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<title>Data (computer science)</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Data (computer science)</span></span>

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</style><div role="note" class="hatnote navigation-not-searchable">For other uses, see <a href="Data_(disambiguation)" class="mw-disambig" title="Data (disambiguation)">Data (disambiguation)</a> and <a href="Datum_(disambiguation)" class="mw-disambig" title="Datum (disambiguation)">Datum (disambiguation)</a>.</div>

<div role="note" class="hatnote navigation-not-searchable">For broader coverage of this topic, see <a href="Data" title="Data">Data</a>.</div>

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<p>In <a href="Computer_science" title="Computer science">computer science</a>, <b>data</b> (treated as singular, plural, or as a <a href="Mass_noun" title="Mass noun">mass noun</a>) is any sequence of one or more <a href="Symbol" title="Symbol">symbols</a>; <b>datum</b> is a single symbol of data. Data requires <a href="Interpretation_(logic)" title="Interpretation (logic)">interpretation</a> to become <a href="Information" title="Information">information</a>. <a href="Digital_data" title="Digital data">Digital data</a> is data that is represented using the <a href="Binary_number" title="Binary number">binary number</a> system of ones (1) and zeros (0), instead of <a href="Analog_signature_analysis" title="Analog signature analysis">analog</a> representation. In modern (post-1960) computer systems, all data is digital.

</p><p>Data exists in three states: <a href="Data_at_rest" title="Data at rest">data at rest</a>, <a href="Data_in_transit" title="Data in transit">data in transit</a> and <a href="Data_in_use" title="Data in use">data in use</a>. Data within a computer, in most cases, <a href="Parallel_communication" title="Parallel communication">moves as parallel data</a>. Data moving to or from a computer, in most cases, <a href="Serial_communication" title="Serial communication">moves as serial data</a>. Data sourced from an analog device, such as a temperature sensor, may be converted to digital using an <a href="Analog-to-digital_converter" title="Analog-to-digital converter">analog-to-digital converter</a>. Data representing <a href="Quantity" title="Quantity">quantities</a>, characters, or symbols on which operations are performed by a <a href="Computer" title="Computer">computer</a> are <a href="Data_storage" title="Data storage">stored</a> and <a href="Record_(computer_science)" title="Record (computer science)">recorded</a> on <a href="Magnetic_tape_data_storage" class="mw-redirect" title="Magnetic tape data storage">magnetic</a>, <a href="Optical_storage" title="Optical storage">optical</a>, electronic, or mechanical recording media, and <a href="Data_communication" title="Data communication">transmitted</a> in the form of digital electrical or optical signals.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> Data pass in and out of computers via <a href="Peripheral" title="Peripheral">peripheral devices</a>.

</p><p>Physical <a href="Computer_memory" title="Computer memory">computer memory</a> elements consist of an address and a byte/word of data storage. Digital data are often stored in <a href="Relational_database#RDBMS" title="Relational database">relational databases</a>, like <a href="Table_(database)" title="Table (database)">tables</a> or SQL databases, and can generally be represented as abstract key/value pairs. Data can be organized in many different types of <a href="Data_structure" title="Data structure">data structures</a>, including arrays, <a href="Graph_(abstract_data_type)" title="Graph (abstract data type)">graphs</a>, and <a href="Object_(computer_science)" title="Object (computer science)">objects</a>. Data structures can store data of many different <a href="Data_type" title="Data type">types</a>, including <a href="Floating-point_arithmetic" title="Floating-point arithmetic">numbers</a>, <a href="String_(computer_science)" title="String (computer science)">strings</a> and even other <a href="Recursive_data_type" title="Recursive data type">data structures</a>.

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<div class="mw-heading mw-heading2"><h2 id="Characteristics">Characteristics</h2></div>

<p><a href="Metadata" title="Metadata">Metadata</a> helps translate data to information. Metadata is data about the data. Metadata may be implied, specified or given.

</p><p>Data relating to physical events or processes will have a temporal component. This temporal component may be implied. This is the case when a device such as a temperature logger receives data from a temperature <a href="Sensor" title="Sensor">sensor</a>. When the temperature is received it is assumed that the data has a temporal reference of <i>now</i>. So the device records the date, time and temperature together. When the data logger communicates temperatures, it must also report the date and time as metadata for each temperature reading.

</p><p>Fundamentally, computers follow a sequence of instructions they are given in the form of data. A set of instructions to perform a given task (or tasks) is called a <i><a href="Computer_program" title="Computer program">program</a></i>. A program is data in the form of coded instructions to control the operation of a computer or other machine.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> In the nominal case, the program, as <a href="Execution_(computing)" title="Execution (computing)">executed</a> by the computer, will consist of <a href="Machine_code" title="Machine code">machine code</a>. The elements of <a href="Computer_data_storage" title="Computer data storage">storage</a> manipulated by the program, but not actually executed by the <a href="Central_processing_unit" title="Central processing unit">central processing unit</a> (CPU), are also data. At its most essential, a single datum is a <a href="Value_(computer_science)" title="Value (computer science)">value</a> stored at a specific location. Therefore, it is possible for computer programs to operate on other computer programs, by manipulating their programmatic data.

</p><p>To store data <a href="Byte" title="Byte">bytes</a> in a file, they have to be <a href="Serialization" title="Serialization">serialized</a> in a <a href="File_format" title="File format">file format</a>. Typically, programs are stored in special file types, different from those used for other data. <a href="Executable" title="Executable">Executable files</a> contain programs; all other files are also <a href="Data_file" title="Data file">data files</a>. However, executable files may also contain data used by the program which is built into the program. In particular, some executable files have a <a href="Data_segment" title="Data segment">data segment</a>, which nominally contains constants and initial values for variables, both of which can be considered data.

</p><p>The line between program and data can become blurry. An <a href="Interpreter_(computing)" title="Interpreter (computing)">interpreter</a>, for example, is a program. The input data to an interpreter is itself a program, just not one expressed in native <a href="Machine_code" title="Machine code">machine language</a>. In many cases, the interpreted program will be a human-readable <a href="Text_file" title="Text file">text file</a>, which is manipulated with a <a href="Text_editor" title="Text editor">text editor</a> program. <a href="Metaprogramming" title="Metaprogramming">Metaprogramming</a> similarly involves programs manipulating other programs as data. Programs like <a href="Compiler" title="Compiler">compilers</a>, <a href="Linker_(computing)" title="Linker (computing)">linkers</a>, <a href="Debugger" title="Debugger">debuggers</a>, program updaters, <a href="Antivirus_software" title="Antivirus software">virus scanners</a> and such use other programs as their data.

</p><p>For example, a <a href="User_(computing)" title="User (computing)">user</a> might first instruct the <a href="Operating_system" title="Operating system">operating system</a> to load a <a href="Word_processor" title="Word processor">word processor</a> program from one file, and then use the running program to open and edit a <a href="Document_file_format" title="Document file format">document</a> stored in another file. In this example, the document would be considered data. If the word processor also features a <a href="Spell_checker" title="Spell checker">spell checker</a>, then the dictionary (word list) for the spell checker would also be considered data. The <a href="Algorithm" title="Algorithm">algorithms</a> used by the spell checker to suggest corrections would be either <a href="Machine_code" title="Machine code">machine code</a> data or text in some interpretable <a href="Programming_language" title="Programming language">programming language</a>.

</p><p>In an alternate usage, <a href="Binary_file" title="Binary file">binary files</a> (which are not <a href="Human-readable_medium" class="mw-redirect" title="Human-readable medium">human-readable</a>) are sometimes called <i>data</i> as distinguished from human-readable <i><a href="Text_file" title="Text file">text</a></i>.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>

</p><p>The total amount of digital data in 2007 was estimated to be 281 billion <a href="Gigabyte" title="Gigabyte">gigabytes</a> (281 <a href="Byte#Multiple-byte_units" title="Byte">exabytes</a>).<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Data_keys_and_values,_structures_and_persistence">Data keys and values, structures and persistence</h2></div>

<p>Keys in data provide the context for values. Regardless of the structure of data, there is always a key component present. Keys in data and data-structures are essential for giving meaning to data values. Without a key that is directly or indirectly associated with a value, or collection of values in a structure, the values become meaningless and cease to be data. That is to say, there has to be a key component linked to a value component in order for it to be considered data.

</p><p>Data can be represented in computers in multiple ways, as per the following examples:

</p>

<ul><li><a href="Random_access_memory" class="mw-redirect" title="Random access memory">Random access memory</a> (RAM) holds data that the CPU has direct access to. A CPU may only manipulate data within its <a href="Processor_register" title="Processor register">processor registers</a> or memory. This is as opposed to data storage, where the CPU must direct the transfer of data between the storage device (disk, tape...) and memory. RAM is an array of linear contiguous locations that a processor may read or write by providing an address for the read or write operation. The processor may operate on any location in memory at any time in any order. In RAM the smallest element of data is the binary <a href="Bit" title="Bit">bit</a>. The capabilities and limitations of accessing RAM are processor specific. In general <a href="Computer_data_storage" title="Computer data storage">main memory</a> is arranged as an array of <a href="Memory_address" title="Memory address">locations</a> beginning at address 0 (<a href="Hexadecimal" title="Hexadecimal">hexadecimal</a> 0). Each location can store usually 8 or 32 bits depending on the <a href="Computer_architecture" title="Computer architecture">computer architecture</a>.</li></ul>

<ul><li>Data keys need not be a direct hardware address in memory. <a href="Indirection" title="Indirection">Indirect</a>, abstract and logical keys codes can be stored in association with values to form a <a href="Data_structure" title="Data structure">data structure</a>. Data structures have predetermined <a href="Offset_(computer_science)" title="Offset (computer science)">offsets</a> (or links or paths) from the start of the structure, in which data values are stored. Therefore, the data key consists of the key to the structure plus the offset (or links or paths) into the structure. When such a structure is repeated, storing variations of the data values and the data keys within the same repeating structure, the result can be considered to resemble a <a href="Table_(information)" title="Table (information)">table</a>, in which each element of the repeating structure is considered to be a column and each repetition of the structure is considered as a row of the table. In such an organization of data, the data key is usually a value in one (or a composite of the values in several) of the columns.</li></ul>

<div class="mw-heading mw-heading3"><h3 id="Organised_recurring_data_structures">Organised recurring data structures</h3></div>

<ul><li>The <a href="Table_(information)" title="Table (information)">tabular</a> view of repeating data structures is only one of many possibilities. Repeating data structures can be organised <a href="Hierarchy" title="Hierarchy">hierarchically</a>, such that nodes are linked to each other in a cascade of parent-child relationships. Values and potentially more complex data-structures are linked to the nodes. Thus the nodal hierarchy provides the key for addressing the data structures associated with the nodes. This representation can be thought of as an <a href="Tree_(data_structure)" class="mw-redirect" title="Tree (data structure)">inverted tree</a>. Modern computer operating system <a href="File_system" title="File system">file systems</a> are a common example; and <a href="XML" title="XML">XML</a> is another.</li></ul>

<div class="mw-heading mw-heading3"><h3 id="Sorted_or_ordered_data">Sorted or ordered data</h3></div>

<ul><li>Data has some inherent features when it is <a href="Collation" title="Collation">sorted on a key</a>. All the values for subsets of the key appear together. When passing sequentially through groups of the data with the same key, or a subset of the key changes, this is referred to in data processing circles as a break, or a <a href="Control_break" title="Control break">control break</a>. It particularly facilitates the aggregation of data values on subsets of a key.</li></ul>

<div class="mw-heading mw-heading3"><h3 id="Peripheral_storage">Peripheral storage</h3></div>

<ul><li>Until the advent of bulk <a href="Non-volatile_memory" title="Non-volatile memory">non-volatile memory</a> like <a href="Flash_memory" title="Flash memory">flash</a>, persistent data storage was traditionally achieved by writing the data to <a href="Peripheral" title="Peripheral">external block devices like magnetic tape and disk drives</a>. These devices typically seek to a location on the magnetic media and then read or write <a href="Block_(data_storage)" title="Block (data storage)">blocks of data</a> of a predetermined size. In this case, the seek location on the media, is the data key and the blocks are the data values. Early used <i>raw disk</i> data file-systems or disc operating systems reserved <a href="Fragmentation_(computing)" title="Fragmentation (computing)">contiguous</a> blocks on the disc drive for <a href="Data_file" title="Data file">data files</a>. In those systems, the files could be filled up, running out of data space before all the data had been written to them. Thus much unused data space was reserved unproductively to ensure adequate free space for each file. Later file-systems introduced <a href="Partition_type" title="Partition type">partitions</a>. They reserved blocks of disc data space for partitions and used the allocated blocks more economically, by dynamically assigning blocks of a partition to a file as needed. To achieve this, the file system had to keep track of which blocks were used or unused by data files in a catalog or file allocation table. Though this made better use of the disc data space, it resulted in fragmentation of files across the disc, and a concomitant performance overhead due additional seek time to read the data. Modern file systems reorganize fragmented files dynamically to optimize file access times. Further developments in file systems resulted in <a href="Virtualization" title="Virtualization">virtualization</a> of disc drives i.e. where a logical drive can be defined as partitions from a number of physical drives.</li></ul>

<div class="mw-heading mw-heading3"><h3 id="Indexed_data">Indexed data</h3></div>

<ul><li>Retrieving a small subset of data from a much larger set may imply inefficiently searching through the data sequentially. <b><a href="Database_index" title="Database index">Indexes</a></b> are a way to copy out keys and location addresses from data structures in files, tables and data sets, then organize them using <a href="Tree_(data_structure)" class="mw-redirect" title="Tree (data structure)">inverted tree structures</a> to reduce the time taken to retrieve a subset of the original data. In order to do this, the key of the subset of data to be retrieved must be known before retrieval begins. The most popular indexes are the <a href="B-tree" title="B-tree">B-tree</a> and the dynamic <a href="Hash_function" title="Hash function">hash</a> key indexing methods. Indexing is overhead for filing and retrieving data. There are other ways of organizing indexes, e.g. sorting the keys and using a <a href="Binary_search_algorithm" class="mw-redirect" title="Binary search algorithm">binary search algorithm</a>.</li></ul>

<div class="mw-heading mw-heading3"><h3 id="Abstraction_and_indirection">Abstraction and indirection</h3></div>

<ul><li><a href="Object-oriented_programming" title="Object-oriented programming">Object-oriented programming</a> uses two basic concepts for understanding data and software:</li></ul>

<ol><li>The taxonomic rank-structure of <i><a href="Class_(computer_programming)" title="Class (computer programming)">classes</a></i>, which is an example of a hierarchical data structure; and</li>

<li>at run time, the creation of references to in-memory data-structures of objects that have been <a href="Instance_(computer_science)" title="Instance (computer science)">instantiated</a> from a <a href="Library_(computing)#Object_libraries" title="Library (computing)">class library</a>.</li></ol>

<p>It is only after instantiation that an object of a specified class exists. After an object's reference is cleared, the object also ceases to exist. The memory locations where the object's data was stored are <a href="Garbage_collection_(computer_science)" title="Garbage collection (computer science)">garbage</a> and are reclassified as unused memory available for reuse.

</p>

<div class="mw-heading mw-heading3"><h3 id="Database_data">Database data</h3></div>

<ul><li>The advent of <a href="Database" title="Database">databases</a> introduced a further <a href="Abstraction_layer" title="Abstraction layer">layer of abstraction</a> for persistent data storage. Databases use <a href="Metadata" title="Metadata">metadata</a>, and a <a href="SQL" title="SQL">structured query language</a> protocol between <a href="Client%E2%80%93server_model" title="Client–server model">client and server</a> systems, communicating over a <a href="Computer_network" title="Computer network">computer network</a>, using a <a href="Two-phase_commit_protocol" title="Two-phase commit protocol">two phase commit</a> logging system to ensure <a href="Database_transaction" title="Database transaction">transactional</a> completeness, when saving data.</li></ul>

<div class="mw-heading mw-heading3"><h3 id="Parallel_distributed_data_processing">Parallel distributed data processing</h3></div>

<ul><li>Modern scalable and high-performance data persistence technologies, such as <a href="Apache_Hadoop" title="Apache Hadoop">Apache Hadoop</a>, rely on massively parallel distributed data processing across many commodity computers on a high bandwidth network. In such systems, the data is distributed across multiple computers and therefore any particular computer in the system must be represented in the key of the data, either directly, or indirectly. This enables the differentiation between two identical sets of data, each being processed on a different computer at the same time.</li></ul>

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<li><a href="Data_dictionary" title="Data dictionary">Data dictionary</a></li>

<li><a href="Data_modeling" title="Data modeling">Data modeling</a></li>

<li><a href="Data_stream" title="Data stream">Data stream</a></li>

<li><a href="Database_index" title="Database index">Database index</a></li>

<li><a href="State_(computer_science)" title="State (computer science)">State (computer science)</a></li>

<li><a href="Tuple" title="Tuple">Tuple</a></li></ul>

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<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><cite id="CITEREFPaul,_Ryan2008" class="citation news cs1">Paul, Ryan (12 March 2008). <a rel="nofollow" class="external text" href="https://arstechnica.com/news.ars/post/20080312-study-amount-of-digital-info-global-storage-capacity.html">"Study: amount of digital info > global storage capacity"</a>. Ars Technics. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20080313111238/http://arstechnica.com/news.ars/post/20080312-study-amount-of-digital-info-global-storage-capacity.html">Archived</a> from the original on 13 March 2008<span class="reference-accessdate">. Retrieved <span class="nowrap">13 March</span> 2008</span>.</cite></span>

</li>

<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><cite id="CITEREFGantz,_John_F.2008" class="citation web cs1">Gantz, John F.; et al. (2008). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20080311234210/http://www.emc.com/leadership/digital-universe/expanding-digital-universe.htm">"The diverse and exploding digital universe"</a>. International Data Corporation via EMC. Archived from <a rel="nofollow" class="external text" href="http://www.emc.com/leadership/digital-universe/expanding-digital-universe.htm">the original</a> on 11 March 2008<span class="reference-accessdate">. Retrieved <span class="nowrap">12 March</span> 2008</span>.</cite></span>

</li>

</ol></div></div>

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</style><div id="Data8" style="font-size:114%;margin:0 4em"><a href="Data" title="Data">Data</a></div></th></tr><tr><td colspan="2" class="navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Data_acquisition" title="Data acquisition">Acquisition</a></li>

<li><a href="Data_augmentation" title="Data augmentation">Augmentation</a></li>

<li><a href="Data_analysis" title="Data analysis">Analysis</a></li>

<li><a href="Data_anonymization" title="Data anonymization">Anonymization</a></li>

<li><a href="Data_archaeology" title="Data archaeology">Archaeology</a></li>

<li><a href="Data_cleansing" title="Data cleansing">Cleansing</a></li>

<li><a href="Data_collection" title="Data collection">Collection</a></li>

<li><a href="Data_compression" title="Data compression">Compression</a></li>

<li><a href="Data_corruption" title="Data corruption">Corruption</a></li>

<li><a href="Data_curation" title="Data curation">Curation</a></li>

<li><a href="Data_deduplication" title="Data deduplication">Deduplication</a></li>

<li><a href="Data_degradation" title="Data degradation">Degradation</a></li>

<li><a href="Data_de-identification" class="mw-redirect" title="Data de-identification">De-identification</a></li>

<li><a href="Data_ecosystem" title="Data ecosystem">Ecosystem</a></li>

<li><a href="Data_editing" title="Data editing">Editing</a></li>

<li><a href="Data_engineering" title="Data engineering">Engineering</a></li>

<li><a href="Data_erasure" title="Data erasure">Erasure</a></li>

<ul><li><a href="Data_extraction" title="Data extraction">Extract</a></li>

<li><a href="Data_transformation" class="mw-redirect" title="Data transformation">Transform</a></li>

<li><a href="Data_ethics" class="mw-redirect" title="Data ethics">Ethics</a></li>

<li><a href="Data_exhaust" title="Data exhaust">Exhaust</a></li>

<li><a href="Data_exploration" title="Data exploration">Exploration</a></li>

<li><a href="Data_farming" title="Data farming">Farming</a></li>

<li><a href="Data_format_management" title="Data format management">Format management</a></li>

<li><a href="Data_fusion" title="Data fusion">Fusion</a></li>

<li><a href="Data_governance" title="Data governance">Governance</a>

<ul><li><a href="Data_cooperative" title="Data cooperative">Cooperatives</a></li></ul></li>

<li><a href="Data_infrastructure" title="Data infrastructure">Infrastructure</a></li>

<li><a href="Data_integration" title="Data integration">Integration</a></li>

<li><a href="Data_integrity" title="Data integrity">Integrity</a></li>

<li><a href="Data_library" class="mw-redirect" title="Data library">Library</a></li>

<li><a href="Data_lineage" title="Data lineage">Lineage</a></li>

<li><a href="Data_management" title="Data management">Management</a></li>

<li><a href="Data_migration" title="Data migration">Migration</a></li>

<li><a href="Data_mining" title="Data mining">Mining</a></li>

<li><a href="Data_philanthropy" title="Data philanthropy">Philanthropy</a></li>

<li><a href="Data_pre-processing" class="mw-redirect" title="Data pre-processing">Pre-processing</a></li>

<li><a href="Data_preservation" title="Data preservation">Preservation</a></li>

<li><a href="Data_processing" title="Data processing">Processing</a></li>

<li><a href="Information_privacy" title="Information privacy">Protection (privacy)</a></li>

<li><a href="Data_publishing" title="Data publishing">Publishing</a>

<li><a href="Data_recovery" title="Data recovery">Recovery</a></li>

<li><a href="Data_reduction" title="Data reduction">Reduction</a></li>

<li><a href="Data_redundancy" title="Data redundancy">Redundancy</a></li>

<li><a href="Data_re-identification" title="Data re-identification">Re-identification</a></li>

<li><a href="Data_remanence" title="Data remanence">Remanence</a></li>

<li><a href="Data_rescue" title="Data rescue">Rescue</a></li>

<li><a href="Data_retention" title="Data retention">Retention</a></li>

<li><a href="Data_quality" title="Data quality">Quality</a></li>

<li><a href="Data_science" title="Data science">Science</a></li>

<li><a href="Data_scraping" title="Data scraping">Scraping</a></li>

<li><a href="Data_scrubbing" title="Data scrubbing">Scrubbing</a></li>

<li><a href="Data_security" title="Data security">Security</a></li>

<li><a href="Data_sharing" title="Data sharing">Sharing</a></li>

<li><a href="Data_steward" title="Data steward">Stewardship</a></li>

<li><a href="Data_storage" title="Data storage">Storage</a></li>

<li><a href="Data_structure" title="Data structure">Structure</a></li>

<li><a href="Data_synchronization" title="Data synchronization">Synchronization</a></li>

<li><a href="Topological_data_analysis" title="Topological data analysis">Topological data analysis</a></li>

<li><a href="Data_validation" title="Data validation">Validation</a></li>

<li><a href="Data_warehouse" title="Data warehouse">Warehouse</a></li>

<li><a href="Data_wrangling" title="Data wrangling">Wrangling/munging</a></li></ul>

</div></td></tr></tbody></table></div></div><div><div class="zim-footer">

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