groff: Setting Registers
5.8.1 Setting Registers
-----------------------
Define registers and update their values with the 'nr' request or the
'\R' escape sequence.
-- Request: .nr ident value
-- Escape sequence: \R'ident value'
Set register IDENT to VALUE. If IDENT doesn't exist, GNU 'troff'
creates it. In the '\R' escape sequence, the delimiter need not be
a neutral apostrophe; see ⇒Delimiters. It also does not
produce an input token in GNU 'troff'. ⇒Gtroff Internals.
.nr a (((17 + (3 * 4))) % 4)
\n[a]
.\R'a (((17 + (3 * 4))) % 4)'
\n[a]
=> 1 1
(Later, we will discuss additional forms of 'nr' and '\R' that can
change a register's value after it is dereferenced but before it is
interpolated. ⇒Auto-increment.)
The complete transparency of '\R' can cause surprising effects if
you use registers like '.k', which get evaluated at the time they
are accessed.
.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
If you process this with the PostScript device ('-Tps'), there will
be a line break eventually after 'ggg' in both input lines.
However, after processing the space after 'ggg', the partially
collected line is not overfull yet, so GNU 'troff' continues to
collect input until it sees the space (or in this case, the
newline) after 'hhh'. At this point, the line is longer than the
line length, and the line gets broken.
In the first input line, since the '\R' escape sequence leaves no
traces, the check for the overfull line hasn't been done yet at the
point where '\R' gets handled, and you get a value for the '.k'
register that is even greater than the current line length.
In the second input line, the insertion of '\h'0'' to cause a
zero-width motion forces GNU 'troff' to check the line length,
which in turn causes the start of a new output line. Now '.k'
returns the expected value.
'nr' and '\R' each have two additional special forms to increment or
decrement a register.
-- Request: .nr ident +value
-- Request: .nr ident -value
-- Escape sequence: \R'ident +value'
-- Escape sequence: \R'ident -value'
Increment (decrement) register IDENT by VALUE. In the '\R' escape
sequence, the delimiter need not be a neutral apostrophe; see ⇒
Delimiters.
.nr a 1
.nr a +1
\na
=> 2
A leading minus sign in VALUE 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 'troff' to interpret '-' as a negation or minus,
rather than decrementation, operator: enclose it with its operand
in parentheses or subtract it from zero.
.nr a 7
.nr b 3
.nr a -\nb
\na
=> 4
.nr a (-\nb)
\na
=> -3
.nr a 0-\nb
\na
=> -3
If a register's prior value does not exist (the register was
undefined), an increment or decrement is applied as if to 0.
-- Request: .rr ident
Remove register IDENT. If IDENT doesn't exist, the request is
ignored. Technically, only the name is removed; the register's
contents are still accessible under aliases created with 'aln', if
any.
-- Request: .rnn ident1 ident2
Rename register IDENT1 to IDENT2. If IDENT1 doesn't exist, the
request is ignored. Renaming a built-in register does not
otherwise alter its properties.
-- Request: .aln new old
Create an alias NEW for an existing register OLD, causing the names
to refer to the same stored object. If OLD is undefined, a warning
in category 'reg' is produced and the request is ignored. ⇒
Warnings, for information about the enablement and suppression of
warnings.
To remove a register alias, invoke 'rr' on its name. A register's
contents do not become inaccessible until it has no more names.
Info Catalog
groff: Registers
groff: Registers
groff: Interpolating Registers