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Field (computer Science)
In data hierarchy, a field (data field) is a Variable (computer science), variable in a record (computer science), record. A record, also known as a data structure, allows logically related data to be identified by a single name. Identifying related data as a single group is central to the construction of understandable computer programs. The individual fields in a record may be accessed by name, just like any variable in a computer program. Each field in a record has two components. One component is the field's data type, datatype Declaration (computer programming), declaration. The other component is the field's identifier. Memory fields Fields may be stored in random access memory (RAM). The following Pascal (programming language), Pascal record definition has three field identifiers: firstName, lastName, and age. The two name fields have a datatype of an Array (data structure), array of Character (computing), character. The age field has a datatype of integer. type PersonRec ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Data Hierarchy
Data hierarchy refers to the systematic organization of data, often in hierarchical form. Data organization involves characters, fields, records, files and so on. This concept is a starting point when trying to see what makes up data and whether data has a structure. For example, how does a person make sense of data such as 'employee', 'name', 'department', 'Marcy Smith', 'Sales Department' and so on, assuming that they are all related? One way to understand them is to see these terms as smaller or larger components in a hierarchy. One might say that Marcy Smith is one of the employees in the Sales Department, or an example of an employee in that Department. The data we want to capture about all our employees, and not just Marcy, is the name, ID number, address etc. Purpose of the data hierarchy "Data hierarchy" is a basic concept in data and database theory and helps to show the relationships between smaller and larger components in a database or data file. It is used to give a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Instance Variable
In class-based, object-oriented programming, an instance variable is a variable defined in a class (i.e., a member variable), for which each instantiated object of the class has a separate copy, or instance. An instance variable has similarities with a class variable, but is non- static. An instance variable is a variable which is declared in a class but outside of constructors, methods, or blocks. Instance variables are created when an object is instantiated, and are accessible to all the constructors, methods, or blocks in the class. Access modifiers can be given to the instance variable. An instance variable is not a class variable, although there are similarities. Both are a type of class attribute (or class property, field, or data member). While an instance variable's value may differ between instances of a class, a class variable can only have one value at any one time, shared between all instances. The same dichotomy between ''instance'' and ''class'' members ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Relational Database
A relational database (RDB) is a database based on the relational model of data, as proposed by E. F. Codd in 1970. A Relational Database Management System (RDBMS) is a type of database management system that stores data in a structured format using rows and columns. Many relational database systems are equipped with the option of using SQL (Structured Query Language) for querying and updating the database. History The concept of relational database was defined by E. F. Codd at IBM in 1970. Codd introduced the term ''relational'' in his research paper "A Relational Model of Data for Large Shared Data Banks". In this paper and later papers, he defined what he meant by ''relation''. One well-known definition of what constitutes a relational database system is composed of Codd's 12 rules. However, no commercial implementations of the relational model conform to all of Codd's rules, so the term has gradually come to describe a broader class of database systems, which at a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
ISAM
Indexed Sequential Access Method (ISAM) is a method for creating, maintaining, and manipulating computer files of data so that records can be retrieved sequentially or randomly by one or more keys. Indexes of key fields are maintained to achieve fast retrieval of required file records in indexed files. IBM originally developed ISAM for mainframe computers, but implementations are available for most computer systems. The term ''ISAM'' is used for several related concepts: *The IBM ISAM product and the algorithm it employs. *A database system where an application developer directly uses an application programming interface to search indexes in order to locate records in data files. In contrast, a relational database uses a query optimizer which automatically selects indexes. *An indexing algorithm that allows both sequential and keyed access to data. Most databases use some variation of the B-tree for this purpose, although the original IBM ISAM and VSAM implementations did not d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Disk Read-and-write Head
A disk read-and-write head is the small part of a disk drive that moves above the disk platter and transforms the platter's magnetic field into electric current (reads the disk) or, vice versa, transforms electric current into magnetic field (writes the disk). The heads have gone through a number of changes over the years. In a hard drive, the heads ''fly'' above the disk surface with clearance of as little as 3 nanometres. The flying height has been decreasing with each new generation of technology to enable higher areal density. The flying height of the head is controlled by the design of an air bearing etched onto the disk-facing surface of the ''slider''. The role of the air bearing is to maintain the flying height constant as the head moves over the surface of the disk. The air bearings are carefully designed to maintain the same height across the entire platter, despite differing speeds depending on the head distance from the center of the platter. If the head hit ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Operating System
An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for efficient use of the system and may also include accounting software for cost allocation of Scheduling (computing), processor time, mass storage, peripherals, and other resources. For hardware functions such as input and output and memory allocation, the operating system acts as an intermediary between programs and the computer hardware, although the application code is usually executed directly by the hardware and frequently makes system calls to an OS function or is interrupted by it. Operating systems are found on many devices that contain a computerfrom cellular phones and video game consoles to web servers and supercomputers. , Android (operating system), Android is the most popular operating system with a 46% market share, followed ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Computer File
A computer file is a System resource, resource for recording Data (computing), data on a Computer data storage, computer storage device, primarily identified by its filename. Just as words can be written on paper, so too can data be written to a computer file. Files can be shared with and transferred between computers and Mobile device, mobile devices via removable media, Computer networks, networks, or the Internet. Different File format, types of computer files are designed for different purposes. A file may be designed to store a written message, a document, a spreadsheet, an Digital image, image, a Digital video, video, a computer program, program, or any wide variety of other kinds of data. Certain files can store multiple data types at once. By using computer programs, a person can open, read, change, save, and close a computer file. Computer files may be reopened, modified, and file copying, copied an arbitrary number of times. Files are typically organized in a file syst ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Random Access
Random access (also called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of addressable elements roughly as easily and efficiently as any other, no matter how many elements may be in the set. In computer science it is typically contrasted to sequential access which requires data to be retrieved in the order it was stored. For example, data might be stored notionally in a single sequence like a row, in two dimensions like rows and columns on a surface, or in multiple dimensions. However, given all the coordinates, a program can access each record about as quickly and easily as any other. In this sense, the choice of datum is arbitrary in the sense that no matter which item is sought, all that is needed to find it is its address, i.e. the coordinates at which it is located, such as its row and column (or its track and record number on a magnetic drum). At first, the term "random access" was used because th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Data Hierarchy Diagram Showing Employee Database Example By JeffTan
Data ( , ) are a collection of discrete or continuous values that convey information, describing the quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted formally. A datum is an individual value in a collection of data. Data are usually organized into structures such as tables that provide additional context and meaning, and may themselves be used as data in larger structures. Data may be used as variables in a computational process. Data may represent abstract ideas or concrete measurements. Data are commonly used in scientific research, economics, and virtually every other form of human organizational activity. Examples of data sets include price indices (such as the consumer price index), unemployment rates, literacy rates, and census data. In this context, data represent the raw facts and figures from which useful information can be extracted. Data are collected using techniques such ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Class (computer Programming)
In object-oriented programming, a class defines the shared aspects of objects created from the class. The capabilities of a class differ between programming languages, but generally the shared aspects consist of state ( variables) and behavior ( methods) that are each either associated with a particular object or with all objects of that class. Object state can differ between each instance of the class whereas the class state is shared by all of them. The object methods include access to the object state (via an implicit or explicit parameter that references the object) whereas class methods do not. If the language supports inheritance, a class can be defined based on another class with all of its state and behavior plus additional state and behavior that further specializes the class. The specialized class is a ''sub-class'', and the class it is based on is its ''superclass''. Attributes Object lifecycle As an instance of a class, an object is constructed from a class via '' ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Java (programming Language)
Java is a High-level programming language, high-level, General-purpose programming language, general-purpose, Memory safety, memory-safe, object-oriented programming, object-oriented programming language. It is intended to let programmers ''write once, run anywhere'' (Write once, run anywhere, WORA), meaning that compiler, compiled Java code can run on all platforms that support Java without the need to recompile. Java applications are typically compiled to Java bytecode, bytecode that can run on any Java virtual machine (JVM) regardless of the underlying computer architecture. The syntax (programming languages), syntax of Java is similar to C (programming language), C and C++, but has fewer low-level programming language, low-level facilities than either of them. The Java runtime provides dynamic capabilities (such as Reflective programming, reflection and runtime code modification) that are typically not available in traditional compiled languages. Java gained popularity sh ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Instance (computer Science)
In computer science, an instance is an occurrence of a software element that is based on a type definition. When created, an occurrence is said to have been ''instantiated'', and both the creation process and the result of creation are called ''instantiation''. Examples ; Class instance: An object-oriented programming (OOP) object created from a class. Each instance of a class shares a data layout but has its own memory allocation. ; Computer instance: An occurrence of a virtual machine which typically includes storage, a virtual CPU. ; Polygonal model: In computer graphics, it can be instantiated in order to be drawn several times in different locations in a scene which can improve the performance of rendering since a portion of the work needed to display each instance is reused. ; Program instance: In a POSIX-oriented operating system, it refers to an executing process A process is a series or set of activities that interact to produce a result; it may occur once-only o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
