/usr/lib/swi-prolog/doc/Manual/chars.html

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<h2 id="sec:chars"><a id="sec:4.2"><span class="sec-nr">4.2</span> <span class="sec-title">Character 
representation</span></a></h2>

<a id="sec:chars"></a>

<p>In traditional (Edinburgh) Prolog, characters are represented using
<em>character codes</em>. Character codes are integer indices into a 
specific character set. Traditionally the character set was 7-bit 
US-ASCII. 8-bit character sets have been allowed for a long time, 
providing support for national character sets, of which iso-latin-1 (ISO 
8859-1) is applicable to many Western languages.

<p>ISO Prolog introduces three types, two of which are used for 
characters and one for accessing binary streams (see <a id="idx:open4:307"></a><a class="pred" href="IO.html#open/4">open/4</a>). 
These types are:

<p>
<ul class="latex">
<li><i>code</i><br>
A <em>character code</em> is an integer representing a single character. 
As files may use multi-byte encoding for supporting different character 
sets (<a id="idx:utf8:308">utf-8</a> encoding for example), reading a 
code from a text file is in general not the same as reading a byte.
<li><i>char</i><br>
Alternatively, characters may be represented as
<em>one-character atoms</em>. This is a natural representation, hiding 
encoding problems from the programmer as well as providing much easier 
debugging.
<li><i>byte</i><br>
Bytes are used for accessing binary streams.
</ul>

<p>In SWI-Prolog, character codes are <em>always</em> the Unicode 
equivalent of the encoding. That is, if <a id="idx:getcode1:309"></a><a class="pred" href="chario.html#get_code/1">get_code/1</a> 
reads from a stream encoded as
<code>KOI8-R</code> (used for the Cyrillic alphabet), it returns the 
corresponding Unicode code points. Similarly, assembling or 
disassembling atoms using <a id="idx:atomcodes2:310"></a><a class="pred" href="manipatom.html#atom_codes/2">atom_codes/2</a> 
interprets the codes as Unicode points. See
<a class="sec" href="widechars.html">section 2.18.1</a> for details.

<p>To ease the pain of the two character representations (code and 
char), SWI-Prolog's built-in predicates dealing with character data work 
as flexible as possible: they accept data in any of these formats as 
long as the interpretation is unambiguous. In addition, for output 
arguments that are instantiated, the character is extracted before 
unification. This implies that the following two calls are identical, 
both testing whether the next input character is an <code>a</code>.

<pre class="code">
peek_code(Stream, a).
peek_code(Stream, 97).
</pre>

<p>The two character representations are handled by a large number of 
built-in predicates, all of which are ISO-compatible. For converting 
between code and character there is <a id="idx:charcode2:311"></a><a class="pred" href="manipatom.html#char_code/2">char_code/2</a>. 
For breaking atoms and numbers into characters there are <a id="idx:atomchars2:312"></a><a class="pred" href="manipatom.html#atom_chars/2">atom_chars/2</a>, <a id="idx:atomcodes2:313"></a><a class="pred" href="manipatom.html#atom_codes/2">atom_codes/2</a>,
<a id="idx:numberchars2:314"></a><a class="pred" href="manipatom.html#number_chars/2">number_chars/2</a> 
and <a id="idx:numbercodes2:315"></a><a class="pred" href="manipatom.html#number_codes/2">number_codes/2</a>. 
For character I/O on streams there are
<a id="idx:getchar12:316"></a><span class="pred-ext">get_char/[1,2]</span>, <a id="idx:getcode12:317"></a><span class="pred-ext">get_code/[1,2]</span>, <a id="idx:getbyte12:318"></a><span class="pred-ext">get_byte/[1,2]</span>, <a id="idx:peekchar12:319"></a><span class="pred-ext">peek_char/[1,2]</span>,
<a id="idx:peekcode12:320"></a><span class="pred-ext">peek_code/[1,2]</span>, <a id="idx:peekbyte12:321"></a><span class="pred-ext">peek_byte/[1,2]</span>, <a id="idx:putcode12:322"></a><span class="pred-ext">put_code/[1,2]</span>, <a id="idx:putchar12:323"></a><span class="pred-ext">put_char/[1,2]</span> 
and
<a id="idx:putbyte12:324"></a><span class="pred-ext">put_byte/[1,2]</span>. 
The Prolog flag <a class="flag" href="flags.html#flag:double_quotes">double_quotes</a> 
controls how text between double quotes is interpreted.

<p></body></html>
swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / chars.html
