[unix-history] / usr / doc / trofftut / tt12

.NH
Conditionals
.PP
Suppose we want the
.BD .SH
macro 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
.BD .SH
macro
whether
the section number is 1,
and add some space if it is.
The
.BD .if
command provides the conditional test
that we can add
just before the heading line is output:
.P1 4
^if \e\en(SH=1 ^sp 2i	\e" first section only
.P2
.PP
The condition after the
.BD .if
can 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.
.PP
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 
.BD .S1
and invoke it
if we are about to do section 1
(as determined by an
.BD .if ).
.P1
^de S1
---  processing for section 1 ---
^^
^de SH
^^^
^if \e\en(SH=1 ^S1
^^^
^^
.P2
.PP
An alternate way is to use the
extended form of the
.BD .if ,
like this:
.P1
^if \e\en(SH=1 \e{--- processing
for section 1 ----\e}
.P2
The braces
.BD \e{
and
.BD \e}
must occur in the positions shown
or you will get unexpected extra lines in your output.
.UL troff
also provides
an `if-else' construction,
which we will not go into here.
.PP
A condition can be negated by preceding it with
.BD ! ;
we get the same effect as above (but less clearly) by using
.P1
^if !\e\en(SH>1 ^S1
.P2
.PP
There are a handful of 
other conditions that can be tested with
.BD .if .
For example, is the current page even or odd?
.P1
^if e ^tl ''even page title''
^if o ^tl ''odd page title''
.P2
gives facing pages different titles
when used inside an appropriate new page macro.
.PP
Two other conditions
are
.BD t
and
.BD n ,
which tell you whether the formatter is
.UL troff
or
.UL nroff . 
.P1
^if t troff stuff ...
^if n nroff stuff ...
.P2
.PP
Finally, string comparisons may be made in an
.BD .if :
.P1
^if  'string1'string2'  stuff
.P2
does `stuff' if
.ul
string1
is the same as
.ul
string2.
The character separating the strings can be anything
reasonable that is
not contained in either string.
The strings themselves can reference strings with
.BD \e* ,
arguments with 
.BD \e$ ,
and so on.
Commit	Line	Data
54b544da BK	1	.NH
	2	Conditionals
	3	.PP
	4	Suppose we want the
	5	.BD .SH
	6	macro to leave two extra inches of space just before section 1,
	7	but nowhere else.
	8	The cleanest way to do that is to test inside the
	9	.BD .SH
	10	macro
	11	whether
	12	the section number is 1,
	13	and add some space if it is.
	14	The
	15	.BD .if
	16	command provides the conditional test
	17	that we can add
	18	just before the heading line is output:
	19	.P1 4
	20	^if \e\en(SH=1 ^sp 2i \e" first section only
	21	.P2
	22	.PP
	23	The condition after the
	24	.BD .if
	25	can be any arithmetic or logical expression.
	26	If the condition is logically true, or arithmetically greater than zero,
	27	the rest of the line is treated as if
	28	it were text _
	29	here a command.
	30	If the condition is false, or zero or negative,
	31	the rest of the line is skipped.
	32	.PP
	33	It is possible to do more than one command if a condition is true.
	34	Suppose several operations are to be done before section 1.
	35	One possibility is to define a macro
	36	.BD .S1
	37	and invoke it
	38	if we are about to do section 1
	39	(as determined by an
	40	.BD .if ).
	41	.P1
	42	^de S1
	43	--- processing for section 1 ---
	44	^^
	45	^de SH
	46	^^^
	47	^if \e\en(SH=1 ^S1
	48	^^^
	49	^^
	50	.P2
	51	.PP
	52	An alternate way is to use the
	53	extended form of the
	54	.BD .if ,
	55	like this:
	56	.P1
	57	^if \e\en(SH=1 \e{--- processing
	58	for section 1 ----\e}
	59	.P2
	60	The braces
	61	.BD \e{
	62	and
	63	.BD \e}
	64	must occur in the positions shown
65	or you will get unexpected extra lines in your output.
66	.UL troff
67	also provides
68	an `if-else' construction,
69	which we will not go into here.
70	.PP
71	A condition can be negated by preceding it with
72	.BD ! ;
73	we get the same effect as above (but less clearly) by using
74	.P1
75	^if !\e\en(SH>1 ^S1
76	.P2
77	.PP
78	There are a handful of
79	other conditions that can be tested with
80	.BD .if .
81	For example, is the current page even or odd?
82	.P1
83	^if e ^tl ''even page title''
84	^if o ^tl ''odd page title''
85	.P2
86	gives facing pages different titles
87	when used inside an appropriate new page macro.
88	.PP
89	Two other conditions
90	are
91	.BD t
92	and
93	.BD n ,
94	which tell you whether the formatter is
95	.UL troff
96	or
97	.UL nroff .
98	.P1
99	^if t troff stuff ...
100	^if n nroff stuff ...
101	.P2
102	.PP
103	Finally, string comparisons may be made in an
104	.BD .if :
105	.P1
106	^if 'string1'string2' stuff
107	.P2
108	does `stuff' if
109	.ul
110	string1
111	is the same as
112	.ul
113	string2.
114	The character separating the strings can be anything
115	reasonable that is
116	not contained in either string.
117	The strings themselves can reference strings with
118	.BD \e* ,
119	arguments with
120	.BD \e$ ,
121	and so on.