5.6.1 Setting Registers

Define or set registers using the nr request or the \R escape.

Although the following requests and escapes can be used to create registers, simply using an undefined register will cause it to be set to zero.

Request: .nr ident value

Escape: \R'ident value'

Set number register ident to value. If ident doesn’t exist, gtroff creates it.

The argument to \R usually has to be enclosed in quotes. See Escapes, for details on parameter delimiting characters.

The \R escape doesn’t produce an input token in gtroff; in other words, it vanishes completely after gtroff has processed it.

For example, the following two lines are equivalent:

.nr a (((17 + (3 * 4))) % 4)
\R'a (((17 + (3 * 4))) % 4)'
    ⇒ 1

Note that the complete transparency of \R can cause surprising effects if you use number 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 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 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 number register that is even greater than the current line length.

In the second input line, the insertion of \h'0' to emit an invisible zero-width space forces troff to check the line length, which in turn causes the start of a new output line. Now .k returns the expected value.

Both nr and \R have two additional special forms to increment or decrement a register.

Request: .nr ident +value

Request: .nr ident -value

Escape: \R'ident +value'

Escape: \R'ident -value'

Increment (decrement) register ident by value.

.nr a 1
.nr a +1
\na
    ⇒ 2

To assign the negated value of a register to another register, some care must be taken to get the desired result:

.nr a 7
.nr b 3
.nr a -\nb
\na
    ⇒ 4
.nr a (-\nb)
\na
    ⇒ -3

The surrounding parentheses prevent the interpretation of the minus sign as a decrementing operator. An alternative is to start the assignment with a ‘0’:

.nr a 7
.nr b -3
.nr a \nb
\na
    ⇒ 4
.nr a 0\nb
\na
    ⇒ -3

Request: .rr ident

Remove number register ident. If ident doesn’t exist, the request is ignored.

Request: .rnn ident1 ident2

Rename number register ident1 to ident2. If either ident1 or ident2 doesn’t exist, the request is ignored.

Request: .aln ident1 ident2

Create an alias ident1 for a number register ident2. The new name and the old name are exactly equivalent. If ident1 is undefined, a warning of type ‘reg’ is generated, and the request is ignored. See Debugging, for information about warnings.