Editing Field (computer science)

{{Short description|Variable in a record}}
In [[data hierarchy]], a '''field''' ('''data field''') is a [[Variable (computer science)|variable]] in a [[record (computer science)|record]].<ref name="pascal_p42_quoted">{{cite book
  | last1 = Jensen
  | first1 = Kathleen
  | last2 = Wirth
  | first2 = Niklaus
  | title = PASCAL User Manual and Report
  | publisher = Springer-Verlag
  | year = 1974
  | page = 42
  | quote = [A] record is a structure consisting of a fixed number of components, called fields.
  | isbn = 0-387-90144-2
}}</ref> 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 program]]s.<ref name="cpl_p169_quote1">{{cite book
  | last1 = Wilson
  | first1 = Leslie B.
  | last2 = Clark
  | first2 = Robert G.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 169
  | quote = Being able to structure data in this way is central to the construction of understandable programs.
  | isbn = 0-201-71012-9
}}</ref> The individual fields in a record may be accessed by name, just like any variable in a computer program.<ref name="cpl_p169">{{cite book
  | last1 = Wilson
  | first1 = Leslie B.
  | last2 = Clark
  | first2 = Robert G.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 169
  | isbn = 0-201-71012-9
}}</ref>

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]].<ref name="pascal_p42">{{cite book
  | last1 = Jensen
  | first1 = Kathleen
  | last2 = Wirth
  | first2 = Niklaus
  | title = PASCAL User Manual and Report
  | publisher = Springer-Verlag
  | year = 1974
  | page = 42
  | isbn = 0-387-90144-2
}}</ref>

== 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]].
<syntaxhighlight lang="pascal">
type PersonRecord =
    record
        lastName : array [ 1 .. 20 ] of Char;
        firstName : array [ 1 .. 20 ] of Char;
        age : Integer
    end;
</syntaxhighlight>

In Pascal, the identifier component precedes a colon, and the datatype component follows the colon. Once a record is defined, [[Instance variable|variables]] of the record can be [[Instance (computer science)|allocated]]. Once the memory of the record is allocated, a field can be accessed like a variable by using the dot notation.
<syntaxhighlight lang="pascal">
var alice : PersonRecord;
alice.firstName := 'Alice';
</syntaxhighlight>

The term ''field'' has been replaced with the terms ''data member''<ref name="stroustrup-p450">{{cite book
  | last = Stroustrup
  | first = Bjarne
  | title = The C++ Programming Language, Fourth Edition
  | publisher = Addison-Wesley
  | year = 2013
  | page = 450
  | isbn = 978-0-321-56384-2
}}</ref> and ''attribute''.<ref name="cpl_p104">{{cite book
  | last1 = Wilson
  | first1 = Leslie B.
  | last2 = Clark
  | first2 = Robert G.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 104
  | isbn = 0-201-71012-9
}}</ref> The following [[java (programming language)|Java]] [[Class (computer programming)|class]] has three attributes: firstName, lastName, and age.
<syntaxhighlight lang="java">
public class PersonRecord
{
	private String firstName;
	private String lastName;
	private int age;
}
</syntaxhighlight>

== File fields ==
[[File:Data Hierarchy diagram showing Employee database example by JeffTan.gif|thumb|File Data Hierarchy diagram]]
Fields may be stored in a [[random access]] [[computer file|file]].<ref name="cpl_p169_quote2">{{cite book
  | last1 = Wilson
  | first1 = Leslie B.
  | last2 = Clark
  | first2 = Robert G.
  | title = Comparative Programming Languages, Third Edition
  | publisher = Addison-Wesley
  | year = 2001
  | page = 169
  | quote = The original use of records was in languages like COBOL, where they were used to define the structure of records held in a file.
  | isbn = 0-201-71012-9
}}</ref> A file may be written to or read from in an arbitrary order. To accomplish the arbitrary access, the [[operating system]] provides a method to quickly ''seek'' around the file.<ref name="upe_p207_quote">{{cite book
  | last1 = Kernighan
  | first1 = Brian W.
  | last2 = Pike
  | first2 = Rob
  | title = The UNIX Programming Environment
  | publisher = Prentice Hall
  | year = 1984
  | page = 207
  | quote = The system call lseek provides a way to move around in a file without actually reading or writing.
  | isbn = 0-13-937699-2
}}</ref> Once the [[Disk read-and-write head|disk head]] is positioned at the beginning of a record, each file field can be read into its corresponding memory field.

File fields are the main storage structure in the [[ISAM|Indexed Sequential Access Method]] (ISAM). In [[relational database]] [[Database theory|theory]], the term ''field'' has been replaced with the terms ''column'' and ''attribute''.<ref name="did_p5_quote">{{cite book
  | last = Date
  | first = C.J.
  | title = Database in Depth
  | publisher = O'Reilly Media, Inc.
  | year = 2005
  | page = 5
  | quote = An n-ary relation can be pictured as a table with n columns; the columns in the picture correspond to attributes of the relation and the rows correspond to tuples.
  | isbn = 0-596-10012-4
}}</ref>

==See also==
* {{Annotated link|Class variable}}
* {{Annotated link|Mutator method}}

==References==
{{Reflist|2}}

{{DEFAULTSORT:Field (Computer Science)}}
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[[Category:Data modeling]]