Circular structure and circular keywords

Loop structure

1. The characteristics of circulation structure

Any cycle requires four essential conditions

• 1. Counter initialization

• 2. The loop condition

• 3. The loop body

• 4. The counter changes

2. while loop

while word ： When ……

while ( The loop condition ) {

Cyclic operation

}

package com.qfedu.test1;
​
import java.util.Scanner;
​
/**
 *   demand ： The teacher checks whether Zhao Si's learning task is qualified every day ,
 *   If it's not qualified , I'm going to continue .
 *   The daily learning task assigned by the teacher to Zhao Si is ：
 *   Read the textbook in the morning , Study the theory part , Programming in the afternoon , Master the code part 
 *
 */
public class Test3 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        
        System.out.println(" Is your academic record qualified ？y/n");
        
        String answer = input.next();
        
        while(answer.equals("n")) {
            System.out.println(" Read the textbook in the morning , Study the theory part ");
            System.out.println(" Programming in the afternoon , Master the code part ");
            
            System.out.println(" Is your academic record qualified ？y/n");
            answer = input.next();
        }
        
        System.out.println(" congratulations , To complete the task ！");
        
    }
}

3. do-while loop

while and do-while The difference between

• do-while loop Execute before judge Whether or not the conditions are valid At least once

• while loop Judge before you execute Conditions not established Not once

package com.qfedu.test2;
​
import java.util.Scanner;
​
/**
 *   difference ：while and do-while The difference between 
 *  do-while loop   Execute before judge   Whether or not the conditions are valid   At least once 
 *  while loop   Judge before you execute   Conditions not established   Not once 
 * 
 *   demand ： After a few days of study , The teacher gave Zhaosi a test question ,
 *   Let him write a program on the computer first , Then the teacher checks whether it is qualified . If it's not qualified , Continue to write …… 
 *
 */
public class Test1 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        
        String answer;
        
        do {
            System.out.println(" Write code , On board test ……");
            
            System.out.println(" Please enter whether it is qualified ？y/n");
            
            answer = input.next();
        }while(answer.equals("n"));
        
        System.out.println(" Congratulations on completing the task ！");
        
    }
}
​

package com.qfedu.test2;
/**
 *   Use do-while Loop printing N Study hard all the time 
 *
 */
public class Test2 {
    public static void main(String[] args) {
        int i = 1;
        do {
            System.out.println(" study hard " + i);
            i++;
        }while(i <= 1000);
    }
}

​

4.for loop

word in order to ……

The first round

1. Counter initialization The first to perform And only once

2. Judge the condition

3. The loop body

4. The counter changes

The second round

Execute directly from the judgment condition

The number of cycles is fixed , In certain cases ,for Cycle ratio while and do-while Simple writing

package com.qfedu.test3;
/**
 *  for loop   
 *   word    in order to  ……
 *  for Loop execution process 
 *   The first round 
 *      1. Counter initialization    The first to perform   And only once 
 *      2. Judge the condition 
 *      3. The loop body 
 *      4. The counter changes 
 *   The second round 
 *       Execute directly from the judgment condition  
 * 
 *   The number of cycles is fixed , In certain cases ,for Cycle ratio while and do-while Simple writing 
 *
 */
public class Test1 {
    public static void main(String[] args) {
        for(int i = 0;i < 15;i++) {
            System.out.println(" study hard " + i);
        }
        
        System.out.println(" Program end ");
        
        
    }
}
​

package com.qfedu.test3;
​
import java.util.Scanner;
​
/**
 *   Loop in a student's final exam 5 Course results , And calculate the average score 
 *
 */
public class Test2 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        
        System.out.println(" Please enter a name ");
        
        String name = input.next();
        
        double sum = 0;
        
        for(int i = 1;i <= 5;i++) {
            System.out.println(" Please enter the first "+ i +" Results ");
            
            double score = input.nextDouble();
            
            sum += score;
        }
        
        System.out.println(" The total score is ：" + sum);
​
        System.out.println(name + " The average score of the students is ：" + sum / 5);
        
        System.out.println(" Program end ");
        
    }
}

4.1 for Circulation FAQs

for Circulation FAQs 1 Counter not initialized

package com.qfedu.test4;
/**
 *  for Circulation FAQs 1   Counter not initialized 
 */
public class Test2 {
    public static void main(String[] args) {
        int i = 0;
        for(;i < 10;i++) {
            System.out.println(i);
        }
    }
}​

for Circulation FAQs 2 Lack of judgment conditions

package com.qfedu.test4;
/**
 *  for Circulation FAQs 2  Lack of judgment conditions 
 *
 */
public class Test3 {
    public static void main(String[] args) {
        for(int i = 0;;i ++) {
            System.out.println(i);
        }
        
        //  System.out.println(" Program end ");  Because of the above for The loop lacks judgment conditions   So the code cannot be executed here 
    }
}

for Circulation FAQs 3 Missing counter change

package com.qfedu.test4;
/**
 *  for Circulation FAQs 3  Missing counter change 
 *
 */
public class Test4 {
    public static void main(String[] args) {
        for(int i = 0;i < 10;) { //  Missing counter change 
            System.out.println(i);
        }
    }
}

​

for Circulation FAQs 4 Missing counter initialization 、 Judge the condition 、 The counter changes

package com.qfedu.test4;
/**
 *  for Circulation FAQs 4   Missing counter initialization  、 Judge the condition 、 The counter changes 
 *
 */
public class Test5 {
    public static void main(String[] args) {
        for(;;) {
            System.out.println("hello world");
        }
    }
}

​

5. Cycle comparison and summary

difference

• difference 1： grammar

  

   

• difference 2： Execution order while loop ： First judge , Re execution do-while loop ： Execute first , To determine for loop ： First judge , Re execution

• difference 3： Applicable scenario

  The number of cycles is determined , Generally selected for loop The number of cycles is uncertain , Generally selected while or do-while loop

6.break keyword

Applicable scenario :

1.switch in To jump out of switch structure

2.3 In a cycle Represents an interrupt loop structure The number of cycles not completed No more execution

Usually used in conjunction with branch structures

package com.qfedu.test5;
/**
 *   Use do-while loop   break  Run exit 
 *
 */
public class Test3 {
    public static void main(String[] args) {
        int i = 1;
        do {
            if(i == 3) {
                System.out.println(" Too tired , sign out ");
                break;
            }
            
            System.out.println(" Running page "+ i +" circle ");
            i++;
​
            
        }while(i <= 10);
        
        System.out.println(" Program end ");
        
    }
}
​

practice

package com.qfedu.test5;
​
import java.util.Scanner;
​
/**
 *   Use circular entry 5 Results   If you enter a negative number   Stop typing   And prompt 
 *
 */
public class Test4 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        
        int sum = 0;
        
        boolean flag = true;
        
        for(int i = 1;i <= 5;i++) {
            System.out.println(" Please enter the first "+ i +" Results ");
            int score = input.nextInt();
            
            if(score < 0) {
                flag = false;
                System.out.println(" Score entry error , Please re-enter ");
                break;
            }
            
            sum += score;
        }
        
        if(flag) {  // flag == true
            System.out.println(" The average score is " + sum / 5);
        }else {
            System.out.println(" Incorrect score entry , No more averaging ");
        }       
    }
}

​

7. continue keyword

continue continue

Applicable scenario ： It can only be used in a cycle , Jump out of this cycle , Continue with next cycle

package com.qfedu.test6;
​
import java.util.Scanner;
​
/**
 *  continue  continue 
 *   Applicable scenario ： It can only be used in a cycle , Jump out of this cycle , Continue with next cycle 
 *   Cycle input Java Students' grades in class , The statistical score is greater than or equal to 80 The proportion of students who get a score 
 *
 */
public class Test2 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
​
        System.out.println(" Please enter the number of people ");
        
        int num = input.nextInt();
        
        double count = 0;
        
        for(int i = 1;i <= num;i++) {
            System.out.println(" Please enter the first "+ i +" Personal achievements ");
            int score = input.nextInt();
            if(score <= 80) {
                continue;
            }
            count ++;
        }
        
        
        System.out.println(" Greater than 80 The total number of points is ：" + count);
        System.out.println(" Greater than 80 The percentage of sub population is ：" + count / num * 100 + "%");
        
        
        
    }
}

8.break and continue The difference between

• Use occasion break be used for switch In structure and cyclic structure continue Used in cyclic structures

• effect （ In cyclic structure ） break Statement terminates a loop , The program jumps to the next statement outside the loop block continue Jump out of this cycle , Enter next cycle The same is true of the double cycle