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diff --git a/doc/groff.html.node/Conditional-Blocks.html b/doc/groff.html.node/Conditional-Blocks.html new file mode 100644 index 0000000..b2b789b --- /dev/null +++ b/doc/groff.html.node/Conditional-Blocks.html @@ -0,0 +1,174 @@ +<!DOCTYPE html> +<html> +<!-- Created by GNU Texinfo 7.0.3, https://www.gnu.org/software/texinfo/ --> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> +<!-- This manual documents GNU troff version 1.23.0. + +Copyright © 1994-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with no +Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A +copy of the license is included in the section entitled "GNU Free +Documentation License". --> +<title>Conditional Blocks (The GNU Troff Manual)</title> + +<meta name="description" content="Conditional Blocks (The GNU Troff Manual)"> +<meta name="keywords" content="Conditional Blocks (The GNU Troff Manual)"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta name="viewport" content="width=device-width,initial-scale=1"> + +<link href="index.html" rel="start" title="Top"> +<link href="Request-Index.html" rel="index" title="Request Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Conditionals-and-Loops.html" rel="up" title="Conditionals and Loops"> +<link href="while.html" rel="next" title="while"> +<link href="Operators-in-Conditionals.html" rel="prev" title="Operators in Conditionals"> +<style type="text/css"> +<!-- +a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em} +div.example {margin-left: 3.2em} +kbd.key {font-style: normal} +span.r {font-family: initial; font-weight: normal; font-style: normal} +span:hover a.copiable-link {visibility: visible} +strong.def-name {font-family: monospace; font-weight: bold; font-size: larger} +--> +</style> + + +</head> + +<body lang="en"> +<div class="subsection-level-extent" id="Conditional-Blocks"> +<div class="nav-panel"> +<p> +Next: <a href="while.html" accesskey="n" rel="next">while</a>, Previous: <a href="Operators-in-Conditionals.html" accesskey="p" rel="prev">Operators in Conditionals</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> +</div> +<hr> +<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> +<a class="index-entry-id" id="index-opening-brace-escape-sequence-_0028_005c_007d_0029"></a> +<a class="index-entry-id" id="index-closing-brace-escape-sequence-_0028_005c_007d_0029"></a> +<a class="index-entry-id" id="index-brace-escape-sequence_002c-opening-_0028_005c_007d_0029"></a> +<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> + + +</div> +<hr> +<div class="nav-panel"> +<p> +Next: <a href="while.html">while</a>, Previous: <a href="Operators-in-Conditionals.html">Operators in Conditionals</a>, Up: <a href="Conditionals-and-Loops.html">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> +</div> + + + +</body> +</html> |