JS Basics-Branching structure-loops-arrays

1. Branching structure
1.if structure
Grammar:
if (condition) {
statement block;
}
Attention:
1. If the condition is as Boolean as possible, if it is not Boolean, the following condition value will be treated as false
if (0) {}
if (0.0) {}
if ("") {}
if (undefined) {}
if (null) {}
if (NaN) {}
Ex
if (35) {...} True
The {} After 2.if can be omitted
When omitted, if only the first statement below is controlled
It is recommended not to omit {}
2.if...else ... Structure
Grammar:
if (condition) {
statement block;
}else{
statement block;
}
3.if...else If...else ... Structure
Grammar:
if (condition) {
Statement Block 1;
}else if (condition) {
Statement Block 2;
}else if (condition) {
Statement Block 3
.. s
}else{
statement block N;
}
Practice:
Enter a 1-7 number from the pop-up box, representing Monday to Sunday, respectively,
Input 1: Print out "Monday eat pork"
2: "Tuesday braised pork Ribs"
3: "Wednesday Eat braised Meatballs"
4: "Thursday Eat braised eggplant"
5: "Friday braised fish with brown sauce"
6 and 7: "Weekend weight Loss"
4.switch...case structure
1. Function: equivalent judgment (= = =)
2. Syntax:
switch (value/expression) {
Case value 1:
Statement Block 1;
Break End switch structure, optional
Case Value 2:
Statement Block 2;
Break
....
Default
statement block N;
Break
}
3. Special usage
When performing the same operation:
switch (value/expression) {
Case value 1:
Case Value 2:
Case Value 3:
statement block;
}
2. Cycle
1. Features
1. Cycle conditions: The start and end of a cycle
2. Loop operation: The same or similar statement to be executed
2. Cyclic-while
Grammar:
while (condition) {
Loop body-cyclic operation
Update loop condition
}
Problem: Print output 1-10 number

Exercise 1: Print all numbers between 1-100
3. Cycle Flow Control
1.break
Function: Terminates the operation of the entire cycle
2.continue
Function: Terminates the cycle and continues the next cycle

Problem:
The loop enters information from the pop-up box and prints until exit is entered.
Practice:
Calculates all the even numbers between 1-100 and
4. Cyclic-do...while
Grammar:
do{
Loop body
}while (conditions);
Execution process:
1. Perform the Loop body first
2. Re-judging the cycle conditions
If the condition is true, the loop body will continue to execute
If the condition is false, jump out of the loop operation
Practice: Revision Cycle entry operation
1. Loop through the data in the popup box and print it in the console
2. Enter exit (exit not output)
Exercise 2:
1. Randomly generate a number between 1-100
Math.random (); Generates a random number between 0-1
var r=parseint (Math.random () *100) +1;
2. Let the user enter a number
If the number entered is larger than the random number, the hint guessed big;
If the input number is smaller than the random number, the hint guess is small;
Otherwise, guess right!
Allow the loop to guess the number
3. Enter exit to exit the game.
5.for Cycle
1.ex:
var I=1; Expression 1
while (i<=100) {//Expression 2
Console.log (i);
i++; Expression 3
}
2.for syntax
for (expression 1; expression 2; expression 3) {
Loop body
}
Ex
for (Var i=1;i<=100;i++) {
Console.log (i);
}
Expression 1: Declaration of a cyclic condition
Expression 2: The judgment of the cyclic condition
Expression 3: Updating the loop variable
Execution process:
1. Execute expression 1 First
2. Determine the result of expression 2 (Boolean type)
3. If the condition is true, execute the loop body, or exit
4. After execution completes the loop body, then executes the expression 3
5. Judging the result of expression 2
Practice:
Define a function to print any row in the multiplication table
Pass the parameter, pass a few lines to print.
1*1=1
1*2=2 2*2=4
1*3=3 2*3=6 3*3=9
1*4=4 2*4=8 3*4=12 4*4=16
...
Ex: Pass 4, Print line 4th
Cycle conditions: From 1 onwards to line end
Cyclic operation: Cyclic variable *line= loop variable *line
3.for three special usages of expressions
for (expression 1; expression 2; expression 3) {}
1. Omitting an expression
Three expressions can be omitted arbitrarily, and semicolons are not omitted.
But be sure to complement the expression inside or outside the loop.
2. Expression 1 and expression 3 allow multiple expressions to be written, separated by commas between multiple expressions.
Ex
function sum () {
for (Var i=1,sum=0;i<=100;i++) {
Sum+=i;
}
Console.log (sum);
}
4. Loop nesting
1. Loop nesting
In one loop, another loop appears
for (Var i=1;i<=10;i++) {//Outer loop
for (Var j=1;j<=10;j++) {//Memory loop
Loop body
}
}
The outer loop walks once, the inner layer loops round
Exercise 1: Print in the console like
*
**
***
****
*****
Exercise 2: Print a 99 multiplication table
3. Arrays
1. What is an array
Save multiple data in a variable
Arrays are arranged in Linetype order-line structure
2. Declaring an array
1. Syntax
1.var array name =[];
Ex
var unames=[];
2.var array name =[element 1, Element 2, element 3];
var unames=["TOM", "Lilei", "Lucy";
3.var array name =new array ();
4.var array name =new Array (element 1, Element 2, element 3);
Ex
var names=new Array ("Lin Daiyu", "Jia Baoyu", "Granny Liu");
Practice:
1. Create an array that holds the names of the 3 learners
2. Create an array that holds the age of 3 trainees
3. In the console, print the values in the output array, respectively.
2. Use of arrays
Both the value and assignment operations are used: array name [subscript]
1. Assigning a value to an array element
Array name [subscript]= value;
2. Get the elements in the array
Array name [subscript];
3. Get the length of the array
Array Length: The number of elements in the array
Property: Length
Syntax: array name. length
1.length represents the length of the array, and it can also indicate the subscript of the element that is about to be inserted
Ex
var unames=["Tom", "Lucy", "Lilei"];
Unames[unames.length]= "Monkey King";
Unames[unames.length]=prompt ();

2. Iterate through each element in the array
for (Var i=0;i<=unames.length-1;i++) {
I: indicates subscript for each element
Unames[i]: Take each element
}

JS Basics-Branching structure-loops-arrays