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5.25 Diversions

In gtroff it is possible to divert text into a named storage area. Due to the similarity to defining macros it is sometimes said to be stored in a macro. This is used for saving text for output at a later time, which is useful for keeping blocks of text on the same page, footnotes, tables of contents, and indices.

For orthogonality it is said that gtroff is in the top-level diversion if no diversion is active (i.e., the data is diverted to the output device).

Although the following requests can be used to create diversions, simply using an undefined diversion will cause it to be defined as empty. See Identifiers.

— Request: .di macro
— Request: .da macro

Begin a diversion. Like the de request, it takes an argument of a macro name to divert subsequent text into. The da macro appends to an existing diversion.

di or da without an argument ends the diversion.

The current partially-filled line is included into the diversion. See the box request below for an example. Note that switching to another (empty) environment (with the ev request) avoids the inclusion of the current partially-filled line.

— Request: .box macro
— Request: .boxa macro

Begin (or append to) a diversion like the di and da requests. The difference is that box and boxa do not include a partially-filled line in the diversion.

Compare this:

          
          Before the box.
          .box xxx
          In the box.
          .br
          .box
          After the box.
          .br
              ⇒ Before the box.  After the box.
          .xxx
              ⇒ In the box.

with this:

          
          Before the diversion.
          .di yyy
          In the diversion.
          .br
          .di
          After the diversion.
          .br
              ⇒ After the diversion.
          .yyy
              ⇒ Before the diversion.  In the diversion.

box or boxa without an argument ends the diversion.

— Register: \n[.z]
— Register: \n[.d]

Diversions may be nested. The read-only number register .z contains the name of the current diversion (this is a string-valued register). The read-only number register .d contains the current vertical place in the diversion. If not in a diversion it is the same as register nl.

— Register: \n[.h]

The high-water mark on the current page. It corresponds to the text baseline of the lowest line on the page. This is a read-only register.

          
          .tm .h==\n[.h], nl==\n[nl]
              ⇒ .h==0, nl==-1
          This is a test.
          .br
          .sp 2
          .tm .h==\n[.h], nl==\n[nl]
              ⇒ .h==40, nl==120

As can be seen in the previous example, empty lines are not considered in the return value of the .h register.

— Register: \n[dn]
— Register: \n[dl]

After completing a diversion, the read-write number registers dn and dl contain the vertical and horizontal size of the diversion. Note that only the just processed lines are counted: For the computation of dn and dl, the requests da and boxa are handled as if di and box had been used – lines which have been already stored in a macro are not taken into account.

          
          .\" Center text both horizontally & vertically
          .
          .\" Enclose macro definitions in .eo and .ec
          .\" to avoid the doubling of the backslash
          .eo
          .\" macro .(c starts centering mode
          .de (c
          .  br
          .  ev (c
          .  evc 0
          .  in 0
          .  nf
          .  di @c
          ..
          
          .\" macro .)c terminates centering mode
          .de )c
          .  br
          .  ev
          .  di
          .  nr @s (((\n[.t]u - \n[dn]u) / 2u) - 1v)
          .  sp \n[@s]u
          .  ce 1000
          .  @c
          .  ce 0
          .  sp \n[@s]u
          .  br
          .  fi
          .  rr @s
          .  rm @s
          .  rm @c
          ..
          .\" End of macro definitions, restore escape mechanism
          .ec
— Escape: \!
— Escape: \?anything\?

Prevent requests, macros, and escapes from being interpreted when read into a diversion. Both escapes take the given text and transparently embed it into the diversion. This is useful for macros which shouldn't be invoked until the diverted text is actually output.

The \! escape transparently embeds text up to and including the end of the line. The \? escape transparently embeds text until the next occurrence of the \? escape. Example:

          
          \?anything\?

anything may not contain newlines; use \! to embed newlines in a diversion. The escape sequence \? is also recognized in copy mode and turned into a single internal code; it is this code that terminates anything. Thus the following example prints 4.

          
          .nr x 1
          .nf
          .di d
          \?\\?\\\\?\\\\\\\\nx\\\\?\\?\?
          .di
          .nr x 2
          .di e
          .d
          .di
          .nr x 3
          .di f
          .e
          .di
          .nr x 4
          .f

Both escapes read the data in copy mode.

If \! is used in the top-level diversion, its argument is directly embedded into the gtroff intermediate output. This can be used for example to control a postprocessor which processes the data before it is sent to the device driver.

The \? escape used in the top-level diversion produces no output at all; its argument is simply ignored.

— Request: .output string

Emit string directly to the gtroff intermediate output (subject to copy mode interpretation); this is similar to \! used at the top level. An initial double quote in string is stripped off to allow initial blanks.

This request can't be used before the first page has started – if you get an error, simply insert .br before the output request.

Without argument, output is ignored.

Use with caution! It is normally only needed for mark-up used by a postprocessor which does something with the output before sending it to the output device, filtering out string again.

— Request: .asciify div

Unformat the diversion specified by div in such a way that ASCII characters, characters translated with the trin request, space characters, and some escape sequences that were formatted and diverted are treated like ordinary input characters when the diversion is reread. It can be also used for gross hacks; for example, the following sets register n to 1.

          
          .tr @.
          .di x
          @nr n 1
          .br
          .di
          .tr @@
          .asciify x
          .x

Note that asciify cannot return all items in a diversion back to their source equivalent, nodes such as \N[...] will still remain as nodes, so the result cannot be guaranteed to be a pure string.

See Copy-in Mode.

— Request: .unformat div

Like asciify, unformat the specified diversion. However, unformat only unformats spaces and tabs between words. Unformatted tabs are treated as input tokens, and spaces are stretchable again.

The vertical size of lines is not preserved; glyph information (font, font size, space width, etc.) is retained.