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<h1>2 Data Types</h1>
<h3><a name="id2265309">2.1
Terms</a></h3>
<p>Erlang provides a number of data types which are listed in this
chapter. A piece of data of any data type is called a
<strong>term</strong>.</p>
<h3><a name="id2253001">2.2
Number</a></h3>
<p>There are two types of numeric literals, <strong>integers</strong> and
<strong>floats</strong>. Besides the conventional notation, there are two
Erlang-specific notations:</p>
<ul>
<li>
<span class="code">$</span><strong><span class="code">char</span></strong> <br>
ASCII value of the character <strong><span class="code">char</span></strong>.</li>
<li>
<strong><span class="code">base</span></strong><span class="code">#</span><strong><span class="code">value</span></strong> <br>
Integer with the base <strong><span class="code">base</span></strong>, which must be an
integer in the range 2..36. <br>
In Erlang 5.2/OTP R9B and earlier versions, the allowed range
is 2..16.</li>
</ul>
<p>Examples:</p>
<div class="example"><pre>
1> <span class="bold_code">42.</span>
42
2> <span class="bold_code">$A.</span>
65
3> <span class="bold_code">$\n.</span>
10
4> <span class="bold_code">2#101.</span>
5
5> <span class="bold_code">16#1f.</span>
31
6> <span class="bold_code">2.3.</span>
2.3
7> <span class="bold_code">2.3e3.</span>
2.3e3
8> <span class="bold_code">2.3e-3.</span>
0.0023</pre></div>
<h3><a name="id2259848">2.3
Atom</a></h3>
<p>An atom is a literal, a constant with name. An atom should be
enclosed in single quotes (') if it does not begin with a
lower-case letter or if it contains other characters than
alphanumeric characters, underscore (_), or @.</p>
<p>Examples:</p>
<div class="example"><pre>
hello
phone_number
'Monday'
'phone number'</pre></div>
<h3><a name="id2257844">2.4
Bit Strings and Binaries</a></h3>
<p>A bit string is used to store an area of untyped memory.</p>
<p>Bit Strings are expressed using the
<span class="bold_code"><a href="expressions.html#bit_syntax">bit syntax</a></span>.</p>
<p>Bit Strings which consists of a number of bits which is evenly
divisible by eight are called Binaries</p>
<p>Examples:</p>
<div class="example"><pre>
1> <span class="bold_code"><<10,20>>.</span>
<<10,20>>
2> <span class="bold_code"><<"ABC">>.</span>
<<"ABC">>
1> <span class="bold_code"><<1:1,0:1>>.</span>
<<2:2>></pre></div>
<p>More examples can be found in Programming Examples.</p>
<h3><a name="id2265338">2.5
Reference</a></h3>
<p>A reference is a term which is unique in an Erlang runtime
system, created by calling <span class="code">make_ref/0</span>.</p>
<h3><a name="id2265020">2.6
Fun</a></h3>
<p>A fun is a functional object. Funs make it possible to create
an anonymous function and pass the function itself -- not its
name -- as argument to other functions.</p>
<p>Example:</p>
<div class="example"><pre>
1> <span class="bold_code">Fun1 = fun (X) -> X+1 end.</span>
#Fun<erl_eval.6.39074546>
2> <span class="bold_code">Fun1(2).</span>
3</pre></div>
<p>Read more about funs in <span class="bold_code"><a href="expressions.html#funs">Fun Expressions</a></span>. More examples can be found in Programming
Examples.</p>
<h3><a name="id2260799">2.7
Port Identifier</a></h3>
<p>A port identifier identifies an Erlang port. <span class="code">open_port/2</span>,
which is used to create ports, will return a value of this type.</p>
<p>Read more about ports in <span class="bold_code"><a href="ports.html">Ports and Port Drivers</a></span>.</p>
<h3><a name="id2263912">2.8
Pid</a></h3>
<p>A process identifier, pid, identifies a process.
<span class="code">spawn/1,2,3,4</span>, <span class="code">spawn_link/1,2,3,4</span> and
<span class="code">spawn_opt/4</span>, which are used to create processes, return
values of this type. Example:</p>
<div class="example"><pre>
1> <span class="bold_code">spawn(m, f, []).</span>
<0.51.0></pre></div>
<p>The BIF <span class="code">self()</span> returns the pid of the calling process.
Example:</p>
<div class="example"><pre>
-module(m).
-export([loop/0]).
loop() ->
receive
who_are_you ->
io:format("I am ~p~n", [self()]),
loop()
end.
1> <span class="bold_code">P = spawn(m, loop, []).</span>
<0.58.0>
2> <span class="bold_code">P ! who_are_you.</span>
I am <0.58.0>
who_are_you</pre></div>
<p>Read more about processes in
<span class="bold_code"><a href="processes.html">Processes</a></span>.</p>
<h3><a name="id2264766">2.9
Tuple</a></h3>
<p>Compound data type with a fixed number of terms:</p>
<div class="example"><pre>
{Term1,...,TermN}</pre></div>
<p>Each term <span class="code">Term</span> in the tuple is called an
<strong>element</strong>. The number of elements is said to be
the <strong>size</strong> of the tuple.</p>
<p>There exists a number of BIFs to manipulate tuples.</p>
<p>Examples:</p>
<div class="example"><pre>
1> <span class="bold_code">P = {adam,24,{july,29}}.</span>
{adam,24,{july,29}}
2> <span class="bold_code">element(1,P).</span>
adam
3> <span class="bold_code">element(3,P).</span>
{july,29}
4> <span class="bold_code">P2 = setelement(2,P,25).</span>
{adam,25,{july,29}}
5> <span class="bold_code">tuple_size(P).</span>
3
6> <span class="bold_code">tuple_size({}).</span>
0</pre></div>
<h3><a name="id2259788">2.10
List</a></h3>
<p>Compound data type with a variable number of terms.</p>
<div class="example"><pre>
[Term1,...,TermN]</pre></div>
<p>Each term <span class="code">Term</span> in the list is called an
<strong>element</strong>. The number of elements is said to be
the <strong>length</strong> of the list.</p>
<p>Formally, a list is either the empty list <span class="code">[]</span> or
consists of a <strong>head</strong> (first element) and a <strong>tail</strong>
(remainder of the list) which is also a list. The latter can
be expressed as <span class="code">[H|T]</span>. The notation
<span class="code">[Term1,...,TermN]</span> above is actually shorthand for
the list <span class="code">[Term1|[...|[TermN|[]]]]</span>.</p>
<p>Example: <br>
<span class="code">[]</span> is a list, thus <br>
<span class="code">[c|[]]</span> is a list, thus <br>
<span class="code">[b|[c|[]]]</span> is a list, thus <br>
<span class="code">[a|[b|[c|[]]]]</span> is a list, or in short <span class="code">[a,b,c]</span>.</p>
<p></p>
<p>A list where the tail is a list is sometimes called a <strong>proper list</strong>. It is allowed to have a list where the tail is not a
list, for example <span class="code">[a|b]</span>. However, this type of list is of
little practical use.</p>
<p>Examples:</p>
<div class="example"><pre>
1> <span class="bold_code">L1 = [a,2,{c,4}].</span>
[a,2,{c,4}]
2> <span class="bold_code">[H|T] = L1.</span>
[a,2,{c,4}]
3> <span class="bold_code">H.</span>
a
4> <span class="bold_code">T.</span>
[2,{c,4}]
5> <span class="bold_code">L2 = [d|T].</span>
[d,2,{c,4}]
6> <span class="bold_code">length(L1).</span>
3
7> <span class="bold_code">length([]).</span>
0</pre></div>
<p>A collection of list processing functions can be found in
the STDLIB module <span class="code">lists</span>.</p>
<h3><a name="id2263762">2.11
String</a></h3>
<p>Strings are enclosed in double quotes ("), but is not a
data type in Erlang. Instead a string <span class="code">"hello"</span> is
shorthand for the list <span class="code">[$h,$e,$l,$l,$o]</span>, that is
<span class="code">[104,101,108,108,111]</span>.</p>
<p>Two adjacent string literals are concatenated into one. This is
done at compile-time and does not incur any runtime overhead.
Example:</p>
<div class="example"><pre>
"string" "42"</pre></div>
<p>is equivalent to</p>
<div class="example"><pre>
"string42"</pre></div>
<h3><a name="id2263803">2.12
Record</a></h3>
<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.
However, record is not a true data type. Instead 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>Examples:</p>
<div class="example"><pre>
-module(person).
-export([new/2]).
-record(person, {name, age}).
new(Name, Age) ->
#person{name=Name, age=Age}.
1> <span class="bold_code">person:new(ernie, 44).</span>
{person,ernie,44}</pre></div>
<p>Read more about records in
<span class="bold_code"><a href="records.html">Records</a></span>. More examples can be
found in Programming Examples.</p>
<h3><a name="id2252704">2.13
Boolean</a></h3>
<p>There is no Boolean data type in Erlang. Instead the atoms
<span class="code">true</span> and <span class="code">false</span> are used to denote Boolean values.</p>
<p>Examples:</p>
<div class="example"><pre>
1> <span class="bold_code">2 =< 3</span>.
true
2> <span class="bold_code">true or false</span>.
true</pre></div>
<h3><a name="id2252738">2.14
Escape Sequences</a></h3>
<p>Within strings and quoted atoms, the following escape sequences
are recognized:</p>
<table border="1" cellpadding="2" cellspacing="0">
<tr>
<td align="left" valign="middle"><strong>Sequence</strong></td>
<td align="left" valign="middle"><strong>Description</strong></td>
</tr>
<tr>
<td align="left" valign="middle">\b</td>
<td align="left" valign="middle">backspace</td>
</tr>
<tr>
<td align="left" valign="middle">\d</td>
<td align="left" valign="middle">delete</td>
</tr>
<tr>
<td align="left" valign="middle">\e</td>
<td align="left" valign="middle">escape</td>
</tr>
<tr>
<td align="left" valign="middle">\f</td>
<td align="left" valign="middle">form feed</td>
</tr>
<tr>
<td align="left" valign="middle">\n</td>
<td align="left" valign="middle">newline</td>
</tr>
<tr>
<td align="left" valign="middle">\r</td>
<td align="left" valign="middle">carriage return</td>
</tr>
<tr>
<td align="left" valign="middle">\s</td>
<td align="left" valign="middle">space</td>
</tr>
<tr>
<td align="left" valign="middle">\t</td>
<td align="left" valign="middle">tab</td>
</tr>
<tr>
<td align="left" valign="middle">\v</td>
<td align="left" valign="middle">vertical tab</td>
</tr>
<tr>
<td align="left" valign="middle">\XYZ, \YZ, \Z</td>
<td align="left" valign="middle">character with octal representation XYZ, YZ or Z</td>
</tr>
<tr>
<td align="left" valign="middle">\xXY</td>
<td align="left" valign="middle">character with hexadecimal representation XY</td>
</tr>
<tr>
<td align="left" valign="middle">\x{X...}</td>
<td align="left" valign="middle">character with hexadecimal representation; X... is one or more hexadecimal characters</td>
</tr>
<tr>
<td align="left" valign="middle">\^a...\^z <br>
\^A...\^Z</td>
<td align="left" valign="middle">control A to control Z</td>
</tr>
<tr>
<td align="left" valign="middle">\'</td>
<td align="left" valign="middle">single quote</td>
</tr>
<tr>
<td align="left" valign="middle">\"</td>
<td align="left" valign="middle">double quote</td>
</tr>
<tr>
<td align="left" valign="middle">\\</td>
<td align="left" valign="middle">backslash</td>
</tr>
</table>
<em>Table
2.1:
Recognized Escape Sequences.</em>
<h3><a name="id2271721">2.15
Type Conversions</a></h3>
<p>There are a number of BIFs for type conversions. Examples:</p>
<div class="example"><pre>
1> <span class="bold_code">atom_to_list(hello).</span>
"hello"
2> <span class="bold_code">list_to_atom("hello").</span>
hello
3> <span class="bold_code">binary_to_list(<<"hello">>).</span>
"hello"
4> <span class="bold_code">binary_to_list(<<104,101,108,108,111>>).</span>
"hello"
5> <span class="bold_code">list_to_binary("hello").</span>
<<104,101,108,108,111>>
6> <span class="bold_code">float_to_list(7.0).</span>
"7.00000000000000000000e+00"
7> <span class="bold_code">list_to_float("7.000e+00").</span>
7.0
8> <span class="bold_code">integer_to_list(77).</span>
"77"
9> <span class="bold_code">list_to_integer("77").</span>
77
10> <span class="bold_code">tuple_to_list({a,b,c}).</span>
[a,b,c]
11> <span class="bold_code">list_to_tuple([a,b,c]).</span>
{a,b,c}
12> <span class="bold_code">term_to_binary({a,b,c}).</span>
<<131,104,3,100,0,1,97,100,0,1,98,100,0,1,99>>
13> <span class="bold_code">binary_to_term(<<131,104,3,100,0,1,97,100,0,1,98,100,0,1,99>>).</span>
{a,b,c}</pre></div>
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