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<div class="subsection-level-extent" id="Input-Line-Traps">
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<h4 class="subsection" id="Input-Line-Traps-1">5.28.2 Input Line Traps</h4>
<a class="index-entry-id" id="index-input-line-traps"></a>
<a class="index-entry-id" id="index-traps_002c-input-line"></a>
<dl class="first-deffn">
<dt class="deffn" id="index-_002eit"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.it</code></strong> <var class="def-var-arguments">[<span class="r"><i class="slanted">n</i></span> <span class="r"><i class="slanted">name</i></span>]</var><a class="copiable-link" href='#index-_002eit'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-it"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_002eitc"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.itc</code></strong> <var class="def-var-arguments">[<span class="r"><i class="slanted">n</i></span> <span class="r"><i class="slanted">name</i></span>]</var><a class="copiable-link" href='#index-_002eitc'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-itc"></a>
<a class="index-entry-id" id="index-setting-input-line-trap-_0028it_002c-itc_0029"></a>
<a class="index-entry-id" id="index-input-line-trap_002c-setting-_0028it_002c-itc_0029"></a>
<a class="index-entry-id" id="index-trap_002c-input-line_002c-setting-_0028it_002c-itc_0029"></a>
<a class="index-entry-id" id="index-clearing-input-line-trap-_0028it_002c-itc_0029"></a>
<a class="index-entry-id" id="index-input-line-trap_002c-clearing-_0028it_002c-itc_0029"></a>
<a class="index-entry-id" id="index-trap_002c-input-line_002c-clearing-_0028it_002c-itc_0029"></a>
<p>Set an input line trap, calling macro <var class="var">name</var> after processing the
next <var class="var">n</var> productive input lines (recall <a class="ref" href="Manipulating-Filling-and-Adjustment.html">Manipulating Filling and Adjustment</a>). Any existing input line trap in the
environment is replaced. Without arguments, <code class="code">it</code> and <code class="code">itc</code>
clear any input line trap that has not yet sprung.
</p>
<p>Consider a macro ‘<samp class="samp">.ST <var class="var">s n</var></samp>’ which sets the next
<var class="var">n</var> input lines in the font style <var class="var">s</var>.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.de ST \" Use style $1 for next $2 text lines.
. it \\$2 ES
. ft \\$1
..
.de ES \" end ST
. ft R
..
.ST I 1
oblique
face
.ST I 1
oblique\c
face
⇒ <i class="i">oblique</i> face <i class="i">oblique</i>face <span class="r">(second “face” upright)</span>
</pre></div></div>
<a class="index-entry-id" id="index-input-line-traps-and-interrupted-lines-_0028itc_0029"></a>
<a class="index-entry-id" id="index-interrupted-lines-and-input-line-traps-_0028itc_0029"></a>
<a class="index-entry-id" id="index-traps_002c-input-line_002c-and-interrupted-lines-_0028itc_0029"></a>
<a class="index-entry-id" id="index-lines_002c-interrupted_002c-and-input-line-traps-_0028itc_0029"></a>
<p>Unlike the <code class="code">ce</code> and <code class="code">rj</code> requests, <code class="code">it</code> counts lines
interrupted with the <code class="code">\c</code> escape sequence separately (see <a class="pxref" href="Line-Continuation.html">Line Continuation</a>); <code class="code">itc</code> does not. To see the difference, let’s
change the previous example to use <code class="code">itc</code> instead.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted"><span class="r">…</span>
. itc \\$2 ES
<span class="r">…</span>
⇒ <i class="i">oblique</i> face <i class="i">obliqueface</i> <span class="r">(second “face” oblique)</span>
</pre></div></div>
<p>You can think of the <code class="code">ce</code> and <code class="code">rj</code> requests as implicitly
creating an input line trap with <code class="code">itc</code> that schedules a break when
the trap is sprung.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.de BR
. br
. <i class="slanted">internal: disable centering-without-filling</i>
..
.
.de ce
. if \\n[.br] .br
. itc \\$1 BR
. <i class="slanted">internal: enable centering-without-filling</i>
..
</pre></div></div>
<p>Let us consider in more detail the sorts of input lines that are or are
not “productive”.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.de Trap
TRAP SPRUNG
..
.de Mac
.if r a \l'5n'
..
.it 2 Trap
.
foo
.Mac
bar
baz
.it 1 Trap
.sp \" moves, but does not write or draw
qux
.itc 1 Trap
\h'5n'\c \" moves, but does not write or draw
jat
</pre></div></div>
<p>When ‘<samp class="samp">Trap</samp>’ gets called depends on whether the ‘<samp class="samp">a</samp>’ register is
defined; the control line with the <code class="code">if</code> request may or may not
produce written output. We also see that the spacing request <code class="code">sp</code>,
while certainly affecting the output, does not spring the input line
trap. Similarly, the horizontal motion escape sequence <code class="code">\h</code> also
affected the output, but was not “written”. Observe that we had to
follow it with <code class="code">\c</code> and use <code class="code">itc</code> to prevent the newline at
the end of the text line from causing a word break, which, like an
ordinary space character, counts as written output.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">$ groff -Tascii input-trap-example.groff
⇒ foo bar TRAP SPRUNG baz
⇒
⇒ qux TRAP SPRUNG jat TRAP SPRUNG
$ groff -Tascii -ra1 input-trap-example.groff
⇒ foo _____ TRAP SPRUNG bar baz
⇒
⇒ qux TRAP SPRUNG jat TRAP SPRUNG
</pre></div></div>
</dd></dl>
<p>Input line traps are associated with the environment
(see <a class="pxref" href="Environments.html">Environments</a>); switching to another environment suspends the
current input line trap, and going back resumes it, restoring the count
of qualifying lines enumerated in that environment.
</p>
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