1. Array
1.1 significance and creation of array
- Array: a collection of data that is stored in a single variable
- Creation method
//1. Create an array with new
var arr = new Array();
//2. Creating arrays with array literals
var arr = [];//Common usage
var arr1 = [1,2,'La La La',true];//The data in the array must be separated by commas
//3. The data in the array is called elements. The elements in the array can be of any type
1.2 accessing array elements
- The index (subscript) of the array, which is used to access the sequence number of array elements (starting from 0)
var week = ['Monday','Tuesday','Wednesday','Thursday','Friday','Saturday','Sunday']
console.log(week[6]);//Sunday
- extend
week.forEach(function(item,index){
console.log(item);
});
1.3 traversal array
- Access the elements in the array from beginning to end
- Array name [index number]
var color = ['red', 'pink', 'blue', 'green','aqua'];
console.log(color[0]);//Output red
console.log(color[3]);//Output green
- Traversing the array through the for loop index
for (var i = 0; i < color.length; i++) { //Use "array name. Length" to access the number of array elements (array length)
console.log(color[i]);
}
- be careful
- Because array index numbers start at 0, i must start at 0
- When outputting, i counter is used as index number
- Here, the length of the array is the number of array elements. Do not confuse it with the index number of the array
- When the number of elements in our array changes, the * * length attribute changes along with it**
- case
//Sum and average the array
var sum = 0;
var num = [2, 6, 1, 7, 4];
for (var i = 0; i < num.length; i++) {
sum = sum + num[i];//We added the array element num[i], not the counter
}
var average = sum / num.length;
console.log(`The sum of this array is ${sum}`);//Output 20
console.log(`The average value of this array is ${average}`);//Output 4
//Find the maximum value of the array
var num1 = [2, 6, 1, 77, 52, 25, 7,87,5,95];
var max = num1[0];
for(var i = 1; i < num1.length; i++){
if(num1[i] > max){
max = num1[i];
}
}
console.log(`The maximum value of this array is ${max}`);//Output 95
1.4 new elements in array
- Add a new element in the array and modify the length
var arr = ['red', 'green', 'blue', 'pink'];
console.log(arr.length);
arr.length = 7;//Change the length of our array to 7. There should be 7 elements in it
console.log(arr);
console.log(arr[4]);//No assignment. The default is undefined
- Adding new elements to the array and modifying index numbers
arr[4] = 'hotpink';
arr[5] = 'lightgreen';
arr[6] = 'aqua';
console.log(arr[4]);
console.log(arr[5]);
console.log(arr[6]);
- Note: if the index number is occupied, the original element will be replaced; Do not assign a value to the array name directly, otherwise there will be no array elements in it
- case
//Filter out the numbers greater than or equal to 10 in the array to form a new array
//Method 1
var str1 = [2, 0, 6, 1, 77, 0, 52, 0, 25, 7];
var str2 = [];
var j = 0
for(var i = 0; i < str1.length; i++){
if(str1[i] >= 10){
//The new array starts at 0 and increases in order
str2[j] = str1[i];
j++;
}
}
console.log(str2);//Output array [77,52,25]
//Method 2
var str1 = [2,6, 1, 77, 52, 0, 25, 7,34,6];
var str2 = [];
for(var i = 0; i < str1.length; i++){
if(str1[i] >= 10){
str2[str2.length] = str1[i];//At first, str2.length is 0. After saving a value, the length will change automatically
}
}
console.log(str2);//Output array [77,52,25]
1.5 array cases
- Remove 0 from array [2, 0, 6, 1, 77, 0, 52, 0, 25, 7] to form a new array without 0
var str = [2,0,6,1,77,0,52,0,25,7];
var newstr = [];
for(var i = 0; i < str.length; i++){
if(str[i] != 0){
newstr[newstr.length] = str[i];
}
}
console.log(newstr);//Output array [2,6, 1,77,52,25,7]
- Requirement: store the contents of the array ['red', 'green', 'blue', 'pink', 'purple] in reverse
var color = ['red', 'green', 'blue', 'pink', 'purple'];
var newcolor = [];
for(var i = color.length - 1; i >= 0; i--){//for(var i = 0; i < color.length; i++){
newcolor[newcolor.length] = color[i]//newcolor[i] = color[color.length - i - 1];
}//}
console.log(newcolor);
1.6 bubble sorting
- Bubble sorting is an algorithm that arranges and displays a series of data in a certain order (from small to large or from large to small)
var str = [5, 4, 3, 2, 1];
for (var i = 0; i < str.length - 1; i++) { //Number of outer circulating pipe trips
for (var j = 0; j < str.length - i - 1; j++) { //Times of inner circulating pipe
if (str[j] > str[j + 1]) { //Internally exchange the values of two variables, and compare the former with the latter
var temp = str[j];
str[j] = str[j + 1];
str[j + 1] = temp;
}
}
}
console.log(str);//Output array [1,2, 3,4, 5]
2. Function
2.1 significance and use of function
- A function encapsulates a block of code that can be repeatedly called and executed. Through this code block, a large amount of code can be reused
- Declarative function
function Function name() {
Function body code
}
- be careful:
- Function is the keyword that declares the function and must be lowercase
- Since functions are generally defined to implement a function, we usually name the function as a verb, such as getSum
- Calling function name ()
function sayHi(){
console.log('hi~');
}
sayHi();
- be careful:
- Don't forget to add parentheses when calling
- Pithy formula: functions are not called, and they do not execute themselves
- Declaring the function itself will not execute the code, and the function body code will be executed only when the function is called
- Function case
//A function that calculates the sum of 1-100
function getsum(){//Declare a function
var sum = 0;
for(var i = 1; i <= 100; i++){
sum += i;
}
console.log(sum);
}
getsum();//Call function
2.2 real and formal parameters in functions
- Definition: when declaring a function, you can add some parameters in parentheses after the function name. These parameters are called formal parameters; When calling this function, you also need to pass the corresponding parameters, which are called arguments
- Function. Some values cannot be fixed inside the function. We can pass different values when calling the function through parameters
// Function declaration with arguments
/* function Function name (formal parameter 1, formal parameter 2, formal parameter 3, etc.) {formal parameters accept arguments, similar to a variable
Function body
}
Function name (argument 1, argument 2, argument 3, etc.); parameters can be or not, unlimited number, separated by commas*/
- case
//Using function to sum two numbers
function getsum(num1, num2, num3) {
console.log(num1 + num2 + num3);
}
getsum(4, 8,8);//12
getsum(5,23);//28
2.3 supplementary function correlation
-
Transfer process of function parameters
- When called, the argument value is passed to the formal parameter
- Formal parameters are simply understood as variables that do not need to be declared
- Multiple parameters of arguments and formal parameters are separated by commas (,)
-
Mismatch between number of the formal and actual parameters of the function
- If the number of arguments is the same as the number of formal parameters, the correct result will be output
- When the number of arguments is greater than the number of formal parameters, only the number of formal parameters is obtained
- When the number of arguments is less than the number of formal parameters, the multiple formal parameters are defined as undefined and the result is NaN
-
Note that in JavaScript, the default value of the formal parameter is undefined
2.4 return value of function return
- Return statement to return the value of the function to the caller
// Declarative function
/* function Function name (){
return The value to be returned;
}
Function name (); at this time, you can call the function to get the value after return in the function body*/
- be careful:
- When using the return statement, the function stops execution and returns the specified value
- If the function does not return, the returned value is undefined
- case
//1. Using function to find the maximum of any two numbers
function getmax(num1,num2){
return num1 > num2? num1 : num2;
/* if(num1 > num2){
return num1;
}else{
return num2;
} */
}
console.log(getmax(34,65));
console.log(getmax(76,23));
//2. Using function to find the maximum value in any array
function getMax(arr){
var max = arr[0];
for(var i = 1; i < arr.length; i++){
if(arr[i] > max){
max = arr[i];
}
}
return max;
}
var arr1 = [3,5,8,12,54,32,18];
// var max1 = getMax(arr1);
// console.log(max1);
console.log(getMax(arr1));
- Supplement:
- When return is used, the function will stop executing and subsequent statements will not be executed
- Return can only return one value. If there are multiple values, the last one will be returned
3. Some cases in operation
- Write a function, the user enters a number to judge whether it is a prime number, and returns the pop-up value (also known as prime number, which can only be used by 1 and its own integer)
var num = +prompt('Please enter a positive integer');
function getprime(num){
for(var i = 2; i < num; i++){
if(num % i == 0){
return 'no';
}
}
return 'yes';
}
console.log(getprime(num));
- Simple calculator, users can choose the calculation method and input numbers
var choose = prompt('Welcome to the easy Calculator:' + '\n' + '1，Addition calculation;' + '\n' + '2，Subtraction calculation' + '\n' + '3，Multiplication calculation' + '\n' + '4，Division calculation' +'\n' + '5，sign out' + '\n' + 'Please enter your options');
if (choose == 5) {
alert('sign out');
} else {
var num1 = +prompt('Please enter the first number');
var num2 = +prompt('Please enter the second number');
alert(getresult(num1, num2));
}
function getresult(num1, num2) {
if (choose == 1) {
return num1 + num2;
} else if (choose == 2) {
return num1 - num2;
} else if (choose == 3) {
return num1 * num2;
} else if (choose == 4) {
return num1 / num2;
}
}