javaScript learning notes "switch statement", "breakpoint", "ternary operation", "while loop" and "for loop"

switch Statements

Swith statement structure:

switch (key) {

case value:

break;

case value:

break;

case value:

break;

default:

break;

}

var x = prompt('Please enter an option A-D');
    switch (x) {
        case 'A':
            alert('A option')
            break;
        case 'B':
            alert('B term')
            break;
        case 'C':
            alert('C option')
            break;
        case 'D':
            alert('D option')
            break;
        default:
            break;
    }

Recognize words:

Swith (selection, conversion) case (option)

break: stop

Value: value

Default: default

Note: 1 The key after switch is usually a variable

2. case is usually followed by a value, "1" 1

3. No break, no break

4. Inside the switch statement body, you can nest some if statements or for statements

Execution process:

1. Take the key after the switch and the value after the case for full proportional pair (= = =)

2. If the match is found, execute the corresponding code segment, go down in turn, and encounter break termination

3. default: when the conditions are not met, default will be taken

//Enter the month to query the days of the current month
var x = prompt('Please enter option month 1-12'); //Enter 2 "2"
    switch (x - 0) {  // x*1   x/1
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
            alert('31 day')
            break;
        case 2:
            alert('28 day')
            break;
        case 4:
        case 6:
        case 9:
        case 11:
            alert('30 day')
            break;
        default:
            alert('No match found')
            break;
    }

switch statement case:

It is known that the monthly interest rates of bank lump sum deposits and withdrawals for different periods are:

Monthly interest rate =

0.63% term = 1 year

0.66% term = 2 years

0.69% term = 3 years

0.75% term = 5 years

0.84% term = 8 years

Enter the principal and term of the deposit to find the total of the interest and principal that can be obtained from the bank at maturity.

The calculation formula of interest is: interest = principal × Monthly interest rate × twelve × Deposit period.

    var money = +prompt('Please enter the stored principal')
    var x = +prompt('Please enter the deposit year');//Positive signs can also be implicitly converted to numbers
    var lx = 0;//Variables for storing interest
    switch (x) {
        case 1:
            lx = money * 12 * x * 0.0063
            alert('The sum of principal and interest is' + (lx + money))
            break;
        case 2:
            lx = money * 12 * x * 0.0066
            alert('The sum of principal and interest is' + (lx + money))
            break;
        case 3:
            lx = money * 12 * x * 0.0075
            alert('The sum of principal and interest is' + (lx + money))
            break;
        case 4:
            lx = money * 12 * x * 0.0084
            alert('The sum of principal and interest is' + (lx + money))
            break;

        default:
            break;
    }

Simplified version:

    var money = +prompt('Please enter the stored principal')
    var x = +prompt('Please enter the deposit year');//Positive signs can also be implicitly converted to numbers
    var lx = 0;//Variables for storing interest
    switch (x) {
        case 1:
            lx = money * 0.0063 * 12 * x; // lx reassignment
            break;
        case 2:
            lx = money * 0.0066 * 12 * x
            break;
        case 3:
            lx = money * 0.0069 * 12 * x
            break;
        case 5:
            lx = money * 0.0075 * 12 * x
            break;
        case 8:
            lx = money * 0.0084 * 12 * x
            break;
    }
         alert(lx + money)

Break points: the order in which code is executed

F12 -- > sources -- > double click the file of the breakpoint -- > find the breakpoint location (decide by yourself) - > refresh the page -- > Press step over next -- > remember to close the breakpoint after the test

Examples are as follows:

var x = "B";
    switch (x) {
        case 'A':
            document.write('A option')
            break;
        case 'B':
            document.write('B term')
            document.write('BB term')
            document.write('BBB term')
            break;
        case 'C':
            document.write('C option')
            break;
        case 'D':
            document.write('D option')
            break;
        default:
            break;
    }

At the console break point, observe the code running sequence

Click the icon on the right

Observe the code running sequence:

Ternary operator

Also known as ternary operator:

Syntax:

Relational expression? Value 1 / statement body: value 2 / statement body

Relational expression: compare the result of a boolean type

Execution process:

If the relationship expression is true, go? Following statement body

If the relational expression is false, go: the following statement body
```
var x = 11;
    x%2==0?document.write('It's an even number'):document.write('It's an odd number');
```
Operator priority

There are many operation symbols, = =?: =

First judge that x%2==0, then carry out three item operation, and the operation result is even, and finally assign a value
```
    var year = 2008;
    // Enter a numeric year to determine whether it is a leap year
    year%4==0&&year%100!=0||year%400==0?alert('It's a leap year'):alert("It's a normal year");
```
while loop

In Code: execute a piece of code repeatedly

Conditions:

Initial value

End point value (condition judgment)

How much do you increase the initial value each time

Format of the loop:

Initial value

While (condition judgment){

Circulatory body;

How much does the initial value change

}

Execution steps:

1. x = 1

2. x < = 20 -- > true -- > enters the loop body, and re judge after the x variable changes. As long as x meets the conditions, it will always loop -- > false -- > out of the program
```
    var x=+prompt('Please enter a number');
    while (x <= 20) {
        document.write(x + '<br>');
        x++;    
    }
```
Format of while loop:

Initial value;

While (condition judgment){

Circulatory body

Change of initial value

}
```
    var x = 2;
    while (x == 1) {
        alert('xx')
        x--;
    }
```
do while() loop format

Initial value;

do{

Circulatory body

Change of initial value

} while (condition judgment);
```
    var x = 2;
    do {
        alert('xx')
        x--;
    } while (x == 1)
```
Note: do while go first {}, judge before going

Question: does the while loop go first {} or conditional judgment? Conditional judgment

Differences between while and do while:

The do while loop will go through the loop body first and execute judgment

for loop

For (initial value; condition judgment; update of initial value){

Statement body

}

Execution process:

1. Execute initial value first

2. Go through condition judgment -- > true -- > update of the initial value of the loop body {} -- > re judgment -- > true -- > update of the initial value of the loop body {} -- > re judgment -- > false -- > exit the program

// Print numbers 1-10
   for(var i=1;i<=10;i++){
       document.write(i + "<br>")
   }

for loop case:

1. Print even numbers inside 1-10

    for (let x = 1; x <= 10; x++) {
        if (x%2==0){
            document.write(x + '<br>')
        }
        
    }

2. Find the sum of all numbers 1-10

    for (var i = 1; i <= 10; i++) {
        // i = 1 2 3 4 .. 
        sum += i; // Through the loop, add each loop variable + to sum
    }
    console.log(sum);

3. Find the number of multiples of 3 in 1-100

    var count = 0;
    for (var i = 1; i <= 100; i++) { //How many cycles (100)
        // i= 1 2 3 4...100
        // Judge i
        if (i % 3 == 0) { // For 1 2 3 4 100 make judgments one by one. Satisfaction is a multiple of 3
            count++;
        }
    }

    console.log(count);

4. Find the sum of all numbers in 100-200 that can be divided by 5

    var sum = 0;
    for (var i = 100; i <= 200; i++) {
        if (i % 5 == 0) {
            sum += i;
        }
    }

    console.log(sum);

5. Print all numbers between 100 – 200 that can be divided by 5

    //Print all numbers between 100 – 200 that can be divided by 5
    for (var x = 100; x <= 200; x++) {
        if (x % 5 == 0) {
            document.write(x + '<br>')
        }
    }

6. Print all numbers that can be divided by 5 between 100 – 200

    //Print all numbers that can be divided by 5 between 100 – 200
    var count = 0;
    for (var x = 100; x < 200; x++) {
        if (x % 5 == 0) {
            count++;
        }
    }
    document.write(count + '<br>');

7. Print all numbers that can be divided by 5 and can be divided by 3 between 100 – 200

    // Print all numbers that can be divided by 5 and can be divided by 3 between 100 – 200
    var count = 0;
    for (var x = 100; x <= 200; x++) {
        if (x % 3 == 0 && x % 5 == 0) {
            count++;
        }
    }
    document.write(count + "<br>" );

8. Print all numbers and numbers between 100 – 200 that can be divided by 5 and 3

    // Print all numbers and numbers between 100 – 200 that can be divided by 5 and 3
    var sum = 0;
    for (var x = 100; x < 200; x++) {
        if (x % 3 == 0 && x % 5 == 0) {
            sum += x;
        }

    }
    document.write(sum + "<br>");

9. Print all numbers and numbers between 100 – 200 that cannot be divided by 5 and can be divided by 3

    //Print all numbers and numbers between 100 – 200 that cannot be divided by 5 and can be divided by 3
    var sum = 0;
    for (var x = 100; x < 200; x++) {
        if (x % 3 == 0 && x % 5 != 0) {
            sum += x;
        }

    }
    document.write(sum + "<br>");

10. Enter month and display days

    var month = +prompt('Please enter the month you want to query')
    switch (month) {
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
            document.write('31 day')
            break;
        case 4:
        case 6:
        case 9:
        case 11:
            document.write('30 day')
            break;
        case 2:
            document.write('29 Or 28 days')
            break;
        default:
            break;
    }

11. Implement operand and operator calculation

Programming to achieve the following functions: read in two operands (data1 and data2) and an operator (OP), and calculate the value of the expression data1 op data2, where OP can be +, -, */

    var data1 = +prompt('Please enter the first number');
    var data2 = +prompt('Please enter the second number');
    var op = prompt('Please enter an operator...');
    var y = 0;
    switch (op) {
        case '+':
            y = data1 + data2
            break;
        case '-':
            y = data1 - data2
            break;
        case '*':
            y = data1 * data2
            break;
        case '/':
            y = data1 / data2
            break;
        case '%':
            y = data1 % data2
            break;

        default:
            break;
    }
    document.write(y)