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1.1 root 1: .\" @(#)tt11 6.1 (Berkeley) 5/23/86
2: .\"
3: .NH
4: Macros with arguments
5: .PP
6: The next step is to define macros that can change from one
7: use to the next
8: according to parameters supplied as arguments.
9: To make this work, we need two things:
10: first, when we define the macro, we have to indicate that some
11: parts of it will be provided as arguments when the macro is called.
12: Then when the macro is
13: called
14: we have to provide actual arguments
15: to be plugged into the definition.
16: .PP
17: Let us illustrate by defining a macro
18: .BD .SM
19: that will print its argument two points
20: smaller than the surrounding text.
21: That is, the macro call
22: .P1
23: ^SM TROFF
24: .P2
25: will produce
26: .UC TROFF .
27: .PP
28: The definition of
29: .BD .SM
30: is
31: .P1
32: ^de SM
33: \es\-2\e\e$1\es+2
34: ^^
35: .P2
36: Within a macro definition,
37: the symbol
38: .BD \e\e$n
39: refers to the
40: .BD n th
41: argument
42: that the macro was called with.
43: Thus
44: .BD \e\e$1
45: is the string to be placed in a smaller point
46: size when
47: .BD .SM
48: is called.
49: .PP
50: As a slightly more complicated version, the following definition of
51: .BD .SM
52: permits optional second and third arguments
53: that will be printed in the normal size:
54: .P1
55: ^de SM
56: \e\e$3\es\-2\e\e$1\es+2\e\e$2
57: ^^
58: .P2
59: Arguments not provided when the macro is called are treated
60: as empty,
61: so
62: .P1
63: ^SM TROFF ),
64: .P2
65: produces
66: .UC TROFF ),
67: while
68: .P1
69: ^SM TROFF ). (
70: .P2
71: produces
72: .UC TROFF ). (
73: It is convenient to reverse
74: the order of arguments because trailing punctuation
75: is much more common than leading.
76: .PP
77: By the way, the number of arguments that a macro was called with
78: is available in number register
79: .BD .$ .
80: .PP
81: The following macro
82: .BD ^BD
83: is the one used to make the
84: `bold roman' we have been using for
85: .UL troff
86: command names in text.
87: It combines horizontal motions, width computations,
88: and argument rearrangement.
89: .P1 2
90: \&.de BD
91: \e&\e\e$3\ef1\e\e$1\eh'\-\ew'\e\e$1'u+1u'\e\e$1\efP\e\e$2
92: \&..
93: .P2
94: The
95: .BD \eh
96: and
97: .BD \ew
98: commands need no extra backslash, as we discussed above.
99: The
100: .BD \e&
101: is there in case the argument begins with a period.
102: .WS
103: .PP
104: Two backslashes are needed with the
105: .BD \e\e$n
106: commands, though,
107: to protect one of them when the macro is
108: being defined.
109: Perhaps a second example will make this clearer.
110: Consider a macro called
111: .BD .SH
112: which
113: produces section headings rather like those in this paper,
114: with the sections numbered automatically,
115: and the title in bold in a smaller size.
116: The use is
117: .P1
118: ^SH "Section title ..."
119: .P2
120: (If the argument to a macro is to contain blanks,
121: then it must be
122: .ul
123: surrounded
124: by double quotes,
125: unlike a string, where only one leading quote is permitted.)
126: .PP
127: Here is the definition of the
128: .BD .SH
129: macro:
130: .P1
131: .ta .75i 1.15i
132: ^nr SH 0 \e" initialize section number
133: ^de SH
134: ^sp 0.3i
135: ^ft B
136: ^nr SH \e\en(SH+1 \e" increment number
137: ^ps \e\en(PS\-1 \e" decrease PS
138: \e\en(SH. \e\e$1 \e" number. title
139: ^ps \e\en(PS \e" restore PS
140: ^sp 0.3i
141: ^ft R
142: ^^
143: .P2
144: The section number is kept in number register SH, which is incremented each
145: time just before it is used.
146: (A number register may have the same name as a macro
147: without conflict but a string may not.)
148: .PP
149: We used
150: .BD \e\en(SH
151: instead of
152: .BD \en(SH
153: and
154: .BD \e\en(PS
155: instead of
156: .BD \en(PS .
157: If we had used
158: .BD \en(SH ,
159: we would get the value of the register at the time the macro was
160: .ul
161: defined,
162: not at the time it was
163: .ul
164: used.
165: If that's what you want, fine,
166: but not here.
167: Similarly,
168: by using
169: .BD \e\en(PS ,
170: we get the point size at the time the macro is called.
171: .WS
172: .PP
173: As an example that does not involve numbers,
174: recall our
175: .BD .NP
176: macro which had a
177: .P1
178: ^tl 'left'center'right'
179: .P2
180: We could make these into parameters by using instead
181: .P1
182: ^tl '\e\e*(LT'\e\e*(CT'\e\e*(RT'
183: .P2
184: so the title comes from three strings called LT, CT and RT.
185: If these are empty, then the title will be a blank line.
186: Normally CT would be set with something like
187: .P1
188: \&^ds CT - % -
189: .P2
190: to give just the page number between hyphens (as on the top of this page),
191: but a user could supply private definitions for
192: any of the strings.
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