Suppose we want the
.SHmacro to leave two extra inches of space just before section 1,
but nowhere else.
The cleanest way to do that is to test inside the
.SHmacro
whether
the section number is 1,
and add some space if it is.
The
.ifcommand provides the conditional test
that we can add
just before the heading line is output:
^if \\n(SH=1 ^sp 2i \" first section only
The condition after the .ifcan be any arithmetic or logical expression. If the condition is logically true, or arithmetically greater than zero, the rest of the line is treated as if it were text _ here a command. If the condition is false, or zero or negative, the rest of the line is skipped.
It is possible to do more than one command if a condition is true.
Suppose several operations are to be done before section 1.
One possibility is to define a macro
.S1and invoke it
if we are about to do section 1
(as determined by an
.if^de S1
--- processing for section 1 ---
^^
^de SH
^^^
^if \\n(SH=1 ^S1
^^^
^^
An alternate way is to use the
extended form of the
.iflike this:
^if \\n(SH=1 \{--- processing
for section 1 ----\}
The braces
\{and
\}must occur in the positions shown
or you will get unexpected extra lines in your output.
troff
also provides
an `if-else' construction,
which we will not go into here.
A condition can be negated by preceding it with
!we get the same effect as above (but less clearly) by using
^if !\\n(SH>1 ^S1
There are a handful of
other conditions that can be tested with
.ifFor example, is the current page even or odd?
^if e ^tl ''even page title''
^if o ^tl ''odd page title''
gives facing pages different titles
when used inside an appropriate new page macro.
Two other conditions
are
tand
nwhich tell you whether the formatter is
troff
or
nroff.
^if t troff stuff ...
^if n nroff stuff ...
Finally, string comparisons may be made in an
.if^if 'string1'string2' stuff
does `stuff' if
string1
is the same as
string2.
The character separating the strings can be anything
reasonable that is
not contained in either string.
The strings themselves can reference strings with
\*arguments with
\$and so on.