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<title>Operators in Conditionals (The GNU Troff Manual)</title>
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<div class="subsection-level-extent" id="Operators-in-Conditionals">
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Next: <a href="if_002dthen.html" accesskey="n" rel="next">if-then</a>, Previous: <a href="Conditionals-and-Loops.html" accesskey="p" rel="prev">Conditionals and Loops</a>, Up: <a href="Conditionals-and-Loops.html" accesskey="u" rel="up">Conditionals and Loops</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Request-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
<h4 class="subsection" id="Operators-in-Conditionals-1">5.23.1 Operators in Conditionals</h4>
<a class="index-entry-id" id="index-if-request_002c-operators-to-use-with"></a>
<a class="index-entry-id" id="index-ie-request_002c-operators-to-use-with"></a>
<a class="index-entry-id" id="index-while-request_002c-operators-to-use-with"></a>
<a class="index-entry-id" id="index-conditional-expressions"></a>
<a class="index-entry-id" id="index-expressions_002c-conditional"></a>
<p>In <code class="code">if</code>, <code class="code">ie</code>, and <code class="code">while</code> requests, in addition to the
numeric expressions described in <a class="ref" href="Numeric-Expressions.html">Numeric Expressions</a>, several
Boolean operators are available; the members of this expanded class are
termed <em class="dfn">conditional expressions</em>.
</p>
<dl class="table">
<dt><code class="code">c <var class="var">glyph</var></code></dt>
<dd><p>True if <var class="var">glyph</var> is available, where <var class="var">glyph</var> is an ordinary
character, a special character ‘<samp class="samp">\(<var class="var">xx</var></samp>’ or ‘<samp class="samp">\[<var class="var">xxx</var>]</samp>’,
‘<samp class="samp">\N'<var class="var">xxx</var>'</samp>’, or has been defined by any of the <code class="code">char</code>,
<code class="code">fchar</code>, <code class="code">fschar</code>, or <code class="code">schar</code> requests.
</p>
</dd>
<dt><code class="code">d <var class="var">name</var></code></dt>
<dd><p>True if a string, macro, diversion, or request called <var class="var">name</var> exists.
</p>
</dd>
<dt><code class="code">e</code></dt>
<dd><p>True if the current page is even-numbered.
</p>
</dd>
<dt><code class="code">F <var class="var">font</var></code></dt>
<dd><p>True if <var class="var">font</var> exists. <var class="var">font</var> is handled as if it were opened
with the <code class="code">ft</code> request (that is, font translation and styles are
applied), without actually mounting it.
</p>
</dd>
<dt><code class="code">m <var class="var">color</var></code></dt>
<dd><p>True if <var class="var">color</var> is defined.
</p>
</dd>
<dt id='index-conditional-output-for-terminal-_0028TTY_0029'><span><code class="code">n</code><a class="copiable-link" href='#index-conditional-output-for-terminal-_0028TTY_0029'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-TTY_002c-conditional-output-for"></a>
<a class="index-entry-id" id="index-terminal_002c-conditional-output-for"></a>
<p>True if the document is being processed in <code class="code">nroff</code> mode.
See <a class="xref" href="troff-and-nroff-Modes.html"><code class="code">troff</code> and <code class="code">nroff</code> Modes</a>.
</p>
</dd>
<dt><code class="code">o</code></dt>
<dd><p>True if the current page is odd-numbered.
</p>
</dd>
<dt><code class="code">r <var class="var">register</var></code></dt>
<dd><p>True if <var class="var">register</var> exists.
</p>
</dd>
<dt><code class="code">S <var class="var">style</var></code></dt>
<dd><p>True if <var class="var">style</var> is available for the current font family. Font
translation is applied.
</p>
</dd>
<dt><code class="code">t</code></dt>
<dd><p>True if the document is being processed in <code class="code">troff</code> mode.
See <a class="xref" href="troff-and-nroff-Modes.html"><code class="code">troff</code> and <code class="code">nroff</code> Modes</a>.
</p>
<a class="index-entry-id" id="index-vtroff"></a>
</dd>
<dt><code class="code">v</code></dt>
<dd><p>Always false. This condition is recognized only for compatibility with
certain other <code class="code">troff</code> implementations.<a class="footnote" id="DOCF88" href="groff.html_fot.html#FOOT88"><sup>88</sup></a>
</p></dd>
</dl>
<p>If the first argument to an <code class="code">if</code>, <code class="code">ie</code>, or <code class="code">while</code>
request begins with a non-alphanumeric character apart from <code class="code">!</code>
(see below); it performs an <i class="slanted">output comparison test</i>.
<a class="footnote" id="DOCF89" href="groff.html_fot.html#FOOT89"><sup>89</sup></a>
</p>
<a class="index-entry-id" id="index-output-comparison-operator"></a>
<dl class="table">
<dt><code class="code"><code class="code">'</code><var class="var">xxx</var><code class="code">'</code><var class="var">yyy</var><code class="code">'</code></code></dt>
<dd><p>True if formatting the comparands <var class="var">xxx</var> and <var class="var">yyy</var> produces the
same output commands. The delimiter need not be a neutral apostrophe:
the output comparison operator accepts the same delimiters as most
escape sequences; see <a class="ref" href="Delimiters.html">Delimiters</a>. This <em class="dfn">output comparison
operator</em> formats <var class="var">xxx</var> and <var class="var">yyy</var> in separate environments;
after the comparison, the resulting data are discarded.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.ie "|"\fR|\fP" true
.el false
⇒ true
</pre></div></div>
<p>The resulting glyph properties, including font family, style, size, and
slant, must match, but not necessarily the requests and/or escape
sequences used to obtain them. In the previous example, ‘<samp class="samp">|</samp>’ and
‘<samp class="samp">\fR|\fP</samp>’ result in ‘<samp class="samp">|</samp>’ glyphs in the same typefaces at the
same positions, so the comparands are equal. If ‘<samp class="samp">.ft I</samp>’ had
been added before the ‘<samp class="samp">.ie</samp>’, they would differ: the first ‘<samp class="samp">|</samp>’
would produce an italic ‘<samp class="samp">|</samp>’, not a roman one. Motions must match
in orientation and magnitude to within the applicable horizontal and
vertical motion quanta of the device, after rounding. ‘<samp class="samp">.if
"\u\d"\v'0'"</samp>’ is false even though both comparands result in zero net
motion, because motions are not interpreted or optimized but sent as-is
to the output.<a class="footnote" id="DOCF90" href="groff.html_fot.html#FOOT90"><sup>90</sup></a> On the other hand, ‘<samp class="samp">.if "\d"\v'0.5m'"</samp>’ is true, because
<code class="code">\d</code> is defined as a downward motion of one-half em.<a class="footnote" id="DOCF91" href="groff.html_fot.html#FOOT91"><sup>91</sup></a>
</p>
<a class="index-entry-id" id="index-string-comparison"></a>
<a class="index-entry-id" id="index-comparison-of-strings"></a>
<p>Surround the comparands with <code class="code">\?</code> to avoid formatting them; this
causes them to be compared character by character, as with string
comparisons in other programming languages.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.ie "\?|\?"\?\fR|\fP\?" true
.el false
⇒ false
</pre></div></div>
<a class="index-entry-id" id="index-_005c_003f_002c-and-copy-mode"></a>
<a class="index-entry-id" id="index-copy-mode_002c-and-_005c_003f"></a>
<a class="index-entry-id" id="index-mode_002c-copy_002c-and-_005c_003f"></a>
<p>Since comparands protected with <code class="code">\?</code> are read in copy mode
(see <a class="pxref" href="Copy-Mode.html">Copy Mode</a>), they need not even be valid <code class="code">groff</code> syntax.
The escape character is still lexically recognized, however, and
consumes the next character.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.ds a \[
.ds b \[
.if '\?\*a\?'\?\*b\?' a and b equivalent
.if '\?\\?'\?\\?' backslashes equivalent
⇒ a and b equivalent
</pre></div></div>
</dd>
</dl>
<p>The above operators can’t be combined with most others, but a leading
‘<samp class="samp">!</samp>’, not followed immediately by spaces or tabs, complements an
expression.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.nr x 1
.ie !r x register x is not defined
.el register x is defined
⇒ register x is defined
</pre></div></div>
<p>Spaces and tabs are optional immediately after the ‘<samp class="samp">c</samp>’, ‘<samp class="samp">d</samp>’,
‘<samp class="samp">F</samp>’, ‘<samp class="samp">m</samp>’, ‘<samp class="samp">r</samp>’, and ‘<samp class="samp">S</samp>’ operators, but right after
‘<samp class="samp">!</samp>’, they end the predicate and the conditional evaluates
true.<a class="footnote" id="DOCF92" href="groff.html_fot.html#FOOT92"><sup>92</sup></a>
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.nr x 1
.ie ! r x register x is not defined
.el register x is defined
⇒ r x register x is not defined
</pre></div></div>
<p>The unexpected ‘<samp class="samp">r x</samp>’ in the output is a clue that our conditional
was not interpreted as we planned, but matters may not always be so
obvious.
</p>
</div>
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