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<h4 class="subsection" id="Setting-Registers-1">5.8.1 Setting Registers</h4>
<a class="index-entry-id" id="index-setting-registers-_0028nr_002c-_005cR_0029"></a>
<a class="index-entry-id" id="index-registers_002c-setting-_0028nr_002c-_005cR_0029"></a>
<p>Define registers and update their values with the <code class="code">nr</code> request or
the <code class="code">\R</code> escape sequence.
</p>
<dl class="first-deffn">
<dt class="deffn" id="index-_002enr-1"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.nr</code></strong> <var class="def-var-arguments">ident value</var><a class="copiable-link" href='#index-_002enr-1'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-nr-1"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_005cR_0027ident-value_0027"><span class="category-def">Escape sequence: </span><span><strong class="def-name"><code class="t">\R<code class="code">'</code></code><span class="r"><i class="slanted">ident value</i></span><code class="t"><code class="code">'</code></code></strong><a class="copiable-link" href='#index-_005cR_0027ident-value_0027'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_005cR"></a>
<p>Set register <var class="var">ident</var> to <var class="var">value</var>. If <var class="var">ident</var> doesn’t exist,
GNU <code class="code">troff</code> creates it. In the <code class="code">\R</code> escape sequence, the
delimiter need not be a neutral apostrophe; see <a class="ref" href="Delimiters.html">Delimiters</a>. It
also does not produce an input token in GNU <code class="code">troff</code>. See <a class="xref" href="Gtroff-Internals.html"><code class="code">gtroff</code> Internals</a>.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.nr a (((17 + (3 * 4))) % 4)
\n[a]
.\R'a (((17 + (3 * 4))) % 4)'
\n[a]
⇒ 1 1
</pre></div></div>
<p>(Later, we will discuss additional forms of <code class="code">nr</code> and <code class="code">\R</code> that
can change a register’s value after it is dereferenced but before it is
interpolated. See <a class="xref" href="Auto_002dincrement.html">Auto-increment</a>.)
</p>
<p>The complete transparency of <code class="code">\R</code> can cause surprising effects if
you use registers like <code class="code">.k</code>, which get evaluated at the time they
are accessed.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.ll 1.6i
.
aaa bbb ccc ddd eee fff ggg hhh\R':k \n[.k]'
.tm :k == \n[:k]
⇒ :k == 126950
.
.br
.
aaa bbb ccc ddd eee fff ggg hhh\h'0'\R':k \n[.k]'
.tm :k == \n[:k]
⇒ :k == 15000
</pre></div></div>
<p>If you process this with the PostScript device (<code class="code">-Tps</code>), there will
be a line break eventually after <code class="code">ggg</code> in both input lines.
However, after processing the space after <code class="code">ggg</code>, the partially
collected line is not overfull yet, so GNU <code class="code">troff</code> continues to
collect input until it sees the space (or in this case, the newline)
after <code class="code">hhh</code>. At this point, the line is longer than the line
length, and the line gets broken.
</p>
<p>In the first input line, since the <code class="code">\R</code> escape sequence leaves no
traces, the check for the overfull line hasn’t been done yet at the
point where <code class="code">\R</code> gets handled, and you get a value for the
<code class="code">.k</code> register that is even greater than the current line length.
</p>
<p>In the second input line, the insertion of <code class="code">\h'0'</code> to cause a
zero-width motion forces GNU <code class="code">troff</code> to check the line length,
which in turn causes the start of a new output line. Now <code class="code">.k</code>
returns the expected value.
</p></dd></dl>
<p><code class="code">nr</code> and <code class="code">\R</code> each have two additional special forms to
increment or decrement a register.
</p>
<dl class="first-deffn">
<dt class="deffn" id="index-_002enr-2"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.nr</code></strong> <var class="def-var-arguments">ident <code class="t">+</code><span class="r"><i class="slanted">value</i></span></var><a class="copiable-link" href='#index-_002enr-2'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-nr-2"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_002enr-3"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.nr</code></strong> <var class="def-var-arguments">ident <code class="t">-</code><span class="r"><i class="slanted">value</i></span></var><a class="copiable-link" href='#index-_002enr-3'> ¶</a></span></dt>
<dt class="deffnx def-cmd-deffn" id="index-_005cR_0027ident-_002bvalue_0027"><span class="category-def">Escape sequence: </span><span><strong class="def-name"><code class="t">\R<code class="code">'</code></code><span class="r"><i class="slanted">ident <code class="t">+</code>value</i></span><code class="t"><code class="code">'</code></code></strong><a class="copiable-link" href='#index-_005cR_0027ident-_002bvalue_0027'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_005cR-1"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_005cR_0027ident-_002dvalue_0027"><span class="category-def">Escape sequence: </span><span><strong class="def-name"><code class="t">\R<code class="code">'</code></code><span class="r"><i class="slanted">ident <code class="t">-</code>value</i></span><code class="t"><code class="code">'</code></code></strong><a class="copiable-link" href='#index-_005cR_0027ident-_002dvalue_0027'> ¶</a></span></dt>
<dd><p>Increment (decrement) register <var class="var">ident</var> by <var class="var">value</var>. In the
<code class="code">\R</code> escape sequence, the delimiter need not be a neutral
apostrophe; see <a class="ref" href="Delimiters.html">Delimiters</a>.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.nr a 1
.nr a +1
\na
⇒ 2
</pre></div></div>
<a class="index-entry-id" id="index-negating-register-values"></a>
<p>A leading minus sign in <var class="var">value</var> is always interpreted as a
decrementation operator, not an algebraic sign. To assign a register a
negative value or the negated value of another register, you can
force GNU <code class="code">troff</code> to interpret ‘<samp class="samp">-</samp>’ as a negation or minus,
rather than decrementation, operator: enclose it with its operand in
parentheses or subtract it from zero.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.nr a 7
.nr b 3
.nr a -\nb
\na
⇒ 4
.nr a (-\nb)
\na
⇒ -3
.nr a 0-\nb
\na
⇒ -3
</pre></div></div>
<p>If a register’s prior value does not exist (the register was undefined),
an increment or decrement is applied as if to 0.
</p></dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-_002err"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.rr</code></strong> <var class="def-var-arguments">ident</var><a class="copiable-link" href='#index-_002err'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-rr"></a>
<a class="index-entry-id" id="index-removing-a-register-_0028rr_0029"></a>
<a class="index-entry-id" id="index-register_002c-removing-_0028rr_0029"></a>
<p>Remove register <var class="var">ident</var>. If <var class="var">ident</var> doesn’t exist, the request
is ignored. Technically, only the name is removed; the register’s
contents are still accessible under aliases created with <code class="code">aln</code>, if
any.
</p></dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-_002ernn"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.rnn</code></strong> <var class="def-var-arguments">ident1 ident2</var><a class="copiable-link" href='#index-_002ernn'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-rnn"></a>
<a class="index-entry-id" id="index-renaming-a-register-_0028rnn_0029"></a>
<a class="index-entry-id" id="index-register_002c-renaming-_0028rnn_0029"></a>
<p>Rename register <var class="var">ident1</var> to <var class="var">ident2</var>. If <var class="var">ident1</var> doesn’t
exist, the request is ignored. Renaming a built-in register does not
otherwise alter its properties.
</p></dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-_002ealn"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.aln</code></strong> <var class="def-var-arguments">new old</var><a class="copiable-link" href='#index-_002ealn'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-aln"></a>
<a class="index-entry-id" id="index-alias_002c-register_002c-creating-_0028aln_0029"></a>
<a class="index-entry-id" id="index-creating-alias-for-register-_0028aln_0029"></a>
<a class="index-entry-id" id="index-register_002c-creating-alias-for-_0028aln_0029"></a>
<p>Create an alias <var class="var">new</var> for an existing register <var class="var">old</var>, causing
the names to refer to the same stored object. If <var class="var">old</var> is
undefined, a warning in category ‘<samp class="samp">reg</samp>’ is produced and the request
is ignored. See <a class="xref" href="Warnings.html">Warnings</a>, for information about the enablement and
suppression of warnings.
</p>
<a class="index-entry-id" id="index-alias_002c-register_002c-removing-_0028rr_0029"></a>
<a class="index-entry-id" id="index-removing-alias-for-register-_0028rr_0029"></a>
<a class="index-entry-id" id="index-register_002c-removing-alias-for-_0028rr_0029"></a>
<p>To remove a register alias, invoke <code class="code">rr</code> on its name. A register’s
contents do not become inaccessible until it has no more names.
</p></dd></dl>
</div>
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