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. See 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. See 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. See 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.