Day05 branch and loop (II)

1. local variable

Concept ： Variables declared inside a method , You must assign a value before using .

	describe
Define the location	Must be defined in the method Location 1234 Can not be
Scope of action	Within the braces closest to the current variable
About duplicate names	In the overlapping scope of action , You can't have the same name
Storage location	Basic data type , All stored on the stack (stack) in Reference data type , The name is stored on the stack (stack) in , Values are stored in the heap (heap) in
Life cycle	As the method is stacked ( Pressing stack ) And take effect , As the method goes out of the stack ( Bomb stack ) And death

package com.qfedu.test1;
/**
 * 	 local variable 
 * 	 Concept ： Variables declared inside a method , You must assign a value before using .
 * 
 * 	 Define the location ： Must be defined in the method   Location 1234  Can not be   
 * 	 Scope of action ： Within the braces closest to the current variable 
 * 	 About duplicate names ： In the overlapping scope of action , You can't have the same name 
 * 
 * 	 Storage location ：
 * 		 Basic data type , All stored on the stack (stack) in 
 * 		 Reference data type , The name is stored on the stack (stack) in , Values are stored in the heap (heap) in 
 * 	 Life cycle ：
 * 		 As the method is stacked ( Pressing stack ) And take effect , As the method goes out of the stack ( Bomb stack ) And death 
 * @author WHD
 *
 */
//  Location 1
public class Test1 {
	//  Location 2
	public static void main(String[] args) {
		int d;
		// System.out.println(d);  Variable d No assignment   Cannot access 
		
		int a = 10;
		
		int b = 20;
		
		if(a > b) {
			System.out.println("a Greater than b Conditions established ");
			// int a = 66;
			int c = 66;
			System.out.println(b);
		}else {
			// System.out.println(c);  You can't visit c   because c The scope of action of   Only in if in 
			System.out.println(a);
			int c = 66;
		}
		
	}
	//  Location 3
}
//  Location 4

2. Variable name

The number of camels in the United States

word ： Letter

Next ： Underline

beautiful ： Dollar symbol $

people ： RMB symbol ￥

Count ： Numbers

camel ： Hump named

Variables can be named with letters , Underline , Dollar symbol , The RMB symbol begins , It can contain numbers , Cannot start with a number , Follow the hump nomenclature

studentNameAndAge

package com.qfedu.test1;
/**
 * 	 Variable name 
 * @author WHD
 *
 */
public class Test2 {
	public static void main(String[] args) {
		int a1 = 10;
		int _a = 20;
		int $c = 33;
		int ￥d = 44;
		
		// int 5d = 66;  Cannot start with a number 
		
		String studentName = " Zhao si ";
		
	}
}

3. loop

3.1 while loop

Counter initialization ;

while ( The loop condition ) {

 Cyclic operation 

The counter changes ;

}

while loop

• while word ： When ……

• All loops must have four conditions

• 1. Counter initialization

• 2. Judge the condition

• 3. The loop body

• 4. The counter changes

package com.qfedu.test2;
/**
 * 	while loop 
 * 	while ：  When  …… 
 * 
 * 	 All loops must have four conditions 
 * 	1. Counter initialization 
 * 	2. Judge the condition 
 * 	3. The loop body  
 * 	4. The counter changes 
 * 
 * 	 demand ： Use while Cycle to achieve 100 Study hard all the time 
 * @author WHD
 *
 */
public class Test2 {
	public static void main(String[] args) {
		int i = 1;
		while(i <= 10000) {
			System.out.println(" The first "+ i +" Study hard all the time ");
			i++;
		}
		
		System.out.println(" Program end ~");
		
		
		
	}
}

package com.qfedu.test2;

import java.util.Scanner;

/**
 * 	 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 
 * @author WHD
 *
 */
public class Test3 {
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		
		System.out.println(" Please enter whether your academic performance is 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(" Please enter whether your academic performance is qualified ？y/n");
			answer = input.next();
		}
		
		
		System.out.println(" congratulations , Complete the learning task ~");
		
		
	}
}

3.2 do-while loop

do-while: do …… When

Counter initialization ;

do {

Cyclic operation

The counter changes ;

} while ( The loop condition );

while and do-while The difference between ？

while It's to judge first After execution Conditions not established Not once

do-while It's to execute first Post judgment Whether or not the conditions are valid At least once

package com.qfedu.test3;

import java.util.Scanner;

/**
 * 	 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 …… 
 * 
 * 	while and do-while The difference between ？
 * 	while It's to judge first   After execution   Conditions not established   Not once 
 * 	do-while  It's to execute first    Post judgment   Whether or not the conditions are valid   At least once 
 * @author WHD
 *
 */
public class Test1 {
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		
		String answer = "";
		
		do {
			System.out.println(" Write test code on the computer ");
			
			System.out.println(" Please enter whether the result is qualified ？y/n");
			
			answer = input.next();
			
		}while(answer.equals("n"));
		
		System.out.println(" Congratulations on completing the task ！");
		
	}
}

package com.qfedu.test3;
/**
 * 	 Use do-while Realization   Print N Study hard all the time 
 * @author WHD
 *
 */
public class Test2 {
	public static void main(String[] args) {
		int i = 1;
		do {
			System.out.println("do-while Loop to realize the "+ i +" Study hard all the time ");
			i++;
		}while(i < 0);
		
		int j = 1;
		while(j < 0) {
			System.out.println("while Loop to realize the "+ j +" Study hard all the time ");
			j++;
		}
		
		System.out.println(" Program end ！");
		
		
	}
}

3.3 for loop

for( Counter initialization ; Judge the condition ; The counter changes ){ The loop body ; }

for Circulation also requires the four essential parts of circulation

Counter initialization

Judge the condition

The loop body

The counter changes

Use for Cycle to achieve 100 Study printing well

for Loop execution order ：

The first round

1. First perform counter initialization And only perform it once

2. Execute judgment conditions

3. Execution loop body

4. Execute counter change

The second round

Execute directly from the judgment condition

package com.qfedu.test4;

import java.util.Scanner;

/**
 * 	for Loop implementation statistics 5 Results , And calculate the average score 
 * @author WHD
 *
 */
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 = sum + score; // sum += score;
		}
		
		System.out.println(name + " Of 5 The average score of the course is " + sum / 5);

	}
}

package com.qfedu.test5;

import java.util.Scanner;

/**
 * 	 Output addition table 
 * @author WHD
 *
 */
public class Test1 {
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		
		System.out.println(" Please enter a value ");
		
		int num = input.nextInt();
		
		for(int i = 0;i <= num;i++) {
			System.out.println(i + "+" + (num - i) + "=" + num);
		}
		
		System.out.println(" Program end ~");
	}
}

4. Cyclic comparison

difference ：

1. The grammar is different

2. Execution order

while loop ： First judge , Re execution

do-while loop ： Execute first , To determine

for loop ： First judge , Re execution

3. Application

The number of cycles is determined , Generally selected for loop

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

5.break keyword

break keyword

break It can be used for switch in , To jump out of switch structure

It can also be used in loops , Means to jump out of the current loop , The number of incomplete executions No more execution

break Keyword in loop It is usually used in combination with branch structure

break After keyword Can't write code

package com.qfedu.test6;
/**
 * 	break keyword 
 * 	break It can be used for switch in , To jump out of switch structure 
 * 	 It can also be used in loops , Means to jump out of the current loop , The number of incomplete executions   No more execution 
 * 
 * 	 run 10 circle   The first 8 circle   Too tired   sign out 
 * @author WHD
 *
 */
public class Test1 {
	public static void main(String[] args) {
		for (int i = 1; i <= 10; i++) {
			System.out.println(" Running page "+ i +" circle ");
			if(i == 8) {
				System.out.println(" Too tired , Don't run away ");
				break; //  This represents the number of subsequent cycles that have not been completed    No more execution   Break the loop 
			}
			
		}
		System.out.println(" Program end ~");
		
	}
}

package com.qfedu.test6;
/**
 * 	while loop    and  do-while Use in loop break
 * @author WHD
 *
 */
public class Test3 {
	public static void main(String[] args) {
		int i = 1;
		while(i <= 10) {
			System.out.println(" Running page "+ i + " circle ");
			if(i == 5) {
				System.out.println(" Too tired , sign out ");
				break;
			}
			i++;
		}
		
		System.out.println("===============================================");
		int j = 1;
		do {
			System.out.println(" Running page "+ j +" circle ");
			if(j == 6) {
				System.out.println(" Too tired , rest ");
				break;
			}
			j++;
			
		}while(j <= 10);
			
	}
}

package com.qfedu.test6;

import java.util.Scanner;

/**
 * 	 Cycle through a student 5 And calculate the average score ,
 * 	 If a score is entered as negative , Stop entry and prompt for entry errors 
 * @author WHD
 *
 */
public class Test4 {
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		
		System.out.println(" Please enter a name ");
		
		String name = input.next();
		
		int sum = 0; 
		
		boolean flag = true; //  Define Boolean variables   It is used to judge whether farmers have entered wrong grades 
		
		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(" Incorrect score entered , Please re-enter ");
				break;
			}
			
			sum += score;
		}
		
//		
//		if(flag) { //  Boolean variables   Write the variable name directly   Equivalent to    Variable  == true
//			System.out.println(" The average score is " + sum / 5);
//		}else {
//			System.out.println(" Incorrect score entry , No more averaging ");
//		}
		
		System.out.println(flag ? " The average score is " + sum / 5 : " Incorrect score entry , No more averaging " );
		
		
		
		System.out.println(" Program end ");
		
	}
}

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