JS day 4 process control (if statement and switch statement)

One 、 What is process control ：

        During the execution of a program , The execution order of each code has a direct impact on the result of the program . A lot of times

        We all need to control the execution order of the code to achieve the functions we want to complete .

        brief introduction ： Process control is to control our code according to what structural order our code executes

Two 、 There are three main structures of process control , Namely Sequential structure 、 Branching structure and Loop structure

1、 Sequential structure

        The sequential structure is the simplest in the program 、 The most basic process control , He has no specific grammatical structure , The sequence of the program is as follows , Execute sequentially ,  Most of the code in the program is executed in this way

2、 Branch process control

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

        js Language provides two branches of structural statements

        if Grammatical structure and switch Grammatical structure

       （1）if The grammatical structure of           if（ If ）

        1） Single branch statements

          Grammatical structure

       if( Conditional expression )

        { Execute statement }

        Execution ideas   If if The condition of the price adjustment expression inside is true true Execute the execution statement in braces

        If if The conditional expression is false The statement in braces is not executed execute if Code after statement

        Code display

if(3<5){
            alert(' Pillow can't wake up ');    // If 3<5 A warning box pops up ‘ Pillow can't wake up ’
        }

Pictures show ：

 2）if else sentence         ( Two branch statements )

         Grammatical structure ：   if If else otherwise

        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

Code display ：
 

<script>
        var age = prompt(' Please enter your age ')
        if (age>=18) {
            alert(' Pillow can't wake up ')
        }
        else{
            alert(' The pillow can't sleep ')
        };
// Enter the age , If greater than or equal to 18  A warning box pops up “ Pillow can't wake up ”
 If less than 18 A warning box pops up “ Can't sleep on the pillow ”

Pictures show ：

 3)if else if        ( Multi branch statement )

 

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

       if else if A statement is a multi branch statement

        Grammatical norms

        if( Conditional expression 1）{

             sentence 1

        } else if（ Conditional expression 2）{

             sentence 2

        }   else {

             sentence 3

        }

        Execution ideas ： If the conditional expression 1, Fulfill it , After execution Quit the whole if sentence

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

                    If all the above conditions are not true , execute else The words in it

        Be careful ：

        （1) Multi branch statement or multi choice 1   Finally, only one statement is executed

        （2）else if In theory, there can be any number of conditions

        （3）else if There's a space in the middle

The case shows ：

        requirement ： User entered score , According to the score （score） Output the corresponding grade letter A,B,C,D,E

        1、90 above , Output A

        2、80( contain )~90（ Not included ）, Output B

        3、70( contain )~80（ Not included ）, Output C

        4、60( contain )~70（ Not included ）, Output D

        5、60（ Not included ） following , Output E

Code display ;

 <script>
        var score = prompt(' Please enter your score :');
        if (score>=90){
          alert('A'); 
        } else if(score >= 80 ){
            alert('B');
        } else if (score>=70) {
            alert('C');
        } else if (score >= 60) {
            alert('D');
        } else {
            alert('E');
            }

     </script>

Result display ：

        2、switch sentence

        （1）Sitch Statement is also a multi branch statement , Multiple choices can also be realized 1, It is used to execute different code based on different conditions , When you want to set a series of specific values for a variable, the option is , You can use switch transformation 、 switch case The meaning of small examples or options

        （2） Grammatical structure

        switch( expression ) {

            case value：

 

 switch( expression ) {
            case value：
             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 Is executed case

        The words in it If there is no match , Then perform default The words in it

        （4） Code display ：

 switch(2){
            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 ');
        }

Result display ：

  Be careful

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

for example

Code ：

var num = '3'; // there 3 It's character type 
        switch (num) {
            case 1:
            console.log(1);
            break;
            case 3:
            console.log(3);
            break;
        }

result ：

 

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

Code display ：

        var num = 3;
        switch (num) {
            case 1:
            console.log(1);
            break;
            case 3:
            console.log(3);
            // Nothing here break
            case 5：
            console.log(5);
            break;
        }

Result display ：

This is because  case3 No, break The end of the statement , So it leads to the next cycle , Know how to meet break To end the cycle .

therefore break Be sure to write