Day 10 算法结构

顺序结构

从上到下依次执行

选择结构

if单选择结构

用于判断是否可行

if(布尔表达式){//如果布尔表达式为true将执行的语句
}

e.g.

package structure;import java.util.Scanner;public class ifDemo01 {public static void main(String[] args) {Scanner scanner = new Scanner(System.in);System.out.println("请输入内容：");String s = scanner.nextLine();//输完scanner.nextLine();Alt+Enter//equals：判断字符串是否相等  此处不能用==if (s.equals("Hello")){System.out.println(s);}System.out.println("End");scanner.close();}
}

if双选择结构

if(boolean){//if boolean = true
}else{//if booolean = false
}

package structure;import java.util.Scanner;public class Demo02 {public static void main(String[] args) {//score>60 pass;<60 failScanner scanner = new Scanner(System.in);System.out.println("Please enter the score:");int score = scanner.nextInt();if (score>60){System.out.println("PASS");}else{System.out.println("FAIL");}scanner.close();}}

if多选择结构

if(boolean 1){//if boolean 1 = true
}else if(boolean 2){//if boolean 2 = true
}else if(boolean 3){//if boolean 3 = true
}else{//if all of these ?= true
}

package structure;import java.util.Scanner;public class Demo03 {public static void main(String[] args) {Scanner scanner = new Scanner(System.in);System.out.println("PLease enter the score:");int socre = scanner.nextInt();if (socre==100){System.out.println("Congratulations!");}else if (socre<100 && socre>=90){System.out.println("A");}else if (socre<90 && socre>=75){System.out.println("B");}else if (socre<75 && socre>=60){System.out.println("C");}else if (socre<60 && socre>=0){System.out.println("D");}else{//别漏了这个！System.out.println("Null!");}scanner.close();}
}

• if语句至多有1个else语句，else语句在所有的else if语句之后
• if语句可以用若干个else if语句，他们必须在else语句之前
• 一旦一个else if语句检测为true，其他的else if以及else语句都将跳过执行

嵌套的if结构

if(boolean 1){//if bollean 1 = trueif(boolean 2){//if boolean 2 = true}
}

switch 多选择结构

switch(expression){case value://statementsbreak;//可选case value://statementsbreak;//可选//你可以有任意数量的case语句default://可选//statements
}

• 变量类型可以是byte，short，int，char
• 支持String
• case标签必须为字符串常量或字面量

package structure;public class switchDemo04 {public static void main(String[] args) {//case穿透  //switch匹配一个具体的值char grade = 'B';switch (grade){case'A':System.out.println("EXCELLENT!");break;//不加break就会全部输出case 'B':System.out.println("GOOD!");break;case 'C':System.out.println("PASS");break;case 'D':System.out.println("FAIL");break;default:System.out.println("UNKNOWN");}}
}

反编译——字符的本质是数字 看源码

循环结构

while循环

while(布尔表达式){//循环内容
}

• 如果不满足条件，则不能进入循环
• 只要布尔表达式正确，循环就会一直执行下去
• 大部分需要一个让表达式失效的方法来结束循环
• 少部分需要循环一直执行，比如服务器的请求响应监听等
• 循环条件一直为true就会无限循环

package structure;public class whileDemo05 {public static void main(String[] args) {int i = 0;while (i<100){i++;System.out.println(i);}}
}

package structure;public class whileDemo06 {public static void main(String[] args) {//calculate 1+2+3+...+100=?int i = 0;int sum = 0;while (i<=100){sum = sum + i;i++;}System.out.println(sum);}
}

do…while循环

• 若不满足条件，也至少会执行一次

do{//code statements
}while(布尔表达式)；

while和do…while的区别

while先判断后执行，dowhile先执行后判断

For循环！

for(初始化；布尔表达式；更新){//statements
}

package structure;public class ForDemo07 {public static void main(String[] args) {int a = 1;//初始化条件while (a<=100){//条件判断System.out.println(a);a+=2;//迭代}System.out.println("while循环结束");//初始化 条件判断 迭代for (int i = 1;i<=100;i++){System.out.println(i);}//快捷键：100.forSystem.out.println("For循环结束");}
}

for(; ; ){//死循环
}

package structure;public class ForDemo08 {public static void main(String[] args) {//calculate the sum of odd and even between 0 and 100int oddSum = 0;int evenSum = 0;for (int i = 0; i <= 100; i++) {if (i%2!=0){oddSum+=i;}else{evenSum+=i;}}System.out.println("The sum of odd:"+oddSum);System.out.println("The sum of even:"+evenSum);}
}

package structure;public class ForDemo09 {public static void main(String[] args) {/*print the number that can be divided by 5 between 1 and 1000print 3 of them every line*/for (int i = 0; i <= 1000; i++) {if (i%5==0){System.out.print(i+"\t");}if (i%(5*3)==0){System.out.println();//System.out.print("\n")}}}
}

• println 输出会换行
• print 输出不会换行