Arrays. Adding and Removing Array Elements

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Let's look at each type in more detail.

When working with associative arrays It is very easy to both add and remove elements. Since this scripting language is not typed and does not have many strict rules, you can create object elements in several ways: list them immediately, initialize them after creating the object, and also create them as the code progresses.

Now I want to show examples of the implementation of such mechanisms. In the first program, I created a patient object and then listed its elements. As you can see, for patient. param I made a nested object that has its own values.

var patient = new Object(); patient.firstName ="Inna" patient.age =34, patient.param =( height:169 , weight: 55, disease: "no" ) alert(patient.firstName) // will display "Inna" alert(patient.param. disease) // will print no

If there is a need to add one more parameter to the existing ones, for example, last name, then this is done as follows. Add the following lines to the code presented above:

patient.surname = "Lutsenko"

alert (patient.surname)

This is how the collection is easily replenished with new properties. If you want to change any value, you just need to assign it to the selected key:

patient.firstName = "Katya"

For objects, there is also an operation for deleting unnecessary properties using the delete command:

delete patient.surname

They are also sometimes referred to as “arrays with numeric indices.” If we compare them with the previous version, here the keys are presented in the form of numbers, but the values ​​can be absolutely anything. The collection may be entered out of order into such an object.

So, one element can be entered into a cell with index “0”, and the next one - with index “4”. Intermediate memory cells will be empty and output “undefined” if accessed.

As an example, I wrote a small application in which an array of products was created.

var goods = ["Beef","Pasta","Hard cheese","Spices"]; alert(goods); alert(goods); goods = "Tomatoes" alert(goods);

I specifically showed you how to display the entire shopping list and how to select one item. In addition, I touched on the length property, which I will talk about specifically later. In the meantime, I’ll give a little explanation.

goods = "Tomatoes"

This is where I will end my story. Join my subscriber group and invite your friends. And I wish you patience and good luck in your studies!

Bye bye!

Best regards, Roman Chueshov

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Arrays

An array is an ordered collection of values. The values ​​in an array are called elements, and each element is characterized by a numeric position in the array, called an index. Arrays in JavaScript are untyped: array elements can be of any type, and different elements The same array can have different types. Array elements can even be objects or other arrays, allowing you to create complex data structures such as arrays of objects and arrays of arrays.

JavaScript array indexes start at zero and use 32-bit integers - the first element of the array has index 0. JavaScript arrays are dynamic: they can grow and shrink in size as needed; there is no need to declare fixed array sizes when creating them, or to re-allocate memory when their sizes change.

Arrays in JavaScript are a specialized form of objects, and array indices mean little more than just property names, which coincidentally are integers.

The easiest way to create an array is to use a literal, which is a simple comma-separated list of array elements surrounded by square brackets. The values ​​in an array literal do not have to be constants - they can be any expressions, including object literals:

Var empty = ; // Empty array var numbers = ; // Array with five numeric elements var misc = [ 1.1, true, "a", ]; // 3 elements different types+ trailing comma var base = 1024; var table = ; // Array with variables var arrObj = [, ]; // 2 arrays inside containing objects

Array literal syntax allows you to insert an optional trailing comma, i.e. the literal [,] matches an array with two elements, not three.

Another way to create an array is to call the Array() constructor. You can call the constructor in three different ways:

Call the constructor without arguments:

Var arr = new Array();

In this case, it will be created empty array, equivalent to the literal .

Call the constructor with a single numeric argument specifying the length of the array:

Var arr = new Array(10);

In this case, an empty array of the specified length will be created. This form of calling the Array() constructor can be used to pre-allocate memory for an array if the number of its elements is known in advance. Note that this does not store any values ​​in the array.

Explicitly specify the values ​​of the first two or more array elements or one non-numeric element in the constructor call:

Var arr = new Array(5, 4, 3, 2, 1, "test");

In this case, the arguments to the constructor become the values ​​of the elements of the new array. Using array literals is almost always easier than using the Array() constructor.

Array elements are accessed using the operator. To the left of the brackets there must be an array reference. Inside the parentheses there must be an arbitrary expression that returns a non-negative integer value. This syntax is useful for both reading and writing the value of an array element. Therefore, all of the following JavaScript instructions are valid:

// Create an array with one element var arr = ["world"]; // Read element 0 var value = arr; // Write the value to element 1 arr = 3.14; // Write the value to element 2 i = 2; arr[i] = 3; // Write the value to element 3 arr = "hello"; // Read elements 0 and 2, write the value to element 3 arr] = arr;

Let me remind you that arrays are a specialized type of object. Square brackets used to access array elements act exactly the same as square brackets used to access object properties. The JavaScript interpreter converts the numeric indexes in parentheses into strings—index 1 becomes the string "1"—and then uses the strings as property names.

There's nothing special about converting numeric indexes to strings: you can do the same with regular objects:

Var obj = (); // Create a simple object obj = "one"; // Index it with integers

The thing about arrays is that when you use property names that are non-negative integers, arrays automatically determine the value of the length property. For example, above we created an array arr with a single element. It then assigned values ​​to its elements at indexes 1, 2, and 3. As a result of these operations, the value of the array's length property changed to 4.

You should clearly distinguish indexes in an array from object property names. All indices are property names, but only properties with names represented by integers are indices. All arrays are objects, and you can add properties to them with any names. However, if you touch properties that are array indices, arrays respond by updating the value of the length property as necessary.

Please note that negative and non-integer numbers can be used as array indices. In this case, numbers are converted to strings, which are used as property names.

We've already seen that the easiest way to add elements to an array is to assign values ​​to new indices. You can also use the push() method to add one or more elements to the end of an array:

Var arr = ; // Create an empty array arr.push("zero"); // Add a value to the end arr.push("one",2); // Add two more values

You can also add an element to the end of the array by assigning a value to the arr element. The unshift() method can be used to insert an element at the beginning of an array, which shifts existing elements in the array to higher index positions.

You can delete array elements using the delete operator, just like regular object properties:

Var arr = ; delete arr; 2 in arr; // false, index 2 in the array is not defined arr.length; // 3: the delete operator does not change the length property of the array

Removing an element is similar (but slightly different) to assigning the value undefined to that element. Note that applying the delete operator to an array element does not change the value of the length property or shift down elements with higher indexes to fill the void left by deleting the element.

It is also possible to remove elements at the end of an array by simply assigning a new value to the length property. Arrays have a pop() method (the opposite of push()), which reduces the length of the array by 1 and returns the value of the removed element. There is also a shift() method (the opposite of unshift()), which removes the element at the beginning of the array. Unlike the delete operator, the shift() method shifts all elements down to a position below their current index.

Finally, there is a multi-purpose splice() method that allows you to insert, remove, and replace elements of arrays. It changes the value of the length property and shifts array elements to lower or higher indexes as needed. We will look at all these methods a little later.

JavaScript doesn't support "true" multidimensional arrays, but it does provide a good way to simulate them using arrays of arrays. To access a data element in an array of arrays, simply use the operator twice.

For example, suppose the variable matrix is ​​an array of arrays of numbers. Each element of matrix[x] is an array of numbers. To access a certain number in an array you can use the expression matrix[x][y]. Below is specific example, Where two-dimensional array used as a multiplication table:

// Create a multidimensional array var table = new Array(10); // There are 10 rows in the table for(var i = 0; i

The ECMAScript 3 standard defines Array.prototype as a set convenient functions for working with arrays, which are available as methods of any array. These methods will be presented in the following subsections.

The Array.join() method converts all array elements into strings, joins them, and returns the resulting string. IN optional argument You can pass a string to the method, which will be used to separate the elements in the result string. If a delimiter string is not specified, a comma is used. For example, the following fragment results in the string "1,2,3":

Var arr = ; arr.join(); // "1,2,3" arr.join("-"); // "1-2-3"

The Array.reverse() method reverses the order of elements in an array and returns a reordered array. The permutation is performed directly in the original array, i.e. this method does not create new array with reordered elements, but reorders them in an already existing array. For example, the following snippet, using the reverse() and join() methods, results in the string "3,2,1":

Var arr = ; arr.reverse().join(); // "3,2,1"

The Array.sort() method sorts the elements in the source array and returns the sorted array. If the sort() method is called without arguments, the sorting is done in alphabetical order (elements are temporarily converted to strings for comparison if necessary). Undefined elements are moved to the end of the array.

To sort in order other than alphabetical, you can pass a comparison function as an argument to the sort() method. This function sets which of its two arguments should come first in the sorted list. If the first argument must come before the second, the comparison function must return a negative number. If the first argument must follow the second in the sorted array, then the function must return a number Above zero. And if two values ​​are equivalent (that is, their order does not matter), the comparison function should return 0:

Var arr = ; arr.sort(); // alphabet order: 1111, 222, 33, 4 arr.sort(function(a,b) ( // Numerical order: 4, 33, 222, 1111 return a-b; // Returns 0 // depending on the sort order of a and b ) ); // Sort in the opposite direction, from largest to smallest arr.sort(function(a,b) (return b-a));

Notice how convenient it is to use an unnamed function in this snippet. The comparison function is only used here, so there is no need to give it a name.

The Array.concat() method creates and returns a new array containing the elements of the original array on which concat() was called and the values ​​of any arguments passed to concat(). If any of these arguments is itself an array, its elements are added to the returned array. It should be noted, however, that there is no recursive transformation of an array of arrays into a one-dimensional array. The concat() method does not change source array. Below are some examples:

Var arr = ; arr.concat(4, 5); // Return arr.concat(); // Return arr.concat(,) // Return arr.concat(4, ]) // Return ]

The Array.slice() method returns a slice, or subarray, of the specified array. The two method arguments specify the start and end of the returned fragment. The returned array contains the element whose number is specified in the first argument, plus all subsequent elements, up to (but not including) the element whose number is specified in the second argument.

If only one argument is given, the returned array contains all elements from the starting position to the end of the array. If any of the arguments is negative, it determines the element number relative to the end of the array. So, argument -1 corresponds to the last element of the array, and argument -3 corresponds to the third element of the array from the end. Here are some examples:

Var arr = ; arr.slice(0,3); // Return arr.slice(3); // Return arr.slice(1,-1); // Return arr.slice(-3,-2); // Return

The Array.splice() method is universal method, which inserts or removes elements from an array. Unlike the slice() and concat() methods, the splice() method modifies the original array on which it was called. Note that the splice() and slice() methods have very similar names, but perform completely different operations.

The splice() method can remove elements from an array, insert new elements, or do both at the same time. Array elements are shifted as necessary to create a continuous sequence after insertion or deletion.

The first argument of the splice() method specifies the position in the array from which insertion and/or deletion will be performed. The second argument specifies the number of elements that should be removed (cut) from the array. If the second argument is omitted, all array elements from the specified to the end of the array are removed. The splice() method returns an array of the removed elements or (if no elements were removed) an empty array.

The first two arguments to the splice() method specify the array elements to be removed. These arguments can be followed by any number of additional arguments specifying the elements to be inserted into the array, starting at the position specified in the first argument.

Var arr = ; arr.splice(4); // Return , arr = arr.splice(1,2); // Return , arr = arr.splice(1,1); // Return ; arr = arr = ; arr.splice(2,0,"a","b"); // Return ; arr =

The push() and pop() methods allow you to work with arrays as if they were stacks. The push() method adds one or more new elements to the end of the array and returns its new length. The pop() method performs the reverse operation - it removes the last element of the array, reduces the length of the array, and returns the value it removed. Note that both of these methods modify the original array rather than creating a modified copy of it.

The unshift() and shift() methods behave almost the same as push() and pop(), except that they insert and remove elements at the beginning of the array rather than at the end. The unshift() method shifts existing elements to larger indices to free up space, adds the element or elements to the beginning of the array, and returns the new length of the array. The shift() method removes and returns the first element of the array, shifting all subsequent elements down one position to take up the space vacated at the beginning of the array.

Developers deal with arrays every day. Being an ordered collection, important property The array to create the request is Array.prototype.length .

In JavaScript, length does not always indicate the number of elements that exist (for sparse arrays). So let's understand this property.

The length of the array is represented as an unsigned 32-bit integer whose value is greater than the maximum index. This property behaves differently with certain array types.

Let's list them: Dense array - its elements have contiguous indices starting at 0. For example: – a dense array because the indices are contiguous: 0, 1 and 2.

A sparse array—its elements do not have adjacent indices, starting at 0. For example: – a sparse array, since the indices of the elements are not adjacent: 0, 2 and 3.

Typically JavaScript array length is used to determine the number of elements. This is correct for a dense collection type:

var fruits = ["orange", "apple", "banana"]; //fruits – dense array fruits.length // outputs 3, the real number of elements fruits.push("mango"); fruits.length // prints 4 because another element was added var empty = ; empty.length // outputs 0, empty array

View example

A dense array has no spaces, and the number of elements corresponds to the maximum index plus one. The maximum index is 3 for the element “8”, and the array size is 3 + 1 = 4.

In a sparse array, JavaScript's arr length is greater than the maximum index, but it does not indicate the actual number of elements. When executing a query, length will be greater than the number of elements. This occurs due to spaces in the array:

var animals = ["cat", "dog", "monkey"]; // animals is a sparse array animals.length // outputs 4, but the actual number of elements is 3 var words = ["hello"]; words = "welcome"; //maximum index 6. words – sparse array words.length //outputs 7, based on maximum index

When adding or removing elements, length only changes the maximum index. Any changes to the array that do not affect the index with the maximum value do not change length . For example, when using delete :

var colors = ["blue", "red", "yellow", "white", "black"]; colors.length // outputs 5 delete colors; // remove the first element "blue". // Array becomes sparse colors.length // still outputs 5 because max index 4 // not changed

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In the previous examples, the length property was read-only. But JavaScript length allows you to change this property as well.

Depending on the new value and the existing maximum index, changing the length affects the array in a specific way. It can remove elements or make the array sparse.

When the new length value is less than or equal to the maximum index, any elements whose index is greater than or equal to the new size are removed. A useful script to remove elements from the end of an array:

var numbers = ; numbers.length = 3; // changing the length of the numbers array // displays, elements 7 and 8 are removed

Using a value greater than the maximum index will cause the array to become sparse:

var osTypes = ["OS X", "Linux", "Windows"]; osTypes.length = 5; // Create a sparse array. Elements with indexes 3 and 4 // missing osTypes // outputs ["OS X", "Linux", "Windows", ]

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Length can be assigned a non-numeric data type. JavaScript converts the primitive to a number. If the result of the conversion is NaN or a number less than 0, then the error Uncaught RangeError: Invalid array length is thrown:

var numbers = ; numbers.length = "2"; // "2" is converted to number 2 numbers.length = "not-number"; // throws an error Uncaught RangeError: Invalid array length numbers.length = -2; // throws Uncaught RangeError: Invalid array length

Changing the JavaScript array length property, deleting elements with delete , adding elements with a new index is a source of problems: this is how sparse arrays are created. And as a result we get a contradictory length value.

JavaScript offers safer alternatives.

To add elements to the end of the array, use Array.prototype.push() , and to remove the last element, use pop() . To insert an element at the beginning, unshift() is used. To remove the first element - shift() .

To perform more complex insertions, deletions, or substitutions, use the fairly powerful splice() method:

var companies = ["Apple", "Dell"]; companies.push("ASUS"); // Inserts an element at the end of the companies array // prints ["Apple", "Dell", "ASUS"] companies.pop(); // Displays "ASUS". Removes the last element of the companies array // prints ["Apple", "Dell"] companies.shift(); // Prints "Apple". Removes the first element of the companies array // Prints ["Dell"] companies.splice(1, 0, "Microsoft", "HP"); // Adds 2 companies // Outputs ["Dell", "Microsoft", "HP"] companies.length // Outputs 3. The array is dense

View example

Sometimes the array can be sparse. Therefore it is not safe to rely on JavaScript property length to determine the number of elements. Better use auxiliary function which handles missing elements:

/** * Number of elements in the sparse array * @param (Array) collection * @return (number) */ function count(collection) ( var totalCount = 0; for (var index = 0; index< collection.length; index++) { if (index in collection) { totalCount++; } } return totalCount; }

The in operator determines whether a property is contained in a specified object. It's great for checking the existence of an element at a specific index.

JavaScript length is a property with complex behavior. It works without any surprises, but it's best to take precautions when working with sparse arrays and changing its value.