Common JavaScript array methods and teach you how to transpose a matrix

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1. Common two-dimensional array operations

creation and traversal

In the previous chapters, you have learned various ways to create one-dimensional arrays. After understanding how to create one-dimensional arrays, Creating a two-dimensional array is very simple, just set the array elements to an array.

After creating a two-dimensional array, how to traverse the elements in the two-dimensional array and operate on them?

• One-dimensional arrays can be traversed using for, for...in or for...of (provided by ES6).
• For a two-dimensional array, you only need to traverse the elements of the array again after traversing the array.

In addition, in web project development, multi-dimensional arrays are often created by adding elements to multi-dimensional empty arrays. The following demonstrates adding a two-dimensional empty array element as an example.

If you want to assign a value to a two-dimensional array element (such as arr[i][0]), you must first ensure that the added element (such as arr[i]) has been created as an array, otherwise the program will report an "Uncaught TypeError..." error.

Note that

when creating a multi-dimensional array, although JavaScript does not limit the number of array dimensions, in actual applications, in order to facilitate code reading, debugging, and maintenance, it is recommended to use three-dimensional or less arrays to save data.

[Case] ​​Transpose of two-dimensional array

The transposition of a two-dimensional array refers to saving the horizontal elements of the two-dimensional array into vertical elements.

Code implementation ideas:

• Find patterns: res[0][0] = arr[0][0], res[0][1] = arr[1][0], res[0][2] = arr[2 ][0].
• Conclusion: res[i][j] = arr[j][i]. ②
• The length of the res array = the length of the arr element (such as arr[0]). ③
• The length of the res element (such as res[0]) = the length of the arr array. ④
• Follow ③ and ④ to complete the creation and traversal of res, and press ② to transpose.

In order to give you a sense of accomplishment, I won’t post the code. If you have any questions, you can ask them in the comment area. In fact, the matrix can be stored in an array. In the future, you can just run the code directly by transposing the matrix.

2. Common array method

stack and queue methods

In JavaScript, in addition to the methods of adding and deleting array elements explained earlier, you can also use the methods provided by the Array object to simulate stack and queue operations.

• Appends a new element of an array at the end or beginning of the array.
• Remove array elements from the end or beginning of the array.

• The return value of the push() and unshift() methods is the length of the new array.
• The pop() and shift() methods return the removed array elements.

The retrieval method

is being developed to detect whether a given value is an array, or to find the position of a specified element in an array.

Except for the Array.isArray() method, all other methods in the table start retrieval from the specified array index by default, and the retrieval method is the same as the operator "===", that is, only when they are congruent will a more successful result be returned. .

includes() and Array.isArray() methods

• The first parameter of the includes() method represents the value to be found.
• The second parameter of the includes() method is used to specify the subscript to be searched in the array.
• When set to greater than the length of the array, the array will not be retrieved and false will be returned directly.
• When set to a number less than 0, the index position retrieved is equal to the length of the array plus the specified negative number. If the result is still a number less than 0, the entire array is retrieved.

The indexOf() method

indexOf() is used to retrieve the first given value from the specified subscript position in the array. If it exists, it returns the corresponding element subscript, otherwise it returns -1.

Note that

the second parameter of the indexOf() method is used to specify the index to start searching:

• when its value is greater than or equal to the length of the array, -1 is returned directly.
• When its value is a negative number, the search subscript position is equal to the length of the array plus the specified negative number. If the result is still a number less than 0, the entire array is retrieved.

lastIndexOf() method

The lastIndexOf() method provided by the Array object is used to retrieve the subscript of the last given value in the array from the specified subscript position. Different from the indexOf() retrieval method, the lastIndexOf() method defaults to reverse retrieval, that is, retrieval from the end of the array to the beginning of the array.

Note that

the second parameter of the lastIndexOf() method is used to specify the search index, and because it uses a reverse search method:

when its value is greater than or equal to the length of the array, the entire array will be searched.

When its value is a negative number, the index position is equal to the length of the array plus the given negative number. If its value is still a negative number, -1 is returned directly.

Array to String

If you need to convert an array into a string during development, you can use the methods provided by JavaScript to achieve this.

The same points between the join() and toString() methods:

• they can convert multi-dimensional arrays into strings and use commas to connect by default.
• When the array element is undefined, null or an empty array, the corresponding element will be converted into an empty string.

The difference between the join() and toString() methods:

• The join() method can specify the symbol for connecting the array elements.

Other methods

In addition to the several common methods explained earlier, JavaScript also provides many other commonly used array methods. For example, merge arrays, shallow copy arrays, reverse the order of array elements, etc.

Note that

• the slice() and concat() methods return a new array after execution and will not affect the original array. The remaining methods will affect the original array after execution.
• When the value of the first parameter of the splice() method is equal to or greater than the length of the array, the operation starts from the end of the array; when the value is a negative number, the subscript position is equal to the length of the array plus the specified negative number, if the value is still a negative number , then the operation starts from the beginning of the array.

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