5.20.2 if-else

gtroff has if-then-else constructs like other languages, although the formatting can be painful.

Request: .if expr anything

Evaluate the expression expr, and executes anything (the remainder of the line) if expr evaluates to a value greater than zero (true). anything is interpreted as though it was on a line by itself (except that leading spaces are swallowed). See Operators in Conditionals, for more info.

.nr xxx 1
.nr yyy 2
.if ((\n[xxx] == 1) & (\n[yyy] == 2)) true
    ⇒ true

Request: .nop anything

Executes anything. This is similar to .if 1.

Request: .ie expr anything

Request: .el anything

Use the ie and el requests to write an if-then-else. The first request is the ‘if’ part and the latter is the ‘else’ part.

.ie n .ls 2 \" double-spacing in nroff
.el   .ls 1 \" single-spacing in troff

Escape: \{

Escape: \}

In many cases, an if (or if-else) construct needs to execute more than one request. This can be done using the escapes \{ (which must start the first line) and \} (which must end the last line).

.ie t \{\
.    ds lq ``
.    ds rq ''
.\}
.el \{\
.    ds lq ""
.    ds rq ""
.\}

See Expressions.