Conditional Blocks (The GNU Troff Manual)

+Next: while, Previous: Operators in Conditionals, Up: Conditionals and Loops [Contents][Index]

5.23.4 Conditional Blocks

+ + + +

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. +

Escape sequence: \{ ¶

Escape sequence: \} ¶

+ + + + + + + + + + + + + +

\{ 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
+.\}\}\}\}\}\}\}\}
+

+ + +