<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">
<chapter>
<header>
<copyright>
<year>2003</year><year>2013</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and limitations
under the License.
</legalnotice>
<title>Records</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
<file>records.xml</file>
</header>
<section>
<title>Records vs Tuples</title>
<p>The main advantage of using records instead of tuples is that
fields in a record are accessed by name, whereas fields in a
tuple are accessed by position. To illustrate these differences,
suppose that we want to represent a person with the tuple
<c>{Name, Address, Phone}</c>.</p>
<p>We must remember that the <c>Name</c> field is the first
element of the tuple, the <c>Address</c> field is the second
element, and so on, in order to write functions which manipulate
this data. For example, to extract data from a variable <c>P</c>
which contains such a tuple we might write the following code
and then use pattern matching to extract the relevant fields.</p>
<code type="none">
Name = element(1, P),
Address = element(2, P),
...</code>
<p>Code like this is difficult to read and understand and errors
occur if we get the numbering of the elements in the tuple wrong.
If we change the data representation by re-ordering the fields,
or by adding or removing a field, then all references to
the person tuple, wherever they occur, must be checked and
possibly modified.</p>
<p>Records allow us to refer to the fields by name and not
position. We use a record instead of a tuple to store the data.
If we write a record definition of the type shown below, we can
then refer to the fields of the record by name.</p>
<code type="none">
-record(person, {name, phone, address}).</code>
<p>For example, if <c>P</c> is now a variable whose value is a
<c>person</c> record, we can code as follows in order to access
the name and address fields of the records.</p>
<code type="none">
Name = P#person.name,
Address = P#person.address,
...</code>
<p>Internally, records are represented using tagged tuples:</p>
<code type="none">
{person, Name, Phone, Address}</code>
</section>
<section>
<title>Defining a Record</title>
<p>This definition of a person will be used in many of
the examples which follow. It contains three fields, <c>name</c>,
<c>phone</c> and <c>address</c>. The default values for
<c>name</c> and <c>phone</c> is "" and [], respectively.
The default value for <c>address</c> is the atom
<c>undefined</c>, since no default value is supplied for this
field:</p>
<pre>
-record(person, {name = "", phone = [], address}).</pre>
<p>We have to define the record in the shell in order to be able
use the record syntax in the examples:</p>
<pre>
> <input>rd(person, {name = "", phone = [], address}).</input>
person</pre>
<p>This is due to the fact that record definitions are available
at compile time only, not at runtime. See <c>shell(3)</c> for
details on records in the shell.
</p>
</section>
<section>
<title>Creating a Record</title>
<p>A new <c>person</c> record is created as follows:</p>
<pre>
> <input>#person{phone=[0,8,2,3,4,3,1,2], name="Robert"}.</input>
#person{name = "Robert",phone = [0,8,2,3,4,3,1,2],address = undefined}</pre>
<p>Since the <c>address</c> field was omitted, its default value
is used.</p>
<p>There is a new feature introduced in Erlang 5.1/OTP R8B,
with which you can set a value to all fields in a record,
overriding the defaults in the record specification. The special
field <c>_</c>, means "all fields not explicitly specified".</p>
<pre>
> <input>#person{name = "Jakob", _ = '_'}.</input>
#person{name = "Jakob",phone = '_',address = '_'}</pre>
<p>It is primarily intended to be used in <c>ets:match/2</c> and
<c>mnesia:match_object/3</c>, to set record fields to the atom
<c>'_'</c>. (This is a wildcard in <c>ets:match/2</c>.)</p>
</section>
<section>
<title>Accessing a Record Field</title>
<pre>
> <input>P = #person{name = "Joe", phone = [0,8,2,3,4,3,1,2]}.</input>
#person{name = "Joe",phone = [0,8,2,3,4,3,1,2],address = undefined}
> <input>P#person.name.</input>
"Joe"</pre>
</section>
<section>
<title>Updating a Record</title>
<pre>
> <input>P1 = #person{name="Joe", phone=[1,2,3], address="A street"}.</input>
#person{name = "Joe",phone = [1,2,3],address = "A street"}
> <input>P2 = P1#person{name="Robert"}.</input>
#person{name = "Robert",phone = [1,2,3],address = "A street"}</pre>
</section>
<section>
<title>Type Testing</title>
<p>The following example shows that the guard succeeds if
<c>P</c> is record of type <c>person</c>.</p>
<pre>
foo(P) when is_record(P, person) -> a_person;
foo(_) -> not_a_person.</pre>
</section>
<section>
<title>Pattern Matching</title>
<p>Matching can be used in combination with records as shown in
the following example:</p>
<pre>
> <input>P3 = #person{name="Joe", phone=[0,0,7], address="A street"}.</input>
#person{name = "Joe",phone = [0,0,7],address = "A street"}
> <input>#person{name = Name} = P3, Name.</input>
"Joe"</pre>
<p>The following function takes a list of <c>person</c> records
and searches for the phone number of a person with a particular
name:</p>
<code type="none">
find_phone([#person{name=Name, phone=Phone} | _], Name) ->
{found, Phone};
find_phone([_| T], Name) ->
find_phone(T, Name);
find_phone([], Name) ->
not_found.</code>
<p>The fields referred to in the pattern can be given in any order.</p>
</section>
<section>
<title>Nested Records</title>
<p>The value of a field in a record might be an instance of a
record. Retrieval of nested data can be done stepwise, or in a
single step, as shown in the following example:</p>
<pre>
-record(name, {first = "Robert", last = "Ericsson"}).
-record(person, {name = #name{}, phone}).
demo() ->
P = #person{name= #name{first="Robert",last="Virding"}, phone=123},
First = (P#person.name)#name.first.</pre>
<p>In this example, <c>demo()</c> evaluates to <c>"Robert"</c>.</p>
</section>
<section>
<title>Example</title>
<pre>
%% File: person.hrl
%%-----------------------------------------------------------
%% Data Type: person
%% where:
%% name: A string (default is undefined).
%% age: An integer (default is undefined).
%% phone: A list of integers (default is []).
%% dict: A dictionary containing various information
%% about the person.
%% A {Key, Value} list (default is the empty list).
%%------------------------------------------------------------
-record(person, {name, age, phone = [], dict = []}).</pre>
<pre>
-module(person).
-include("person.hrl").
-compile(export_all). % For test purposes only.
%% This creates an instance of a person.
%% Note: The phone number is not supplied so the
%% default value [] will be used.
make_hacker_without_phone(Name, Age) ->
#person{name = Name, age = Age,
dict = [{computer_knowledge, excellent},
{drinks, coke}]}.
%% This demonstrates matching in arguments
print(#person{name = Name, age = Age,
phone = Phone, dict = Dict}) ->
io:format("Name: ~s, Age: ~w, Phone: ~w ~n"
"Dictionary: ~w.~n", [Name, Age, Phone, Dict]).
%% Demonstrates type testing, selector, updating.
birthday(P) when record(P, person) ->
P#person{age = P#person.age + 1}.
register_two_hackers() ->
Hacker1 = make_hacker_without_phone("Joe", 29),
OldHacker = birthday(Hacker1),
% The central_register_server should have
% an interface function for this.
central_register_server ! {register_person, Hacker1},
central_register_server ! {register_person,
OldHacker#person{name = "Robert",
phone = [0,8,3,2,4,5,3,1]}}.</pre>
</section>
</chapter>
