多个变量。

跳转语句

break可以提前从循环中退出，一般与if语句搭配。

continue可以直接跳到当前循环体的结尾。作用与if语句类似。

多层循环

for (int i = 0; i < 10; i ++ ) {
    for (int j = 0; j <= i; j++) {
        System.out.printf("%d ", i + j);
    }
    System.out.println();
}

习题

AcWing 708. 偶数

public class Main {

    public static void main(String[] args) {
        for (int i = 1; i <= 100; i++)
            if (i % 2 == 0) System.out.println(i);
    }
}

AcWing 709. 奇数

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int x = scanner.nextInt();
        scanner.close();
        for (int i = 1; i <= x; i++)
            if (i % 2 != 0) System.out.println(i);
    }
}

AcWing 712. 正数

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        int cnt = 0;
        Scanner scanner = new Scanner(System.in);
        for (int i = 0; i < 6; i++) {
            double x = scanner.nextDouble();
            if (x > 0) cnt++;
        }
        scanner.close();
        System.out.println(cnt + " positive numbers");
    }
}

AcWing 714. 连续奇数的和 1

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        int x = scanner.nextInt(), y = scanner.nextInt();
        scanner.close();

        if (x > y) {
            int tmp = x;
            x = y;
            y = tmp;
        }

        int sum = 0;
        for (int i = x + 1; i < y; i++)
            if (i % 2 != 0) sum += i;
        System.out.println(sum);
    }
}

AcWing 716. 最大数和它的位置

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        int max = -1, pos = -1;
        for (int i = 1; i <= 100; i++) {
            int x = scanner.nextInt();
            if (x > max) {
                max = x;
                pos = i;
            }
        }
        scanner.close();

        System.out.println(max);
        System.out.println(pos);
    }
}

AcWing 721. 递增序列

import java.util.Scanner;

public class Main {

    public static void print(int x) {
        for (int i = 1; i <= x; i++)
            System.out.printf(i + " ");
        System.out.println();
    }

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        while(true) {
            int x = scanner.nextInt();
            if (x == 0) break;
            else print(x);
        }
        scanner.close();
    }
}

AcWing 720. 连续整数相加

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        int a = scanner.nextInt();
        int n = scanner.nextInt();
        while(n <= 0) n = scanner.nextInt();
        scanner.close();

        int sum = 0;
        for (int i = a; i < a + n; i++) sum += i;
        System.out.println(sum);
    }
}

AcWing 724. 约数

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        scanner.close();
        for (int i = 1; i <= n; i++)
            if (n % i == 0) System.out.println(i);
    }
}

AcWing 723. PUM

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        int m = scanner.nextInt();
        int n = scanner.nextInt();
        scanner.close();
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n - 1; j++) {
                System.out.printf("%d ", i * n + j + 1);
            }
            System.out.println("PUM");
        }
    }
}

AcWing 715. 余数

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        scanner.close();
        for (int i = 2; i < 10000; i++)
            if (i % n == 2) System.out.println(i);
    }
}

AcWing 710. 六个奇数

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        int x = scanner.nextInt();
        scanner.close();
        if (x % 2 == 0) x++;
        for (int i = 0; i < 6; i++) {
            System.out.println(x);
            x += 2;
        }
    }
}

AcWing 711. 乘法表

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        scanner.close();
        for (int i = 1; i <= 10; i++)
            System.out.println(i + " x " + n + " = " + (i * n));
    }
}

AcWing 718. 实验

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        int coney = 0, rat = 0, frog = 0;
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        for (int i = 0; i < n; i++) {
            int num = scanner.nextInt();
            String type = scanner.next();
            if (type.equals("C")) coney += num;
            else if (type.equals("R")) rat += num;
            else frog += num;
        }
        scanner.close();
        int total = coney + rat + frog;
        System.out.printf("Total: %d animals\n", total);
        System.out.printf("Total coneys: %d\n", coney);
        System.out.printf("Total rats: %d\n", rat);
        System.out.printf("Total frogs: %d\n", frog);
        System.out.printf("Percentage of coneys: %.2f %%\n", coney * 100.0 / total);
        System.out.printf("Percentage of rats: %.2f %%\n", rat * 100.0 / total);
        System.out.printf("Percentage of frogs: %.2f %%\n", frog * 100.0 / total);
    }
}

AcWing 713. 区间 2

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        int cnt = 0;
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        for (int i = 0; i < n; i++) {
            int x = scanner.nextInt();
            if (x >= 10 && x <= 20) cnt++;
        }
        scanner.close();
        System.out.println(cnt + " in");
        System.out.println((n - cnt) + " out");
    }
}

AcWing 719. 连续奇数的和 2

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        for (int i = 0; i < n; i++) {
            int x = scanner.nextInt();
            int y = scanner.nextInt();

            if (x > y) {
                int tmp = x;
                x = y;
                y = tmp;
            }

            int sum = 0;
            for (int j = x + 1; j < y; j++)
                if (j % 2 != 0) sum += j;
            System.out.println(sum);
        }
        scanner.close();
    }
}

AcWing 717. 简单斐波那契

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        scanner.close();
        int[] a = {0, 1, 1};
        for (int i = 0; i < Math.min(3, n); i++)
            System.out.printf(a[i] + " ");
        for (int i = 3; i < n; i++) {
            a[0] = a[1];
            a[1] = a[2];
            a[2] = a[0] + a[1];
            System.out.printf(a[2] + " ");
        }
        System.out.println("");
    }
}

AcWing 722. 数字序列和它的和

import java.util.Scanner;

public class Main {

    public static void print(int m, int n) {
        if (m > n) {
            int tmp = m;
            m = n;
            n = tmp;
        }

        int sum = 0;
        for (int i = m; i <= n; i++) {
            System.out.printf(i + " ");
            sum += i;
        }
        System.out.println("Sum=" + sum);
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        while(true) {
            int m = scanner.nextInt();
            int n = scanner.nextInt();
            if (m <= 0 || n <= 0) break;
            print(m, n);
        }
        scanner.close();
    }
}

AcWing 725. 完全数

import java.util.Scanner;

public class Main {

    public static boolean judge(int x) {
        int sum = -x;
        for (int i = 1; i <= x / i; i++) {
            if (x % i == 0) {
                sum += i;
                if (i != x / i) sum += x / i;
            }
        }
        return sum == x;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        for (int i = 0; i < n; i++) {
            int x = scanner.nextInt();
            if (judge(x)) System.out.println(x + " is perfect");
            else System.out.println(x + " is not perfect");
        }
        scanner.close();
    }
}

笔记

约数成对出现。如果 $i$ 是 $x$ 的约数，那么 $\frac{x}{i}$ 亦是 $x$ 的约数。因此不必从 $1$ 遍历到 $n$ ，而只需从 $1$ 遍历到 $\sqrt{x}$ 。但要注意

如果 $\sqrt{x}$ 是 $x$ 的约数时，只能计算一次；
$1$ 和 $x$ 都是 $x$ 的约数，遍历到 $1$ 时，会把 $x$ 也加入到总和中，因此可把总和初值设为 $-x$ ，抵消掉约数 $x$

AcWing 726. 质数

import java.util.Scanner;

public class Main {

    public static boolean judge(int x) {
        for (int i = 2; i * i <= x; i++)
            if (x % i == 0) return false;
        return true;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        for (int i = 0; i < n; i++) {
            int x = scanner.nextInt();
            if (judge(x)) System.out.println(x + " is prime");
            else System.out.println(x + " is not prime");
        }
        scanner.close();
    }
}

笔记

利用约数成对出现的特点，优化找约数的时间复杂度。

AcWing 727. 菱形

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        scanner.close();

        int x0 = n / 2, y0 = n / 2;
        for (int y = 0; y < n; y++) {
            for (int x = 0; x < n; x++)
                if (Math.abs(x - x0) + Math.abs(y - y0) <= n / 2)
                    System.out.printf("*");
                else
                    System.out.printf(" ");
            System.out.println("");
        }
    }
}

笔记

用曼哈顿距离描述何时输出*。假设菱形中心坐标为 $(x_0,y_0)$ ，当位置 $(x,y)$ 满足 $|x-x_0|+|y-y_0| = d \leqslant \frac{a}{2}$ 时，输出*，否则输出空格。其中 $a$ 是菱形对角线长度。

注意本题限制 $n$ 为奇数，因此可用曼和顿距离描述。

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