Перевод не завершен. Пожалуйста, помогите перевести эту статью с английского.

В финальной статье этого раздела, мы познакомимся с массивами — классный способ хранения списка элементов под одним именем. Здесь мы посмотрим почему это полезно, затем узнаем, как создать массив, получить, добавить и удалить элементы хранящиеся в массиве.

Предварительные требования: Базовая компьютерная грамотность, базовое понимание HTML и CSS, понимание о том, что такое JavaScript.
Цель: Понять что такое массивы и как использовать их в JavaScript.

Что такое массив?

Arrays are generally described as "list-like objects"; they are basically single objects that contain multiple values stored in a list. Array objects can be stored in variables and dealt with in much the same way as any other type of value, the difference being that we can access each value inside the list individually, and do super useful and efficient things with the list, like loop through it and do the same thing to every value. Maybe we've got a series of product items and their prices stored in an array, and we want to loop through them all and print them out on an invoice, while totaling all the prices together and printing out the total price at the bottom.

If we didn't have arrays, we'd have to store every item in a separate variable, then call the code that does the printing and adding separately for each item. This would be much longer to write out, less efficient, and more error-prone. If we had 10 items to add to the invoice it would be bad enough, but what about 100 items, or 1000? We'll return to this example later on in the article.

As in previous articles, let's learn about the real basics of arrays by entering some examples into a JavaScript console. We've provided one below (you can also open this console in a separate tab or window, or use the browser developer console if you prefer).

Создание массива

Массиваы создаются из квадратных скобок , которые содержат список элементов, разделённых запятыми.

  1. Допустим, мы бы хотели хранить список покупок в массиве — мы бы сделали что-то вроде этого. Введите следующие строчки в вашу консоль:
    var shopping = ['bread', 'milk', 'cheese', 'hummus', 'noodles'];
    shopping;
  2. В таком случае, каждый элемент в массиве — это строка , но имейте в виду, что вы можете хранить любой элемент в массиве — строку, чило, объект, другую переменную, даже другой массив. Вы также можете перемешивать типы элементов — они не должны все быть числами, строками, и так далее. Попробуйте это:
    var sequence = [1, 1, 2, 3, 5, 8, 13];
    var random = ['tree', 795, [0, 1, 2]];
  3. Попробуйте сами создать несколько массивов, перед тем как двигаться дальше.

Получение и изменение элементов массива

Вы можете после этого получать доступ к отдельным элементам в массиве, используя квадратные скобки, таким же способом каким вы получаете доступ к буквам в строке.

  1. Введите следующее в вашу консоль:
    shopping[0];
    // возвращает "bread"
  2. Вы также можете изменять элемент в массиве, просто дав отдельному элементу массива новое значение. Попробуйте это:
    shopping[0] = 'tahini';
    shopping;
    // shopping теперь возвратит [ "tahini", "milk", "cheese", "hummus", "noodles" ]
    Заметка: Мы говорили это прежде, но просто как напоминание — компьютеры начинают считать с нуля!
  3. Заметьте, что массив внутри массива называется многомерным массивом. You can access an item inside an array that is itself inside another array by chaining two sets of square brackets together. For example, to access one of the items inside the array that is the third item inside the random array (see previous section), we could do something like this:
    random[2][2];
  4. Try further playing with, and making some more modifications to, your array examples before moving on.

Нахождение длины массива

Вы можете найти длину массива (количество элементов в нём) точно таким же способом, как вы находите длину строки (в символах) — используя свойство length. Попробуйте следующее:

sequence.length;
// должно возвратить 7

This has other uses, but it is most commonly used to tell a loop to keep going until it has looped through all the items in an array. So for example:

var sequence = [1, 1, 2, 3, 5, 8, 13];
for (var i = 0; i < sequence.length; i++) {
  console.log(sequence[i]);
}

You'll learn about loops properly in a future article, but briefly, this code is saying:

  1. Start looping at item number 0 in the array.
  2. Stop looping at the item number equal to the length of the array. This will work for an array of any length, but in this case it will stop looping at item number 7 (this is good, as the last item — which we want the loop to cover — is 6.
  3. For each item, print it out to the browser console with console.log().

Некоторые полезные методы массивов

In this section we'll look at some rather useful array-related methods that allow us to split strings into array items and vice versa, and add new items into arrays.

Converting between strings and arrays

Often you'll be presented with some raw data contained in a big long string, and you might want to separate the useful items out into a more useful form and then do things to them, like display them in a data table. To do this, we can use the split() method. In its simplest form, this takes a single parameter, the character you want to separate the string at, and returns the substrings between the separator as items in an array.

Note: Okay, this is technically a string method, not an array method, but we've put it in with arrays as it goes well here.

  1. Let's play with this, to see how it works. First, create a string in your console:
    var myData = 'Manchester,London,Liverpool,Birmingham,Leeds,Carlisle';
  2. Now let's split it at each comma:
    var myArray = myData.split(',');
    myArray;
  3. Finally, try finding the length of your new array, and retrieving some items from it:
    myArray.length;
    myArray[0]; // первый элемент в массиве
    myArray[1]; // второй элемент в массиве
    myArray[myArray.length-1]; // последний элемент в массиве
  4. You can also go the opposite way using the join() method. Try the following:
    var myNewString = myArray.join(',');
    myNewString;
  5. Another way of converting an array to a string is to use the toString() method. toString() is arguably simpler than join() as it doesn't take a parameter, but more limiting. With join() you can specify different separators (try running Step 4 with a different character than a comma).
    var dogNames = ["Rocket","Flash","Bella","Slugger"];
    dogNames.toString(); //Rocket,Flash,Bella,Slugger

Добавление и удаление элементов массива

We've not yet covered adding and removing array items — let us look at this now. We'll use the myArray array we ended up with in the last section. If you've not already followed that section, create the array first in your console:

var myArray = ['Manchester', 'London', 'Liverpool', 'Birmingham', 'Leeds', 'Carlisle'];

Прежде всего, чтобы добавить или удалить элемент с конца массива, мы можем использовать push() и pop() соответственно.

  1. Давайте используем push() сначала — заметьте, что вам нужно включить один или более элемнтов, которые вы хотите добавить в конец своего массива. Попробуйте это:
    myArray.push('Cardiff');
    myArray;
    myArray.push('Bradford', 'Brighton');
    myArray;
    
  2. При завершении вызова метода возвращается новая длина массива. Если бы вы хотели сохранить новую длину массива в переменной, вы бы могли сделать что-то вроде этого:
    var newLength = myArray.push('Bristol');
    myArray;
    newLength;
  3. Удаление последнего элемента массива можно совершить с помощью вызова метода pop(). Попробуйте это:
    myArray.pop();
  4. Когда вызов метода завершается, возвращается удалённый элемент. Вы бы могли также сделать такое:
    var removedItem = myArray.pop();
    myArray;
    removedItem;

unshift() и shift() работают точно таким же способом, за исключением того что они работают в начале массива, не в конце.

  1. Во-первых unshift() — попробуйте следующие команды:
    myArray.unshift('Edinburgh');
    myArray;
  2. Теперь shift(); попробуйте эти!
    var removedItem = myArray.shift();
    myArray;
    removedItem;

Active learning: Printing those products!

Let's return to the example we described earlier — printing out product names and prices on an invoice, then totaling the prices and printing them at the bottom. In the editable example below there are comments containing numbers — each of these marks a place where you have to add something to the code. They are as follows:

  1. Below the // number 1 comment are a number of strings, each one containing a product name and price separated by a colon. We'd like you to turn this into an array and store it in an array called products.
  2. On the same line as the // number 2 comment is the beginning of a for loop. In this line we currently have i <= 0, which is a conditional test that causes the for loop to stop immediately, because it is saying "stop when i is no longer less than or equal to 0", and i starts at 0. We'd like you to replace this with a conditional test that stops the loop when i is no longer less than the products array's length.
  3. Just below the // number 3 comment we want you to write a line of code that splits the current array item (name:price) into two separate items, one containing just the name and one containing just the price. If you are not sure how to do this, consult the Useful string methods article for some help, or even better, look at the Converting between strings and arrays section of this article.
  4. As part of the above line of code, you'll also want to convert the price from a string to a number. If you can't remember how to do this, check out the first strings article.
  5. There is a variable called total that is created and given a value of 0 at the top of the code. Inside the loop (below // number 4) we want you to add a line that adds the current item price to that total in each iteration of the loop, so that at the end of the code the correct total is printed onto the invoice. You might need an assignment operator to do this.
  6. We want you to change the line just below // number 5 so that the itemText variable is made equal to "current item name — $current item price", for example "Shoes — $23.99" in each case, so the correct information for each item is printed on the invoice. This is just simple string concatenation, which should be familiar to you.

Active learning: Top 5 searches

A good use for array methods like push() and pop() is when you are maintaining a record of currently active items in a web app. In an animated scene for example, you might have an array of objects representing the background graphics currently displayed, and you might only want 50 displayed at once, for performance or clutter reasons. As new objects are created and added to the array, older ones can be deleted from the array to maintain the desired number.

In this example we're going to show a much simpler use — here we're giving you a fake search site, with a search box. The idea is that when terms are entered in the search box, the top 5 previous search terms are displayed in the list. When the number of terms goes over 5, the last term starts being deleted each time a new term is added to the top, so the 5 previous terms are always displayed.

Note: In a real search app, you'd probably be able to click the previous search terms to return to previous searches, and it would display actual search results! We are just keeping it simple for now.

To complete the app, we need you to:

  1. Add a line below the // number 1 comment that adds the current value entered into the search input to the start of the array. This can be retrieved using searchInput.value.
  2. Add a line below the // number 2 comment that removes the value currently at the end of the array.

Заключение

After reading through this article, we are sure you will agree that arrays seem pretty darn useful; you'll see them crop up everywhere in JavaScript, often in association with loops in order to do the same thing to every item in an array. We'll be teaching you all the useful basics there are to know about loops in the next module, but for now you should give yourself a clap and take a well-deserved break; you've worked through all the articles in this module!

The only thing left to do is work through this module's assessment, which will test your understanding of the articles that came before it.

Посмотрите также

  • Indexed collections — an advanced level guide to arrays and their cousins, typed arrays.
  • Array — the Array object reference page — for a detailed reference guide to the features discussed in this page, and many more.

 

В этом разделе

 

Метки документа и участники

 Внесли вклад в эту страницу: ArtyomIv
 Обновлялась последний раз: ArtyomIv,