When data objects are stored in an array In computer science, an array data structure or simply array is a data structure consisting of a collection of elements , each identified by one or more integer indices, stored so that the address of each element can be computed from its index tuple by a simple mathematical formula. For example, an array of 10 integer variables, with indices 0, individual objects are selected by an index which is usually a non-negative scalar In computing, a scalar is a variable or field that can hold only one value at a time; as opposed to composite variables like array, list, record, etc. In some contexts, a scalar value may be understood to be numeric. A scalar data type is the type of a scalar variable. For example, char, int, float, and double are the most common scalar data types integer The integers are formed by the natural numbers including 0 (0, 1, 2, 3, ...) together with the negatives of the non-zero natural numbers (−1, −2, −3, ...). Viewed as subset of the real numbers, they are numbers that can be written without a fractional or decimal component, and fall within the set {... −2, −1, 0, 1, 2, ...}. For example, 6. Indices are also called subscripts.
There are three ways in which the elements of an array can be indexed:
- 0 (zero-based indexing)
- The first element of the array is indexed by subscript of 0.
- 1 (one-based indexing)
- The first element of the array is indexed by subscript of 1.
- n (n-based indexing)
- The base index of an array can be freely chosen. Usually programming languages allowing n-based indexing also allow negative index values and other scalar In computing, a scalar is a variable or field that can hold only one value at a time; as opposed to composite variables like array, list, record, etc. In some contexts, a scalar value may be understood to be numeric. A scalar data type is the type of a scalar variable. For example, char, int, float, and double are the most common scalar data types data types like enumerations In computer programming, an enumerated type is a data type consisting of a set of named values called elements, members or enumerators of the type. The enumerator names are usually identifiers that behave as constants in the language. A variable that has been declared as having an enumerated type can be assigned any of the enumerators as a value, or characters In computer and machine-based telecommunications terminology, a character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language may be used as an array index.
Arrays can have multiple dimensions, thus it is not uncommon to access an array using multiple indices. For example a two dimensional array A with three rows and four columns might provide access to the element at the 2nd row and 4th column by the expression: A[1,3] (in a row major language) and A[3,1] (in a column major language) in the case of a zero-based indexing system. Thus two indices are used for a two dimensional array, three for a three dimensional array, and n for an n dimensional array.
For details on programming language support of the various features see Comparison of programming languages (array).
Support for fast lookup
Suppose a data store contains N data objects, and it is desired to retrieve one of them based on the value of one of the object's fields. A naive implementation would retrieve and examine each object until a match was found. A successful lookup would retrieve half the objects on average; an unsuccessful lookup all of them for each attempt. Performance is O In mathematics, computer science, and related fields, big O notation describes the limiting behavior of a function when the argument tends towards a particular value or infinity, usually in terms of simpler functions. Big O notation allows its users to simplify functions in order to concentrate on their growth rates: different functions with the(N) or linear time. Since data stores commonly contain millions of objects and since lookup is a common operation, it is often desirable to improve on this performance.
An index is any data structure which improves the performance of lookup. There are many different data structures used for this purpose, and in fact a substantial proportion of the field of Computer Science is devoted to the design and analysis of index data structures. There are complex design trade-offs involving lookup performance, index size, and index update performance. Many index designs exhibit logarithmic (O In mathematics, computer science, and related fields, big O notation describes the limiting behavior of a function when the argument tends towards a particular value or infinity, usually in terms of simpler functions. Big O notation allows its users to simplify functions in order to concentrate on their growth rates: different functions with the(log(N)) lookup performance and in some applications it is possible to achieve flat (O In mathematics, computer science, and related fields, big O notation describes the limiting behavior of a function when the argument tends towards a particular value or infinity, usually in terms of simpler functions. Big O notation allows its users to simplify functions in order to concentrate on their growth rates: different functions with the(1)) performance.
All database A database consists of an organized collection of data for one or more uses, typically in digital form. One way of classifying databases involves the type of their contents, for example: bibliographic, document-text, statistical. Digital databases are managed using database management systems, which store database contents, allowing data creation software includes indexing technology in the interests of improving performance. See Index (database) A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of slower writes and increased storage space. Indexes can be created using one or more columns of a database table, providing the basis for both rapid random look ups and efficient access of ordered records. The disk space.
One specific and very common application is in the domain of information retrieval Information retrieval is the science of searching for documents, for information within documents, and for metadata about documents, as well as that of searching relational databases and the World Wide Web. There is overlap in the usage of the terms data retrieval, document retrieval, information retrieval, and text retrieval, but each also has, where the application of a full-text index Search engine indexing collects, parses, and stores data to facilitate fast and accurate information retrieval. Index design incorporates interdisciplinary concepts from linguistics, cognitive psychology, mathematics, informatics, physics and computer science. An alternate name for the process in the context of search engines designed to find web enables rapid identification of documents based on their textual content.
See also
- Binary search algorithm In computer science, a binary search is an algorithm for locating the position of an element in a sorted list. It inspects the middle element of the sorted list: if equal to the sought value, then the position has been found; otherwise, the upper half or lower half is chosen for further searching based on whether the sought value is greater than Fast lookup for sorted lists sometimes known as the "binary chop" method
- Hash table In computer science, a hash table or hash map is a data structure that uses a hash function to map identifying values, known as keys, to their associated values (e.g., their telephone number). The hash function is used to transform the key into the index (the hash) of an array element (the slot or bucket) where the corresponding value is to be Creating an index for using indexed 'lookup' where keys are not sequential
Categories: Technical communication Categories: Written communication | Communication | Technology | Arrays
Jeff McRitchie
Mon, 14 Dec 2009 14:59:28 GM
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