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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.

.if 0 \{ B
    ⇒ A F
.if 1 \{ O
.  if 0 \{ P
R\} S\} T
    ⇒ 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

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