It is frequently desirable for a control structure to govern more than
one request, macro call, text line, or a combination of the foregoing.
The opening and closing brace escape sequences \{ and \}
define such groups. These conditional blocks can furthermore be
nested.
\{ ¶\} ¶\{ begins a conditional block; it must appear (after optional
spaces and tabs) immediately subsequent to the conditional expression of
an if, ie, or while
request,93 or as the argument to an el
request.
\} ends a condition block and should appear on a line with other
occurrences of itself as necessary to match \{ sequences. It
can be preceded by a control character, spaces, and tabs. Input after
any quantity of \} sequences on the same line is processed only
if all of the preceding conditions to which they correspond are true.
Furthermore, a \} closing the body of a while request
must be the last such escape sequence on an input line.
Brace escape sequences outside of control structures have no meaning and produce no output.
Caution: Input lines using \{ often end with
\RET, especially in macros that consist primarily of control
lines. Forgetting to use \RET on an input line after \{
is a common source of error.
We might write the following in a page header macro. If we delete
\RET, the header will carry an unwanted extra empty line (except
on page 1).
.if (\\n[%] != 1) \{\
. ie ((\\n[%] % 2) = 0) .tl \\*[even-numbered-page-title]
. el .tl \\*[odd-numbered-page-title]
.\}
Let us take a closer look at how conditional blocks nest.
A
.if 0 \{ B
C
D
\}E
F
⇒ A F
N
.if 1 \{ O
. if 0 \{ P
Q
R\} S\} T
U
⇒ N O U
The above behavior may challenge the intuition; it was implemented to
retain compatibility with AT&T troff. For clarity, it
is idiomatic to end input lines with \{ (followed by
\RET if appropriate), and to precede \} on an input
line with nothing more than a control character, spaces, tabs, and other
instances of itself.
We can use ie, el, and conditional blocks to simulate the
multi-way “switch” or “case” control structures of other languages.
The following example is adapted from the groff man
package. Indentation is used to clarify the logic.
.\" Simulate switch/case in roff.
. ie '\\$2'1' .ds title General Commands\"
.el \{.ie '\\$2'2' .ds title System Calls\"
.el \{.ie '\\$2'3' .ds title Library Functions\"
.el \{.ie '\\$2'4' .ds title Kernel Interfaces\"
.el \{.ie '\\$2'5' .ds title File Formats\"
.el \{.ie '\\$2'6' .ds title Games\"
.el \{.ie '\\$2'7' .ds title Miscellaneous Information\"
.el \{.ie '\\$2'8' .ds title System Management\"
.el \{.ie '\\$2'9' .ds title Kernel Development\"
.el .ds title \" empty
.\}\}\}\}\}\}\}\}