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<h2 id="sec:chartype"><a id="sec:4.23"><span class="sec-nr">4.23</span> <span class="sec-title">Character
properties</span></a></h2>
<a id="sec:chartype"></a>
<p>SWI-Prolog offers two comprehensive predicates for classifying
characters and character codes. These predicates are defined as built-in
predicates to exploit the C-character classification's handling of
<em>locale</em> (handling of local character sets). These predicates are
fast, logical and deterministic if applicable.
<p>In addition, there is the library <code>library(ctype)</code>
providing compatibility with some other Prolog systems. The predicates
of this library are defined in terms of <a id="idx:codetype2:1173"></a><a class="pred" href="chartype.html#code_type/2">code_type/2</a>.
<dl class="latex">
<dt class="pubdef"><a id="char_type/2"><strong>char_type</strong>(<var>?Char,
?Type</var>)</a></dt>
<dd class="defbody">
Tests or generates alternative <var>Type</var>s or <var>Char</var>s. The
character types are inspired by the standard C <code><ctype.h></code>
primitives.
<dl class="latex">
<dt><strong>alnum</strong></dt>
<dd class="defbody">
<var>Char</var> is a letter (upper- or lowercase) or digit.
</dd>
<dt><strong>alpha</strong></dt>
<dd class="defbody">
<var>Char</var> is a letter (upper- or lowercase).
</dd>
<dt><strong>csym</strong></dt>
<dd class="defbody">
<var>Char</var> is a letter (upper- or lowercase), digit or the
underscore (<code>_</code>). These are valid C and Prolog symbol
characters.
</dd>
<dt><strong>csymf</strong></dt>
<dd class="defbody">
<var>Char</var> is a letter (upper- or lowercase) or the underscore (<code>_</code>).
These are valid first characters for C and Prolog symbols.
</dd>
<dt><strong>ascii</strong></dt>
<dd class="defbody">
<var>Char</var> is a 7-bit ASCII character (0..127).
</dd>
<dt><strong>white</strong></dt>
<dd class="defbody">
<var>Char</var> is a space or tab, i.e. white space inside a line.
</dd>
<dt><strong>cntrl</strong></dt>
<dd class="defbody">
<var>Char</var> is an ASCII control character (0..31).
</dd>
<dt><strong>digit</strong></dt>
<dd class="defbody">
<var>Char</var> is a digit.
</dd>
<dt><strong>digit</strong>(<var>Weight</var>)</dt>
<dd class="defbody">
<var>Char</var> is a digit with value <var>Weight</var>. I.e. <code>char_type(X,
digit(6)</code> yields <var>X</var> = <code>'6'</code>. Useful for
parsing numbers.
</dd>
<dt><strong>xdigit</strong>(<var>Weight</var>)</dt>
<dd class="defbody">
<var>Char</var> is a hexadecimal digit with value <var>Weight</var>.
I.e. <code>char_type(a, xdigit(X)</code> yields <var>X</var> = <code>'10'</code>.
Useful for parsing numbers.
</dd>
<dt><strong>graph</strong></dt>
<dd class="defbody">
<var>Char</var> produces a visible mark on a page when printed. Note
that the space is not included!
</dd>
<dt><strong>lower</strong></dt>
<dd class="defbody">
<var>Char</var> is a lowercase letter.
</dd>
<dt><strong>lower</strong>(<var>Upper</var>)</dt>
<dd class="defbody">
<var>Char</var> is a lowercase version of <var>Upper</var>. Only true if
<var>Char</var> is lowercase and <var>Upper</var> uppercase.
</dd>
<dt><strong>to_lower</strong>(<var>Upper</var>)</dt>
<dd class="defbody">
<var>Char</var> is a lowercase version of <var>Upper</var>. For
non-letters, or letter without case, <var>Char</var> and <var>Lower</var>
are the same. See also <a id="idx:upcaseatom2:1174"></a><a class="pred" href="chartype.html#upcase_atom/2">upcase_atom/2</a>
and <a id="idx:downcaseatom2:1175"></a><a class="pred" href="chartype.html#downcase_atom/2">downcase_atom/2</a>.
</dd>
<dt><strong>upper</strong></dt>
<dd class="defbody">
<var>Char</var> is an uppercase letter.
</dd>
<dt><strong>upper</strong>(<var>Lower</var>)</dt>
<dd class="defbody">
<var>Char</var> is an uppercase version of <var>Lower</var>. Only true
if
<var>Char</var> is uppercase and <var>Lower</var> lowercase.
</dd>
<dt><strong>to_upper</strong>(<var>Lower</var>)</dt>
<dd class="defbody">
<var>Char</var> is an uppercase version of <var>Lower</var>. For
non-letters, or letter without case, <var>Char</var> and <var>Lower</var>
are the same. See also <a id="idx:upcaseatom2:1176"></a><a class="pred" href="chartype.html#upcase_atom/2">upcase_atom/2</a>
and <a id="idx:downcaseatom2:1177"></a><a class="pred" href="chartype.html#downcase_atom/2">downcase_atom/2</a>.
</dd>
<dt><strong>punct</strong></dt>
<dd class="defbody">
<var>Char</var> is a punctuation character. This is a <code>graph</code>
character that is not a letter or digit.
</dd>
<dt><strong>space</strong></dt>
<dd class="defbody">
<var>Char</var> is some form of layout character (tab, vertical tab,
newline, etc.).
</dd>
<dt><strong>end_of_file</strong></dt>
<dd class="defbody">
<var>Char</var> is -1.
</dd>
<dt><strong>end_of_line</strong></dt>
<dd class="defbody">
<var>Char</var> ends a line (ASCII: 10..13).
</dd>
<dt><strong>newline</strong></dt>
<dd class="defbody">
<var>Char</var> is a newline character (10).
</dd>
<dt><strong>period</strong></dt>
<dd class="defbody">
<var>Char</var> counts as the end of a sentence (.,!,?).
</dd>
<dt><strong>quote</strong></dt>
<dd class="defbody">
<var>Char</var> is a quote character (<code>"</code>, <code>'</code>, <code>`</code>).
</dd>
<dt><strong>paren</strong>(<var>Close</var>)</dt>
<dd class="defbody">
<var>Char</var> is an open parenthesis and <var>Close</var> is the
corresponding close parenthesis.
</dd>
<dt><strong>prolog_var_start</strong></dt>
<dd class="defbody">
<var>Char</var> can start a Prolog variable name.
</dd>
<dt><strong>prolog_atom_start</strong></dt>
<dd class="defbody">
<var>Char</var> can start a unquoted Prolog atom that is not a symbol.
</dd>
<dt><strong>prolog_identifier_continue</strong></dt>
<dd class="defbody">
<var>Char</var> can continue a Prolog variable name or atom.
</dd>
<dt><strong>prolog_prolog_symbol</strong></dt>
<dd class="defbody">
<var>Char</var> is a Prolog symbol character. Sequences of Prolog symbol
characters glue together to form an unquoted atom. Examples are <code><code>=..</code></code>, <code><code>\=</code></code>,
etc.
</dd>
</dl>
</dd>
<dt class="pubdef"><a id="code_type/2"><strong>code_type</strong>(<var>?Code,
?Type</var>)</a></dt>
<dd class="defbody">
As <a id="idx:chartype2:1178"></a><a class="pred" href="chartype.html#char_type/2">char_type/2</a>,
but uses character codes rather than one-character atoms. Please note
that both predicates are as flexible as possible. They handle either
representation if the argument is instantiated and will instantiate only
with an integer code or a one-character atom, depending of the version
used. See also the Prolog flag
<a class="flag" href="flags.html#flag:double_quotes">double_quotes</a>, <a id="idx:atomchars2:1179"></a><a class="pred" href="manipatom.html#atom_chars/2">atom_chars/2</a>
and <a id="idx:atomcodes2:1180"></a><a class="pred" href="manipatom.html#atom_codes/2">atom_codes/2</a>.
</dd>
</dl>
<p><h3 id="sec:case"><a id="sec:4.23.1"><span class="sec-nr">4.23.1</span> <span class="sec-title">Case
conversion</span></a></h3>
<a id="sec:case"></a>
<p>There is nothing in the Prolog standard for converting case in
textual data. The SWI-Prolog predicates <a id="idx:codetype2:1181"></a><a class="pred" href="chartype.html#code_type/2">code_type/2</a>
and <a id="idx:chartype2:1182"></a><a class="pred" href="chartype.html#char_type/2">char_type/2</a>
can be used to test and convert individual characters. We have started
some additional support:
<dl class="latex">
<dt class="pubdef"><a id="downcase_atom/2"><strong>downcase_atom</strong>(<var>+AnyCase,
-LowerCase</var>)</a></dt>
<dd class="defbody">
Converts the characters of <var>AnyCase</var> into lowercase as <a id="idx:chartype2:1183"></a><a class="pred" href="chartype.html#char_type/2">char_type/2</a>
does (i.e. based on the defined <em>locale</em> if Prolog provides
locale support on the hosting platform) and unifies the lowercase atom
with <var>LowerCase</var>.</dd>
<dt class="pubdef"><a id="upcase_atom/2"><strong>upcase_atom</strong>(<var>+AnyCase,
-UpperCase</var>)</a></dt>
<dd class="defbody">
Converts, similar to <a id="idx:downcaseatom2:1184"></a><a class="pred" href="chartype.html#downcase_atom/2">downcase_atom/2</a>,
an atom to uppercase.
</dd>
</dl>
<p><h3 id="sec:whitespace"><a id="sec:4.23.2"><span class="sec-nr">4.23.2</span> <span class="sec-title">White
space normalization</span></a></h3>
<a id="sec:whitespace"></a>
<dl class="latex">
<dt class="pubdef"><a id="normalize_space/2"><strong>normalize_space</strong>(<var>-Out,
+In</var>)</a></dt>
<dd class="defbody">
Normalize white space in <var>In</var>. All leading and trailing white
space is removed. All non-empty sequences for Unicode white space
characters are replaced by a single space (<code>\u0020</code>)
character. <var>Out</var> uses the same conventions as <a id="idx:withoutputto2:1185"></a><a class="pred" href="IO.html#with_output_to/2">with_output_to/2</a>
and <a id="idx:format3:1186"></a><a class="pred" href="format.html#format/3">format/3</a>.
</dd>
</dl>
<p><h3 id="sec:collate"><a id="sec:4.23.3"><span class="sec-nr">4.23.3</span> <span class="sec-title">Language-specific
comparison</span></a></h3>
<a id="sec:collate"></a>
<p><a id="idx:locale:1187"></a><a id="idx:collate:1188"></a><a id="idx:comparelanguagespecific:1189"></a>This
section deals with predicates for language-specific string comparison
operations.
<dl class="latex">
<dt class="pubdef"><a id="collation_key/2"><strong>collation_key</strong>(<var>+Atom,
-Key</var>)</a></dt>
<dd class="defbody">
Create a <var>Key</var> from <var>Atom</var> for locale-specific
comparison. The key is defined such that if the key of atom <var>A</var>
precedes the key of atom <var>B</var> in the standard order of terms, <var>A</var>
is alphabetically smaller than <var>B</var> using the sort order of the
current locale.
<p>The predicate <a id="idx:collationkey2:1190"></a><a class="pred" href="chartype.html#collation_key/2">collation_key/2</a>
is used by <a id="idx:localesort2:1191"></a><a class="pred" href="chartype.html#locale_sort/2">locale_sort/2</a>
from library(sort). Please examine the implementation of <a id="idx:localesort2:1192"></a><a class="pred" href="chartype.html#locale_sort/2">locale_sort/2</a>
as an example of using this call.
<p>The <var>Key</var> is an implementation-defined and generally
unreadable string. On systems that do not support locale handling, <var>Key</var>
is simply unified with <var>Atom</var>.</dd>
<dt class="pubdef"><a id="locale_sort/2"><strong>locale_sort</strong>(<var>+List,
-Sorted</var>)</a></dt>
<dd class="defbody">
Sort a list of atoms using the current locale. <var>List</var> is a list
of atoms or string objects (see <a class="sec" href="strings.html">section
5.2</a>). <var>Sorted</var> is unified with a list containing all atoms
of <var>List</var>, sorted to the rules of the current locale. See also <a id="idx:collationkey2:1193"></a><a class="pred" href="chartype.html#collation_key/2">collation_key/2</a>
and
<a id="idx:setlocale3:1194"></a><a class="pred" href="system.html#setlocale/3">setlocale/3</a>.
</dd>
</dl>
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