A brief introduction to JavaScript arrays

Introduction to Arrays
Array literals
Two-dimensional array
Summarize

Introduction to Arrays

Array - Array array is also an object

It is similar to our ordinary object function and is also used to store some values

The difference is that ordinary objects use strings as attribute values, while arrays use numbers as index operation elements.

Index: integer starting from 0

The storage performance of arrays is better than that of ordinary objects. In development, we often use arrays to store some data.

Create an array:

var arr = new Array();
When you use typeof to check an array, it returns object.

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Adding elements to an array

Syntax: array[index] = value

Read elements from an array

Syntax: array[index]

If you read a non-existent index, it will not report an error but return undefined

Get the length of an array

You can use length property to get the length of the array (the number of elements)

Syntax:數組.length

For continuous arrays, use length to get the length of the array (the number of elements)

For non-contiguous arrays, using length will get the maximum index of the array + 1

Try not to create non-contiguous arrays.

Modify length

If the modified length is less than the original length, the extra part will be left blank.

If the modified length is less than the original length, the extra elements will be deleted.

Add an element to the last position of an array

Syntax:數組[數組.length] = 值

arr[arr.length] = 70;
arr[arr.length] = 80;
arr[arr.length] = 90;

Array literals

Creating arrays using array literals

grammar: []

var arr = [] ;

When creating an array using a literal, you can specify the elements in the array when creating it.

var arr = [1,2,3,4,5];

When using a constructor to create an array, you can also add elements at the same time. Pass the elements to be added as parameters of the constructor, and separate the elements with

var arr = new Array(1,2,3,4,5);

Notice:

Use [] to create an array with an element 10

var arr = [10];

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When using the constructor to create an array with one parameter, an empty array with a length of 10 is created;

var arr = new Array(10);
console.log(arr);
console.log("arr.length="+arr.length);

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The array can contain any data type.

var arr = ["Sun Wukong", 1, true, null, undefined];
console.log(arr);

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Can be an object

var arr = [{name:"Sun Wukong"}, {name:"Zhu Bajie"}, {name:"Sha Wujing"}];
console.log(arr[0].name);

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Can be a function

var arr = [
    function () { alert(1) },
    function () { alert(2) }];

Calling functions via arr[0]()

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Two-dimensional array

create:

use []

var arr = [[1,2,3],[4,5,6],[7,8,9]]; 
//3 rows and 3 columns of a two-dimensional array

Using new Array

  var a = new Array(
			new Array(10,20,30),
			new Array(11,22,33),
			new Array(45,56,67)
		)

Element access array name [row subscript] [column subscript]

(1) Transpose of a two-dimensional array:

var a = [
    ['a','b','c'],
    ['d','e','f'],
    ['g','h','i'],
    ['i','k','I']
]
var str = ''
for(var i=0;i<a.length;i++){
    for(var j=0;j<a[i].length;j++){
        str += a[i][j]+'\t';
    }
    str += '\n';
}
console.log("Before transposition:\n",str);
var res = []
for(var i=0;i<a[0].length;i++){
    res[i] = []
    for(var j=0;j<a.length;j++){
        res[i][j] = a[j][i];
    }
}
console.log("After transposition:",res);

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(2) Define a two-dimensional array and output the maximum value of each row of the array

var str = ''
for(var i=0;i<a.length;i++){ //Outer loop: a.length represents the number of rows in a two-dimensional array for(var j=0;j<a[i].length;j++){ //Inner loop: a[i].length represents the number of elements (columns) in row i
        str += a[i][j]+'\t'
    }
    str += '\n'; //Add a newline character at the end of each line}
console.log(str);
for(var i=0;i<a.length;i++){
    var max = a[i][0]
    for(var j=1;j<a[i].length;j++){
        if(max<a[i][j]){
            max = a[i][j];
        }
    }
    console.log("The maximum value of the line "+(i+1)+" is: "+max)
}

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