<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html xmlns:fn="http://www.w3.org/2005/02/xpath-functions"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <link rel="stylesheet" href="../otp_doc.css" type="text/css"> <title>Erlang -- Records</title> </head> <body bgcolor="white" text="#000000" link="#0000ff" vlink="#ff00ff" alink="#ff0000"><div id="container"> <script id="js" type="text/javascript" language="JavaScript" src="../js/flipmenu/flipmenu.js"></script><script id="js2" type="text/javascript" src="../js/erlresolvelinks.js"></script><script language="JavaScript" type="text/javascript"> <!-- function getWinHeight() { var myHeight = 0; if( typeof( window.innerHeight ) == 'number' ) { //Non-IE myHeight = window.innerHeight; } else if( document.documentElement && ( document.documentElement.clientWidth || document.documentElement.clientHeight ) ) { //IE 6+ in 'standards compliant mode' myHeight = document.documentElement.clientHeight; } 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href="javascript:openAllFlips()">Expand All</a><br><a href="javascript:closeAllFlips()">Contract All</a><p><small><strong>Chapters</strong></small></p> <ul class="flipMenu" imagepath="../js/flipmenu"> <li id="no" title="Introduction" expanded="false">Introduction<ul> <li><a href="introduction.html"> Top of chapter </a></li> <li title="Purpose"><a href="introduction.html#id73748">Purpose</a></li> <li title="Prerequisites"><a href="introduction.html#id66106">Prerequisites</a></li> <li title="Document Conventions"><a href="introduction.html#id67296">Document Conventions</a></li> <li title="Complete List of BIFs"><a href="introduction.html#id68121">Complete List of BIFs</a></li> <li title="Reserved Words"><a href="introduction.html#id60679">Reserved Words</a></li> <li title="Character Set"><a href="introduction.html#id61758">Character Set</a></li> </ul> </li> <li id="no" title="Data Types" expanded="false">Data Types<ul> <li><a href="data_types.html"> Top of chapter </a></li> <li 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Expressions</a></li> <li title="List Comprehensions"><a href="expressions.html#id79401">List Comprehensions</a></li> <li title="Bit String Comprehensions"><a href="expressions.html#id79541">Bit String Comprehensions</a></li> <li title="Guard Sequences"><a href="expressions.html#id79673">Guard Sequences</a></li> <li title="Operator Precedence"><a href="expressions.html#id80211">Operator Precedence</a></li> </ul> </li> <li id="no" title="The Preprocessor" expanded="false">The Preprocessor<ul> <li><a href="macros.html"> Top of chapter </a></li> <li title="File Inclusion"><a href="macros.html#id80530">File Inclusion</a></li> <li title="Defining and Using Macros"><a href="macros.html#id80659">Defining and Using Macros</a></li> <li title="Predefined Macros"><a href="macros.html#id80782">Predefined Macros</a></li> <li title="Macros Overloading"><a href="macros.html#id80850">Macros Overloading</a></li> <li title="Flow Control in Macros"><a href="macros.html#id80919">Flow Control in Macros</a></li> <li title="Stringifying Macro Arguments"><a href="macros.html#id81056">Stringifying Macro Arguments</a></li> </ul> </li> <li id="loadscrollpos" title="Records" expanded="true">Records<ul> <li><a href="records.html"> Top of chapter </a></li> <li title="Defining Records"><a href="records.html#id81178">Defining Records</a></li> <li title="Creating Records"><a href="records.html#id81209">Creating Records</a></li> <li title="Accessing Record Fields"><a href="records.html#id81264">Accessing Record Fields</a></li> <li title="Updating Records"><a href="records.html#id81303">Updating Records</a></li> <li title="Records in Guards"><a href="records.html#id81336">Records in Guards</a></li> <li title="Records in Patterns"><a href="records.html#id81368">Records in Patterns</a></li> <li title="Nested records"><a href="records.html#id81395">Nested records</a></li> <li title="Internal Representation of Records"><a href="records.html#id81430">Internal Representation of Records</a></li> </ul> </li> <li id="no" title="Errors and Error Handling" expanded="false">Errors and Error Handling<ul> <li><a href="errors.html"> Top of chapter </a></li> <li title="Terminology"><a href="errors.html#id81550">Terminology</a></li> <li title="Exceptions"><a href="errors.html#id81678">Exceptions</a></li> <li title="Handling of Run-Time Errors in Erlang"><a href="errors.html#id81851">Handling of Run-Time Errors in Erlang</a></li> <li title="Exit Reasons"><a href="errors.html#id81913">Exit Reasons</a></li> </ul> </li> <li id="no" title="Processes" expanded="false">Processes<ul> <li><a href="processes.html"> Top of chapter </a></li> <li title="Processes"><a href="processes.html#id82387">Processes</a></li> <li title="Process Creation"><a href="processes.html#id82400">Process Creation</a></li> <li title="Registered Processes"><a href="processes.html#id82449">Registered Processes</a></li> <li title="Process Termination"><a href="processes.html#id82560">Process Termination</a></li> <li title="Message Sending"><a href="processes.html#id82650">Message Sending</a></li> <li title="Links"><a href="processes.html#id82679">Links</a></li> <li title="Error Handling"><a href="processes.html#id82744">Error Handling</a></li> <li title="Monitors"><a href="processes.html#id82854">Monitors</a></li> <li title="Process Dictionary"><a href="processes.html#id82946">Process Dictionary</a></li> </ul> </li> <li id="no" title="Distributed Erlang" expanded="false">Distributed Erlang<ul> <li><a href="distributed.html"> Top of chapter </a></li> <li title="Distributed Erlang System"><a href="distributed.html#id83013">Distributed Erlang System</a></li> <li title="Nodes"><a href="distributed.html#id83044">Nodes</a></li> <li title="Node Connections"><a href="distributed.html#id83114">Node Connections</a></li> <li title="epmd"><a href="distributed.html#id83164">epmd</a></li> <li title="Hidden Nodes"><a href="distributed.html#id83184">Hidden Nodes</a></li> <li title="C Nodes"><a href="distributed.html#id83230">C Nodes</a></li> <li title="Security"><a href="distributed.html#id83254">Security</a></li> <li title="Distribution BIFs"><a href="distributed.html#id83364">Distribution BIFs</a></li> <li title="Distribution Command Line Flags"><a href="distributed.html#id83662">Distribution Command Line Flags</a></li> <li title="Distribution Modules"><a href="distributed.html#id83799">Distribution Modules</a></li> </ul> </li> <li id="no" title="Compilation and Code Loading" expanded="false">Compilation and Code Loading<ul> <li><a href="code_loading.html"> Top of chapter </a></li> <li title="Compilation"><a href="code_loading.html#id84002">Compilation</a></li> <li title="Code Loading"><a href="code_loading.html#id84112">Code Loading</a></li> <li title="Code Replacement"><a href="code_loading.html#id84159">Code Replacement</a></li> <li title="Running a function when a module is loaded"><a href="code_loading.html#id84234">Running a function when a module is loaded</a></li> </ul> </li> <li id="no" title="Ports and Port Drivers" expanded="false">Ports and Port Drivers<ul> <li><a href="ports.html"> Top of chapter </a></li> <li title="Ports"><a href="ports.html#id84374">Ports</a></li> <li title="Port Drivers"><a href="ports.html#id84409">Port Drivers</a></li> <li title="Port BIFs"><a href="ports.html#id84449">Port BIFs</a></li> </ul> </li> </ul> </div></div> <div id="content"> <div class="innertube"> <h1>9 Records</h1> <p>A record is a data structure for storing a fixed number of elements. It has named fields and is similar to a struct in C. Record expressions are translated to tuple expressions during compilation. Therefore, record expressions are not understood by the shell unless special actions are taken. See <span class="code">shell(3)</span> for details.</p> <p>More record examples can be found in <strong>Programming Examples</strong>.</p> <h3><a name="id81178">9.1 Defining Records</a></h3> <p>A record definition consists of the name of the record, followed by the field names of the record. Record and field names must be atoms. Each field can be given an optional default value. If no default value is supplied, <span class="code">undefined</span> will be used.</p> <div class="example"><pre> -record(Name, {Field1 [= Value1], ... FieldN [= ValueN]}).</pre></div> <p>A record definition can be placed anywhere among the attributes and function declarations of a module, but the definition must come before any usage of the record.</p> <p>If a record is used in several modules, it is recommended that the record definition is placed in an include file.</p> <h3><a name="id81209">9.2 Creating Records</a></h3> <p>The following expression creates a new <span class="code">Name</span> record where the value of each field <span class="code">FieldI</span> is the value of evaluating the corresponding expression <span class="code">ExprI</span>:</p> <div class="example"><pre> #Name{Field1=Expr1,...,FieldK=ExprK}</pre></div> <p>The fields may be in any order, not necessarily the same order as in the record definition, and fields can be omitted. Omitted fields will get their respective default value instead.</p> <p>If several fields should be assigned the same value, the following construction can be used:</p> <div class="example"><pre> #Name{Field1=Expr1,...,FieldK=ExprK, _=ExprL}</pre></div> <p>Omitted fields will then get the value of evaluating <span class="code">ExprL</span> instead of their default values. This feature was added in Erlang 5.1/OTP R8 and is primarily intended to be used to create patterns for ETS and Mnesia match functions. Example:</p> <div class="example"><pre> -record(person, {name, phone, address}). ... lookup(Name, Tab) -> ets:match_object(Tab, #person{name=Name, _='_'}).</pre></div> <h3><a name="id81264">9.3 Accessing Record Fields</a></h3> <div class="example"><pre> Expr#Name.Field</pre></div> <p>Returns the value of the specified field. <span class="code">Expr</span> should evaluate to a <span class="code">Name</span> record.</p> <p>The following expression returns the position of the specified field in the tuple representation of the record:</p> <div class="example"><pre> #Name.Field</pre></div> <p>Example:</p> <div class="example"><pre> -record(person, {name, phone, address}). ... lookup(Name, List) -> lists:keysearch(Name, #person.name, List).</pre></div> <h3><a name="id81303">9.4 Updating Records</a></h3> <div class="example"><pre> Expr#Name{Field1=Expr1,...,FieldK=ExprK}</pre></div> <p><span class="code">Expr</span> should evaluate to a <span class="code">Name</span> record. Returns a copy of this record, with the value of each specified field <span class="code">FieldI</span> changed to the value of evaluating the corresponding expression <span class="code">ExprI</span>. All other fields retain their old values.</p> <p></p> <h3><a name="id81336">9.5 Records in Guards</a></h3> <p>Since record expressions are expanded to tuple expressions, creating records and accessing record fields are allowed in guards. However all subexpressions, for example for field initiations, must of course be valid guard expressions as well. Examples:</p> <div class="example"><pre> handle(Msg, State) when Msg==#msg{to=void, no=3} -> ... handle(Msg, State) when State#state.running==true -> ...</pre></div> <p>There is also a type test BIF <span class="code">is_record(Term, RecordTag)</span>. Example:</p> <div class="example"><pre> is_person(P) when is_record(P, person) -> true; is_person(_P) -> false.</pre></div> <h3><a name="id81368">9.6 Records in Patterns</a></h3> <p>A pattern that will match a certain record is created the same way as a record is created:</p> <div class="example"><pre> #Name{Field1=Expr1,...,FieldK=ExprK}</pre></div> <p>In this case, one or more of <span class="code">Expr1</span>...<span class="code">ExprK</span> may be unbound variables.</p> <h3><a name="id81395">9.7 Nested records</a></h3> <p>Beginning with R14 parentheses when accessing or updating nested records can be omitted. Assuming we have the following record definitions:</p> <div class="example"><pre> -record(nrec0, {name = "nested0"}). -record(nrec1, {name = "nested1", nrec0=#nrec0{}}). -record(nrec2, {name = "nested2", nrec1=#nrec1{}}). N2 = #nrec2{}, </pre></div> <p>Before R14 you would have needed to use parentheses as following:</p> <div class="example"><pre> "nested0" = ((N2#nrec2.nrec1)#nrec1.nrec0)#nrec0.name, N0n = ((N2#nrec2.nrec1)#nrec1.nrec0)#nrec0{name = "nested0a"}, </pre></div> <p>Since R14 you can also write:</p> <div class="example"><pre> "nested0" = N2#nrec2.nrec1#nrec1.nrec0#nrec0.name, N0n = N2#nrec2.nrec1#nrec1.nrec0#nrec0{name = "nested0a"},</pre></div> <h3><a name="id81430">9.8 Internal Representation of Records</a></h3> <p>Record expressions are translated to tuple expressions during compilation. A record defined as</p> <div class="example"><pre> -record(Name, {Field1,...,FieldN}).</pre></div> <p>is internally represented by the tuple</p> <div class="example"><pre> {Name,Value1,...,ValueN}</pre></div> <p>where each <span class="code">ValueI</span> is the default value for <span class="code">FieldI</span>.</p> <p>To each module using records, a pseudo function is added during compilation to obtain information about records:</p> <div class="example"><pre> record_info(fields, Record) -> [Field] record_info(size, Record) -> Size</pre></div> <p><span class="code">Size</span> is the size of the tuple representation, that is one more than the number of fields.</p> <p>In addition, <span class="code">#Record.Name</span> returns the index in the tuple representation of <span class="code">Name</span> of the record <span class="code">Record</span>. <span class="code">Name</span> must be an atom.</p> </div> <div class="footer"> <hr> <p>Copyright © 2003-2012 Ericsson AB. 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