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

青灯夜游
Release: 2021-08-17 12:15:59
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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
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Use the length of the array Property to get the array length, the return value will be 0.

console.log(a.length);
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JS array learning: Creation and access of one-dimensional and two-dimensional arrays

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

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

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
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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.

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

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

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

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

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

var a = [1, , 2];
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var a = [1, 2, 3];
delete a[1];
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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]
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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";
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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
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JS array learning: Creation and access of one-dimensional and two-dimensional arrays

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;
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Method 2 to define a two-dimensional array:

var a = [  //定义二维数组
    [1.1, 1.2],
    [2.1, 2.2]
];
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JS array learning: Creation and access of one-dimensional and two-dimensional arrays

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

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]
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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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