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<title>Conditional Blocks (The GNU Troff Manual)</title>
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<h4 class="subsection" id="Conditional-Blocks-1">5.23.4 Conditional Blocks</h4>
<a class="index-entry-id" id="index-conditional-blocks"></a>
<a class="index-entry-id" id="index-blocks_002c-conditional"></a>
<p>It is frequently desirable for a control structure to govern more than
one request, macro call, text line, or a combination of the foregoing.
The opening and closing brace escape sequences <code class="code">\{</code> and <code class="code">\}</code>
define such groups. These <em class="dfn">conditional blocks</em> can furthermore be
nested.
</p>
<dl class="first-deffn">
<dt class="deffn" id="index-_005c_007b-2"><span class="category-def">Escape sequence: </span><span><strong class="def-name"><code class="t">\{</code><span class="r"><i class="slanted"></i></span><code class="t"></code></strong><a class="copiable-link" href='#index-_005c_007b-2'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_005c_007b"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_005c_007d-1"><span class="category-def">Escape sequence: </span><span><strong class="def-name"><code class="t">\}</code><span class="r"><i class="slanted"></i></span><code class="t"></code></strong><a class="copiable-link" href='#index-_005c_007d-1'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_005c_007b-1"></a>
<a class="index-entry-id" id="index-_005c_007d"></a>
<a class="index-entry-id" id="index-beginning-of-conditional-block-_0028_005c_007b_0029"></a>
<a class="index-entry-id" id="index-end-of-conditional-block-_0028_005c_007d_0029"></a>
<a class="index-entry-id" id="index-conditional-block_002c-beginning-_0028_005c_007b_0029"></a>
<a class="index-entry-id" id="index-conditional-block_002c-end-_0028_005c_007d_0029"></a>
<a class="index-entry-id" id="index-block_002c-conditional_002c-beginning-_0028_005c_007b_0029"></a>
<a class="index-entry-id" id="index-block_002c-conditional_002c-end-_0028_005c_007d_0029"></a>
<a class="index-entry-id" id="index-brace-escape-sequences-_0028_005c_007b_002c-_005c_007d_0029"></a>
<a class="index-entry-id" id="index-escape-sequences_002c-brace-_0028_005c_007b_002c-_005c_007d_0029"></a>
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<a class="index-entry-id" id="index-brace-escape-sequence_002c-closing-_0028_005c_007d_0029"></a>
<p><code class="code">\{</code> begins a conditional block; it must appear (after optional
spaces and tabs) immediately subsequent to the conditional expression of
an <code class="code">if</code>, <code class="code">ie</code>, or <code class="code">while</code>
request,<a class="footnote" id="DOCF93" href="groff.html_fot.html#FOOT93"><sup>93</sup></a> or as the argument to an <code class="code">el</code>
request.
</p>
<p><code class="code">\}</code> ends a condition block and should appear on a line with other
occurrences of itself as necessary to match <code class="code">\{</code> sequences. It
can be preceded by a control character, spaces, and tabs. Input after
any quantity of <code class="code">\}</code> sequences on the same line is processed only
if all of the preceding conditions to which they correspond are true.
Furthermore, a <code class="code">\}</code> closing the body of a <code class="code">while</code> request
must be the last such escape sequence on an input line.
</p>
<p>Brace escape sequences outside of control structures have no meaning and
produce no output.
</p>
<p><strong class="strong">Caution:</strong> Input lines using <code class="code">\{</code> often end with
<code class="code">\RET</code>, especially in macros that consist primarily of control
lines. Forgetting to use <code class="code">\RET</code> on an input line after <code class="code">\{</code>
is a common source of error.
</p></dd></dl>
<p>We might write the following in a page header macro. If we delete
<code class="code">\RET</code>, the header will carry an unwanted extra empty line (except
on page 1).
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.if (\\n[%] != 1) \{\
. ie ((\\n[%] % 2) = 0) .tl \\*[even-numbered-page-title]
. el .tl \\*[odd-numbered-page-title]
.\}
</pre></div></div>
<p>Let us take a closer look at how conditional blocks nest.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">A
.if 0 \{ B
C
D
\}E
F
⇒ A F
</pre></div></div>
<div class="example">
<div class="group"><pre class="example-preformatted">N
.if 1 \{ O
. if 0 \{ P
Q
R\} S\} T
U
⇒ N O U
</pre></div></div>
<p>The above behavior may challenge the intuition; it was implemented to
retain compatibility with <abbr class="acronym">AT&T</abbr> <code class="code">troff</code>. For clarity, it
is idiomatic to end input lines with <code class="code">\{</code> (followed by
<code class="code">\<kbd class="key">RET</kbd></code> if appropriate), and to precede <code class="code">\}</code> on an input
line with nothing more than a control character, spaces, tabs, and other
instances of itself.
</p>
<p>We can use <code class="code">ie</code>, <code class="code">el</code>, and conditional blocks to simulate the
multi-way “switch” or “case” control structures of other languages.
The following example is adapted from the <code class="code">groff</code> <samp class="file">man</samp>
package. Indentation is used to clarify the logic.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.\" Simulate switch/case in roff.
. ie '\\$2'1' .ds title General Commands\"
.el \{.ie '\\$2'2' .ds title System Calls\"
.el \{.ie '\\$2'3' .ds title Library Functions\"
.el \{.ie '\\$2'4' .ds title Kernel Interfaces\"
.el \{.ie '\\$2'5' .ds title File Formats\"
.el \{.ie '\\$2'6' .ds title Games\"
.el \{.ie '\\$2'7' .ds title Miscellaneous Information\"
.el \{.ie '\\$2'8' .ds title System Management\"
.el \{.ie '\\$2'9' .ds title Kernel Development\"
.el .ds title \" empty
.\}\}\}\}\}\}\}\}
</pre></div></div>
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