JS ------ Chapter II JS logic control

review

ECMA（javascript The official Standardization Organization ） Basic grammar ：

notes , Variable , Operator , data type , Process control , object

data type : character string , The number ( Integers , decimal ), Boolean value (true/false),null,undefined

Process control : The order , Branch , loop

The objectives of this chapter

Process control :

The order : Execute in sequence according to the writing order of the code

Branch : The content of this lesson is

1. if Branch : Single branch , Double branch , Multiple branches
2. switch Branch

1. Process control

1.1 What is process control

Process control : The program is in progress , Control the execution sequence of the program through specific conditions .

1.2 Three program structures

1. Sequential structure ： From the top down ,（ equation ） From right to left （ If there is an expression on the right （ Left to right ））
2. Branching structure ： When is the control code ( According to the condition ) perform , Whether to carry out .
3. Loop structure ： Control code execution ( According to the condition ) How many times .

2. Branching structure --if If

2.1 single if conditional : Single branch

grammar :

if( Conditional expression ){
     Code block ;
}

Execution process : If if The conditions in the brackets are true, Then execute the code block in braces . Otherwise, do not execute .

Be careful : In this code block , Don't add ";"

<script type="text/javascript">
	// Enter the score 
	var score = Number(prompt(" How many points did you get in Xiaoming's exam ?",""));
	// if Single branch judgment 
	//  If (if)  A score greater than or equal to 90( Conditions )   establish : Execute the code inside 
	if(score>=90){
		alert(" Reward you with an electric car !");
	}
  </script>

 

2.2 if else Judge : Double branch

Double branch ： You must choose one of the two

grammar :

if( Conditional expression ){
     Code block 1;
}else{
     Code block 2;
}

Execute the process :

1. If if The following conditional expression holds , Then execute the code block 1

2. If if The following conditional expression does not hold , Then execute the code block 2

Be careful : else No conditions

 // if else  Double branch 
    // Enter the score 
	var score = Number(prompt(" How many points did you get in Xiaoming's exam ?",""));
	// if Double branch judgment : A score greater than or equal to 90 Reward electric cars , otherwise , Whips 
	if(score>=90){
		alert(" Reward an electric car ");
	}else{
		alert(" Whips ");
	}

2.3 multiple if Judge

grammar :

if( Conditional expression 1){
     Code block 1;
}else if( Conditional expression 2){
     Code block 2;
}else if( Conditional expression 3){
     Code block 3;
}
.... //  Omit some conditions 
else{ // else It can be omitted 
     Code block n;
}

Execute the process :

1. Code execution from top to bottom , If the conditional expression 1 establish , Then execute the code block 1, Code block 1 After performing , Other branches are no longer executed （ Each branch is mutually exclusive ）.
2. If the conditional expression 1 Don't set up , Then judge the conditional expression 2 Is it true , If set up , Then execute the code block 2
3. If the conditional expression 2 Cannot be established , Then judge the conditional expression 3 Is it true , If set up , Then execute the code block 3
4. If the conditional expression 3 Cannot be established , Then judge the subsequent conditions in turn .
5. If all the conditions don't hold , execute else Code block in .

Be careful ：else It can be omitted , Means there is no default choice

 // if.. else if..else  Multiple branches 
      var score = Number(prompt(" How many points did you get in Xiaoming's exam ?",""));
      if(score>=90){ //  fraction >=90
	     alert(" good ");
	  }else if(score>=80){ // fraction >=80
		 alert(" good ")
	  }else if(score>=60){ // fraction >=60
	     alert(" secondary ");
	  }else{ 
	     alert(" Bad ")
	  }      
     

<script type="text/javascript">
	//  Input math scores and Chinese scores 
	//  If mathematics is greater than 80, And Chinese is greater than 90
	//  Or math equals 100  And Chinese is greater than 70
	//  Reward electric cars 
	//  Other situations ： Whips 
	var chinese = Number(prompt(" Please input Chinese score :",0));
	var math = Number(prompt(" Please enter the math score ",0));


    //  If the score is invalid , To prompt errors 
	//  Mathematics is less than 0 Greater than 100   Chinese less than 0 Greater than 100
	//  Wrong writing : math<0 && math>100
	if(math<0 || math>100 || chinese<0 || chinese>100){
		document.write(" Wrong score ")
	}else if(math>=80 && chinese>=90 ||
	         math==100 && chinese>=70){
		document.write(" Reward an electric car ");		 
	}else{
		document.write(" Whips ");
	}


  </script>

 2.4 if nesting

var score = Number(prompt(" Enter the score ",""));
if(score>=90){
  
}else {


  if(score>=80)(){
    
  }else{
    
  }
}

Summary :

if The conditions of branching are all judgments aimed at a certain range

3. Branching structure switch - case

switch: For equivalence judgment

if : For range judgment

grammar :

switch( Variable / data ){
    case  data 1:
        Code block 1;
       break;
    case  data 2:
        Code block 2;
       break;
    case  data 3:
        Code block 3;
       break;
    default:
        Default code block ;
       break;
}

Execute the process :

1. according to switch The data in parentheses , With the following case Compare the corresponding values one by one .
2. Satisfy a certain case Branch , The corresponding code block is executed
3. If the above branches are not satisfied , execute default The following code block
4. among break（ End statement execution ） Represents the whole switch End of sentence , If a branch does not break, Then the code will continue to execute downward , Until I met break Or all statements are executed ,switch The statement will end .
5. default Sentences can be omitted .

<script type="text/javascript">
	//  Linghuchong takes part in the martial arts competition :
	// If you are the first, you will be the leader of Wulin 
	// If he is the second, he will be the leader of Wudang 
	// If he is the third, he will be the leader of Emei 
	// Other situations , Expel from the school 
	var mc = Number(prompt(" Please enter linghuchong's martial arts ranking :",""));

	//  For a variable value , Judge whether it conforms to that situation 
	switch(mc){
		case 1: //  If mc yes 1 The situation of 
			document.write(" Leader of Wulin ");
			break;
		case 2: //  If mc yes 2 The situation of 
			document.write(" Wudang leader ");
			break;
		case 3: //  If mc yes 3 The situation of 
			document.write(" Emei leader ");
			break;
		default: //  If mc It's something else 
			document.write(" Expel from the school ");
			break;
	}


	/*
	//  For a variable value , Judge whether it conforms to that situation 
	switch(mc){
		case 1: //  If mc yes 1 The situation of 
			document.write(" Leader of Wulin ");
			break;
		case 2: //  If mc yes 2 The situation of 
			document.write(" Wudang leader ");
			break;
		case 3: //  If mc yes 3 The situation of 
			document.write(" Emei leader ");
			break;
		default: //  If mc It's something else 
			document.write(" Expel from the school ");
			break;
	}*/

	/*
	if(mc==1){
		document.write(" Leader of Wulin ");
	}else if(mc==2){
		document.write(" Wudang leader ");
	}else if(mc==3){
		document.write(" Emei leader ");
	}else{
		document.write(" Expel from the school ");
	}*/
  </script>
        
//  According to the day of today , Output what to eat 
//  week 1,2  Eat rice  3,4  Eat noodles  5,6  dumplings  7  To lose weight 
//  According to the day of today , Output what to eat 
    //  week 1,2  Eat rice  3,4  Eat noodles  5,6  dumplings  7  To lose weight 
	var weekDay = prompt(" Please enter the day of today ?","");
	switch(weekDay){
		case " Monday ":
			//document.write(" Eat rice ");
			//break;
		case " Tuesday ":
			document.write(" Eat rice ");
			break;
		case " Wednesday ":
			//document.write(" Eat noodles ");
			//break;
		case " Thursday ":
			document.write(" Eat noodles ");
			break;
		case " Friday ":
			//document.write(" Eat dumplings ");
			//break;
		case " Saturday ":
			document.write(" Eat dumplings ");
			break;
		case " Sunday ":
			//document.write(" To lose weight ");
			//break;
		case " Sunday ":
			document.write(" To lose weight ");
			break;
		default:
			document.write(" Please input !!!");
			break;
	}


  </script>

 

summary ：

Process control : Sequential structure , Branching structure (if,switch), Loop structure