Process control - Branch

Catalog

Process control

Sequence flow control

Branch process control

1、 Branching structure

2、if Branch statement

3、if  else Branch statement  

4、 Judgement of leap year

5、if else if sentence （ Multi branch statement ）

6、 Sentence for judging grades

7、 Ternary expression  

 8、 Ternary expression — Number complement 0

9、switch sentence  

10、 Query fruit cases  

11、switch Statement and if else if The difference between sentences  

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Process control

During the execution of a program , The execution order of each code has a direct impact on the result of the program . Many times we need to control the execution order of the code to realize the functions we want to complete .
Simple understanding ︰ Process control is to control what structural order our code technology executes
There are three main structures of process control , Namely Sequential structure 、 Branch and loop structures , These three structures represent the order in which the three codes are executed .

Sequence flow control

The sequential structure is the simplest in the program 、 The most basic process control , It has no specific grammatical structure , The program will follow Sequence of codes , Execute sequentially , Most of the code in the program is executed in this way .

Branch process control

1、 Branching structure

In the process of executing code from top to bottom , According to different path codes （ Execute the process of selecting one more code ）, So we can get different results

JS Language provides two branch structure statements

• if sentence
• switch sentence  

2、if Branch statement

If if The result of the conditional expression is true true Execute... In braces Execute statement  
If if The result of the conditional expression is false The statement in braces is not executed execute if Code after statement

// 2.  Execution ideas    If  if  The result of the conditional expression is true  true  Execute... In braces   Execute statement  
//  If if  The result of the conditional expression is false   The statement in braces is not executed   execute if  Code after statement 
// 3.  Code experience 
if (3 < 5) {
    alert(' Desert camel ');
}

3、if  else Branch statement  

if ( Conditional expression ) {

            // Execute statement 1

        } else {

            // Execute statement 2

        }

Execution ideas If the expression turns out to be true Then execute the statement 1   otherwise   Execute statement 2

var age = prompt(' Please enter your score :');
if (age >= 80) {
alert(' Reward you for playing mobile ');
} else {
alert(' roll ,  Go home and do your homework ');
}

4、 Judgement of leap year

The year entered by the user , If it's a leap year, it pops up a leap year , Otherwise, the normal year will pop up

// Algorithm ： Can be 4 Divisible and not divisible 100 Leap year （ Such as 2004 Year is leap year ,1901 Year is not a leap year ） Or can be  400  Divisible is leap year 
//  eject prompt  Input box , Let the user enter the year , Take this value and save it in a variable 
//  Use  if  Statement to determine whether it is a leap year , If it's a leap year , Is executed  if  The output statement in braces , Otherwise, it will be executed  else The output statement inside 
//  Be sure to pay attention to the inside and  &&  And or  ||  Writing , At the same time, note that the way to judge the division is to take the remainder as  0
var year = prompt(' Please enter the year ：');
if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0) {
alert(year + ' It's a leap year ');
} else {
alert(year + ' It's the year of the year ');
}

5、if else if sentence （ Multi branch statement ）

1. Multi branch statement   Is to use multiple conditions to select different statements to execute Get different results   multi-select 1 The process of

2. if else if A statement is a multi branch statement

 3. Grammatical norms

        if ( Conditional expression 1) {

            // sentence 1;

        } else if ( Conditional expression 2) {

            // sentence 2;

        } else if ( Conditional expression 3) {

            // sentence 3;

        } else {

            // Last statement ;

        }

 4. Execution ideas

      If the conditional expression 1 Fulfill it sentence 1 After execution , Quit the whole if Branch statement  

      If the conditional expression 1 dissatisfaction , Then judge the conditional expression 2   If you are satisfied , Execute statement 2 And so on

      If none of the above conditional expressions holds , execute else The words in it

 

6、 Sentence for judging grades

//  Pseudo code   According to the idea of judging from big to small 
//  eject prompt Input box , Let the user enter a score （score）, Take this value and save it in a variable 
//  Use multiple branches  if else if  Statement to judge the output of different values 
var score = prompt(' Please enter the score :');
if (score >= 90) {
alert('A');
} else if (score<90 && score >= 80) {
alert('B');
} else if (score < 80 && score >= 70) {
alert('C');
} else if (score < 70 && score >= 60) {
alert('D');
} else {
alert('E');
}

7、 Ternary expression  

Grammatical structure ： Conditional expression ？ expression 1 ： expression 2

Execution ideas

         If the conditional expression turns out to be true be return expression 1 Value If the conditional expression turns out to be false Then return to expression 2 Value

var num = 10;
var result = num > 5 ? ' Yes ' : ' No, it isn't '; //  We know that an expression has a return value 
console.log(result);
// if (num > 5) {
// result = ' Yes ';
// } else {
// result = ' No, it isn't ';
// }

 8、 Ternary expression — Number complement 0

// User input 0~59 A number between 
//  If the number is less than 10, Then add... Before this number 0,（ Add 0  Splicing ）  otherwise    No operation 
//  Accept the return value with a variable , Output 
var time = prompt(' Please enter a  0 ~ 59  A number between ');
//  Ternary expression     expression  ？  expression 1 ： expression 2 
var result = time < 10 ? '0' + time : time; //    Assign the return value to a variable 
alert(result);

9、switch sentence  

1、switch Statements use

1. switch Statement is also a multi branch statement Multiple choices can also be realized 1

2. Grammatical structure switch transformation 、 switch case The meaning of small examples or options

switch ( expression ) {

    case value1:

    Execute statement 1;

    break;

    case value2:

    Execute statement 2;

    break;

    ...

    default:

    Execute the last statement ;

}

3. Execution ideas Using the value of our expression and case The following option values match If match up , It's time to do it case The words in it If none of them match , Then perform default The words in it

switch (8) {
    case 1:
        console.log(' This is a 1');
        break;
    case 2:
        console.log(' This is a 2');
        break;
    case 3:
        console.log(' This is a 3');
        break;
    default:
        console.log(' No match ');

}

2、switch matters needing attention

var num = 1;
switch (num) {
    case 1:
        console.log(1);

    case 2:
        console.log(2);

    case 3:
        console.log(3);
        break;

}

1. We develop inside Expressions are often written as variables

2. We num Value and case When the values inside match is Congruence   The value must be consistent with the data type num === 1

3. break If the current case There's no break Will not quit switch Is to move on to the next case

10、 Query fruit cases  

// eject  prompt  Input box , Let the user enter the name of the fruit , Take this value and save it in a variable .
//  Take this variable as  switch  The expression in parentheses .
// case  The following values write several different fruit names , Be sure to put quotation marks  , Because it must be a congruent match .
//  Pop up different prices . Also pay attention to each  case  Then add  break , In order to exit  switch  sentence .
//  take  default  Set to no such fruit .
var fruit = prompt(' Please enter the fruit you want to query :');
switch (fruit) {
    case ' Apple ':
    alert(' The price of apple is  3.5/ Jin ');
    break;
    case ' durian ':
    alert(' The price of durian is  35/ Jin ');
    break;
    default:
    alert(' Without this fruit ');
}

11、switch Statement and if else if The difference between sentences  

• In general , They can be replaced by each other
• switch..case Statements usually deal with case In order to compare the determined values , and i...els... More flexible sentences , It is often used to judge the range ( Greater than 、 Equal to a certain country )
• switch A conditional statement that executes directly into a program after conditional judgment , More efficient . and if...else There are several conditions for a statement , You have to judge how many times .
• When there are fewer branches ,if...else Statement execution efficiency ratio switch Sentence height .
• When there are more branches ,switch The execution efficiency of the statement is relatively high , And the structure is clearer .