257 lines
6.6 KiB
Plaintext
257 lines
6.6 KiB
Plaintext
.\" This module is believed to contain source code proprietary to AT&T.
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.\" Use and redistribution is subject to the Berkeley Software License
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.\" Agreement and your Software Agreement with AT&T (Western Electric).
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.\"
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.\" @(#)tt10 8.1 (Berkeley) 6/8/93
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.\" Copyright (C) Caldera International Inc. 2001-2002. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions are
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.\" met:
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.\"
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.\" Redistributions of source code and documentation must retain the above
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.\" copyright notice, this list of conditions and the following
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.\" disclaimer.
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.\"
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.\" Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\"
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.\" This product includes software developed or owned by Caldera
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.\" International, Inc. Neither the name of Caldera International, Inc.
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.\" nor the names of other contributors may be used to endorse or promote
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.\" products derived from this software without specific prior written
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.\" permission.
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.\"
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.\" USE OF THE SOFTWARE PROVIDED FOR UNDER THIS LICENSE BY CALDERA
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.\" INTERNATIONAL, INC. AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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.\" DISCLAIMED. IN NO EVENT SHALL CALDERA INTERNATIONAL, INC. BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
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.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
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.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
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.\" OR OTHERWISE) RISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
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.\" IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.NH
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Number Registers and Arithmetic
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.PP
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.UL troff
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has a facility for doing arithmetic,
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and for defining and using variables with numeric values,
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called
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.ul
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number registers.
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Number registers, like strings and macros, can be useful in setting up a document
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so it is easy to change later.
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And of course they serve for any sort of arithmetic computation.
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.PP
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Like strings, number registers have one or two character names.
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They are set by the
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.BD .nr
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command,
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and are referenced anywhere by
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.BD \enx
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(one character name) or
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.BD \en(xy
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(two character name).
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.PP
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There are quite a few pre-defined number registers maintained by
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.UL troff ,
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among them
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.BD %
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for the current page number;
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.BD nl
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for the current vertical position on the page;
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.BD dy ,
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.BD mo
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and
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.BD yr
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for the current day, month and year; and
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.BD .s
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and
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.BD .f
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for the current size and font.
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(The font is a number from 1 to 4.)
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Any of these can be used in computations like any other register,
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but some, like
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.BD .s
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and
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.BD .f ,
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cannot be changed with
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.BD .nr .
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.PP
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As an example of the use of number registers,
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in the
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.BD \-ms
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macro package [4],
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most significant parameters are defined in terms of the values
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of a handful of number registers.
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These include the point size for text, the vertical spacing,
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and the line and title lengths.
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To set the point size and vertical spacing for the following paragraphs, for example, a user may say
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.P1
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^nr PS 9
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^nr VS 11
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.P2
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The paragraph macro
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.BD .PP
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is defined (roughly) as follows:
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.P1
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.ta 1i
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^de PP
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^ps \e\en(PS \e" reset size
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^vs \e\en(VSp \e" spacing
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^ft R \e" font
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^sp 0.5v \e" half a line
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^ti +3m
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^^
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.P2
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This sets the font to Roman and the point size and line spacing
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to whatever values are stored in the number registers
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.BD PS
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and
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.BD VS .
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.PP
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Why are there two backslashes?
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This is the eternal problem of how to quote a quote.
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When
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.UL troff
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originally reads the macro definition,
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it peels off one backslash
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to see what's coming next.
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To ensure that another is left in the definition when the
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macro is
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.ul
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used,
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we have to put in two backslashes in the definition.
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If only one backslash is used,
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point size and vertical spacing will be frozen at the time the macro
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is defined, not when it is used.
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.PP
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Protecting by an extra layer of backslashes
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is only needed for
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.BD \en ,
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.BD \e* ,
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.BD \e$
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(which we haven't come to yet),
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and
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.BD \e
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itself.
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Things like
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.BD \es ,
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.BD \ef ,
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.BD \eh ,
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.BD \ev ,
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and so on do not need an extra backslash,
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since they are converted by
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.UL troff
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to an internal code immediately upon being seen.
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.WS
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.PP
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Arithmetic expressions can appear anywhere that
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a number is expected.
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As a trivial example,
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.P1
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^nr PS \e\en(PS\-2
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.P2
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decrements PS by 2.
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Expressions can use the arithmetic operators +, \-, *, /, % (mod),
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the relational operators >, >=, <, <=, =, and != (not equal),
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and parentheses.
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.PP
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Although the arithmetic we have done so far
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has been straightforward,
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more complicated things are somewhat tricky.
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First,
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number registers hold only integers.
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.UL troff
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arithmetic uses truncating integer division, just like Fortran.
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Second, in the absence of parentheses,
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evaluation is done left-to-right
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without any operator precedence
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(including relational operators).
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Thus
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.P1
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7*\-4+3/13
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.P2
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becomes `\-1'.
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Number registers can occur anywhere in an expression,
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and so can scale indicators like
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.BD p ,
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.BD i ,
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.BD m ,
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and so on (but no spaces).
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Although integer division causes truncation,
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each number and its scale indicator is converted
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to machine units (1/432 inch) before any arithmetic is done,
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so
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1i/2u
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evaluates to
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0.5i
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correctly.
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.PP
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The scale indicator
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.BD u
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often has to appear
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when you wouldn't expect it _
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in particular, when arithmetic is being done
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in a context that implies horizontal or vertical dimensions.
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For example,
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.P1
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^ll 7/2i
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.P2
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would seem obvious enough _
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3\(12 inches.
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Sorry.
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Remember that the default units for horizontal parameters like
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.BD .ll
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are ems.
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That's really `7 ems / 2 inches',
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and when translated into machine units, it becomes zero.
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How about
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.P1
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^ll 7i/2
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.P2
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Sorry, still no good _
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the `2' is `2 ems', so `7i/2' is small,
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although not zero.
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You
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.ul
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must
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use
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.P1
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^ll 7i/2u
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.P2
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So again, a safe rule is to
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attach a scale indicator to every number,
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even constants.
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.PP
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For arithmetic done within a
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.BD .nr
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command,
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there is no implication of horizontal or vertical dimension,
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so the default units are `units',
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and 7i/2 and 7i/2u
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mean the same thing.
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Thus
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.P1
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^nr ll 7i/2
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^ll \e\en(llu
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.P2
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does just what you want,
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so long as you
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don't forget the
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.BD u
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on the
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.BD .ll
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command.
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