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9.2. Array Methods

In addition to the [] operator, arrays can be manipulated through various methods provided by the Array class. The following sections introduce these methods. Many of the methods were inspired in part by the Perl programming language; Perl programmers may find them comfortingly familiar. As usual, this is an overview only; complete details can be found in the core reference section of this book.

9.2.3. sort( )

Array.sort( ) sorts the elements of an array in place and returns the sorted array. When sort( ) is called with no arguments, it sorts the array elements in alphabetical order (temporarily converting them to strings to perform the comparison, if necessary):

var a = new Array("banana", "cherry", "apple");
a.sort( );
var s = a.join(", ");  // s == "apple, banana, cherry"  

If an array contains undefined elements, they are sorted to the end of the array.

To sort an array into some order other than alphabetical, you must pass a comparison function as an argument to sort( ). This function decides which of its two arguments should appear first in the sorted array. If the first argument should appear before the second, the comparison function should return a number less than zero. If the first argument should appear after the second in the sorted array, the function should return a number greater than zero. And if the two values are equivalent (i.e., if their order is irrelevant), the comparison function should return 0. So, for example, to sort array elements into numerical rather than alphabetical order, you might do this:

var a = [33, 4, 1111, 222];
a.sort( );               // Alphabetical order:  1111, 222, 33, 4
a.sort(function(a,b) {    // Numerical order: 4, 33, 222, 1111
           return a-b;    // Returns < 0, 0, or > 0, depending on order
       }); 

Note the convenient use of a function literal in this code. Since the comparison function is used only once, there is no need to give it a name.

As another example of sorting array items, you might perform a case-insensitive alphabetical sort on an array of strings by passing a comparison function that converts both of its arguments to lowercase (with the toLowerCase( ) method) before comparing them. You can probably think of other comparison functions that sort numbers into various esoteric orders: reverse numerical order, odd numbers before even numbers, etc. The possibilities become more interesting, of course, when the array elements you are comparing are objects, rather than simple types like numbers or strings.

9.2.5. slice( )

The Array.slice( ) method returns a slice, or subarray, of the specified array. Its two arguments specify the start and end of the slice to be returned. The returned array contains the element specified by the first argument and all subsequent elements up to, but not including, the element specified by the second argument. If only one argument is specified, the returned array contains all elements from the start position to the end of the array. If either argument is negative, it specifies an array element relative to the last element in the array. An argument of -1, for example, specifies the last element in the array, and an argument of -3 specifies the third from last element of the array. Here are some examples:

var a = [1,2,3,4,5];
a.slice(0,3);    // Returns [1,2,3]
a.slice(3);      // Returns [4,5]
a.slice(1,-1);   // Returns [2,3,4]
a.slice(-3,-2);  // Returns [3] 

9.2.6. splice( )

The Array.splice( ) method is a general-purpose method for inserting or removing elements from an array. splice( ) modifies the array in place; it does not return a new array, as slice( ) and concat( ) do. Note that splice( ) and slice( ) have very similar names but perform substantially different operations.

splice( ) can delete elements from an array, insert new elements into an array, or perform both operations at the same time. Array elements that appear after the insertion or deletion are moved as necessary so that they remain contiguous with the rest of the array. The first argument to splice( ) specifies the array position at which the insertion and/or deletion is to begin. The second argument specifies the number of elements that should be deleted from (spliced out of ) the array. If this second argument is omitted, all array elements from the start element to the end of the array are removed. splice( ) returns an array of the deleted elements, or an empty array if no elements were deleted. For example:

var a = [1,2,3,4,5,6,7,8];
a.splice(4);    // Returns [5,6,7,8]; a is [1,2,3,4]
a.splice(1,2);  // Returns [2,3]; a is [1,4]
a.splice(1,1);  // Returns [4]; a is [1] 

The first two arguments to splice( ) specify which array elements are to be deleted. These arguments may be followed by any number of additional arguments that specify elements to be inserted into the array, starting at the position specified by the first argument. For example:

var a = [1,2,3,4,5];
a.splice(2,0,'a','b');  // Returns []; a is [1,2,'a','b',3,4,5]
a.splice(2,2,[1,2],3);  // Returns ['a','b']; a is [1,2,[1,2],3,3,4,5]

Note that, unlike concat( ), splice( ) does not flatten array arguments that it inserts. That is, if it is passed an array to insert, it inserts the array itself, not the elements of that array.



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