JS array learning: Creation and access of one-dimensional and two-dimensional arrays

In the previous article "JS Array Learning: 4 Ways to Clear All Elements (Detailed Code Explanation)", we deleted all array elements - that is, 4 ways to clear the array. Let's continue the study of JavaScript arrays. Let's talk about the creation and access of one-dimensional arrays and two-dimensional arrays, and expand on the simple discussion of multi-dimensional arrays.

First, let’s take a look at how one-dimensional arrays are created and accessed.

Creation and access of one-dimensional array

There are two ways to create a one-dimensional array: "array direct quantity" and " Array() function".

1. Array direct quantity "[]"

1) We can directly use "[]" to assign a value to a variable. "[]" can be empty, then an empty array is defined:

var a = [];  //空数组

Use the length of the array Property to get the array length, the return value will be 0.

console.log(a.length);

At this time we can use the form array name[subscript]=value to assign a value to the empty array

a[0]=1;
a[1]="hello";

The array subscript can not only be a number, but also a text string, then the array is an associative array

a["姓名"]="张三";
a["年龄"]=23;

If the subscript value of the array exceeds the range, such as a negative number, floating point number, Boolean value, object or other value, JavaScript will automatically convert it to a string and define it as an associative array.

2)You can also directly define and initialize the array

Contain multiple values ​​directly in "[]" List, separated by commas.

var a = [1, 2, 3];  //包含具体元素的数组

Use the length attribute of the array to obtain the length of the array. The return value is the specifically defined number of array elements.

2. Use new Array() to construct the array

1) If you call the Array() function directly without passing parameters, you can create an empty array.

var a = new Array();  //空数组

Use the length property of the array to get the length of the array, and the return value will be 0.

2) If you pass multiple values ​​(separated by commas) when calling the Array() function, you can create a real array.

var a = new Array(1,2,3);

3)If only one numeric parameter is passed when calling the Array() function, it means that only the length of the array is defined, that is, the number of elements included; the array element is empty , the default value of each element is undefined. (The element values ​​in the array are not defined. Syntactically, elements without any value between two commas are called gap elements)

var a = new Array(3);

Contains an or An array with multiple gap elements is also called a gap array.

var a = [1, , 2];

var a = [1, 2, 3];
delete a[1];

It can also generate gap elements to form a gap array.

3. Accessing an array

Accessing an array means accessing array elements. We can use the form array name[subscript expression] to access, read and write the array.

Note: A subscript expression is an expression whose value is a non-negative integer.

For example,

delete a[1]

in the above example is to use the delete keyword to delete the element with subscript 1 in the a array (that is, delete the second element).

Also:

var a = [];  //空数组
a[0]=1;
a[2]="hello";

is to assign values ​​to the elements with subscripts 0 and 2 in the a array respectively. If the value of the element with subscript 1 is not assigned, it will be undefined. When we want to output the specified array elements, we can also use this form

console.log(a[0]);  //读取第一个元素，返回值为1
console.log(a[1]);  //读取第二个元素，返回值为undefined
console.log(a[2]);  //读取第三个元素，返回值为hello

Creation and access of two-dimensional array

JavaScript does not directly support two-dimensional arrays, but you can set the value of an array element equal to the array, so that you can simulate the structure of a two-dimensional array.

1. Define a two-dimensional array

Method of defining a two-dimensional array 1:

var a = [];
a[0,0] = 1;
a[0,1] = 2;
a[1,0] = 3;
a[1,1] = 4;

Method 2 to define a two-dimensional array:

var a = [  //定义二维数组
    [1.1, 1.2],
    [2.1, 2.2]
];

定义二维数组的方法3：

var a = new Array(
[1.1, 1.2],
[2.1, 2.2]);  //定义二维数组

按照这种思想，如果二维数组中每个元素的值也为数组，则可以模拟三维数组，以此类推，通过数组嵌套的形式可以定义多维数组。

2、访问二维数组

访问二维数组甚至三维数组、多维数组的方法，和访问一维数组的方法一样，都是使用中括号进行访问：

数组名[下标表达式1] [下标表达式2]

数组名[下标表达式1] [下标表达式2] [下标表达式3]

...

数组名[下标表达式1] [下标表达式2] [下标表达式3]...[下标表达式N]

更高维度的数组以此类推。

好了，就说到这里了，有需要的可以看：javascript高级教程

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