bc.1

Unix操作系统minix 2.0源码

.\"
.\" bc.1 - the *roff document processor source for the bc manual
.\"
.\" This file is part of bc written for MINIX.
.\" Copyright (C) 1991, 1992 Free Software Foundation, Inc.
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.\" You may contact the author by:
.\" e-mail: [e-mail]
.\" us-mail: Philip A. Nelson
.\" Computer Science Department, 9062
.\" Western Washington University
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.\"
.TH bc 1 .\" "Command Manual" v1.02 "Feb 3, 1992"
.SH NAME
bc - An arbitrary precision calculator language
.SH SYNTAX
\fBbc\fR [ \fB-lws\fR ] [ \fI file ...\fR ]
.SH VERSION
This man page documents GNU bc version 1.02.
.SH DESCRIPTION
\fBbc\fR is a language that supports arbitrary precision numbers
with interactive execution of statements.  There are some similarities
in the syntax to the C programming language. 
A standard math library is available by command line option.
If requested, the math library is defined before processing any files.
\fBbc\fR starts by processing code from all the files listed
on the command line in the order listed.  After all files have been
processed, \fBbc\fR reads from the standard input.  All code is
executed as it is read.  (If a file contains a command to halt the
processor, \fBbc\fR will never read from the standard input.)
.PP
This version of \fBbc\fR contains several extensions beyond
traditional \fBbc\fR implementations and the POSIX draft standard.
Command line options can cause these extensions to print a warning 
or to be rejected.  This 
document describes the language accepted by this processor.
Extensions will be identified as such.
.SS OPTIONS
.IP -l
Define the standard math library.
.IP -w
Give warnings for extensions to POSIX \fBbc\fR.
.IP -s
Process exactly the POSIX \fBbc\fR language.
.SS NUMBERS
The most basic element in \fBbc\fR is the number.  Numbers are
arbitrary precision numbers.  This precision is both in the integer
part and the fractional part.  All numbers are represented internally
in decimal and all computation is done in decimal.  (This version
truncates results from divide and multiply operations.)  There are two
attributes of numbers, the length and the scale.  The length is the
total number of significant decimal digits in a number and the scale
is the total number of decimal digits after the decimal point.  For
example:
.nf
.RS
 .000001 has a length of 6 and scale of 6.
 1935.000 has a length of 7 and a scale of 3.
.RE
.fi
.SS VARIABLES
Numbers are stored in two types of variables, simple variables and
arrays.  Both simple variables and array variables are named.  Names
begin with a letter followed by any number of letters, digits and
underscores.  All letters must be lower case.  (Full alpha-numeric
names are an extension. In POSIX \fBbc\fR all names are a single
lower case letter.)  The type of variable is clear by the context
because all array variable names will be followed by brackets ([]).
.PP
There are four special variables, \fBscale, ibase, obase,\fR and
\fBlast\fR.  \fBscale\fR defines how some operations use digits after the
decimal point.  The default value of \fBscale\fR is 0. \fBibase\fR
and \fBobase\fR define the conversion base for input and output
numbers.  The default for both input and output is base 10.
\fBlast\fR (an extension) is a variable that has the value of the last
printed number.  These will be discussed in further detail where
appropriate.  All of these variables may have values assigned to them
as well as used in expressions.
.SS COMMENTS
Comments in \fBbc\fR start with the characters \fB/*\fR and end with
the characters \fB*/\fR.  Comments may start anywhere and appear as a
single space in the input.  (This causes comments to delimit other
input items.  For example, a comment can not be found in the middle of
a variable name.)  Comments include any newlines (end of line) between
the start and the end of the comment.
.SS EXPRESSIONS
The numbers are manipulated by expressions and statements.  Since
the language was designed to be interactive, statements and expressions
are executed as soon as possible.  There is no "main" program.  Instead,
code is executed as it is encountered.  (Functions, discussed in
detail later, are defined when encountered.)
.PP
A simple expression is just a constant. \fBbc\fR converts constants
into internal decimal numbers using the current input base, specified
by the variable \fBibase\fR. (There is an exception in functions.)
The legal values of \fBibase\fR are 2 through 16 (F).  Assigning a
value outside this range to \fBibase\fR will result in a value of 2
or 16.  Input numbers may contain the characters 0-9 and A-F. (Note:
They must be capitals.  Lower case letters are variable names.)
Single digit numbers always have the value of the digit regardless of
the value of \fBibase\fR. (i.e. A = 10.)  For multi-digit numbers,
\fBbc\fR changes all input digits greater or equal to ibase to the
value of \fBibase\fR-1.  This makes the number \fBFFF\fR always be
the largest 3 digit number of the input base.
.PP
Full expressions are similar to many other high level languages.
Since there is only one kind of number, there are no rules for mixing
types.  Instead, there are rules on the scale of expressions.  Every
expression has a scale.  This is derived from the scale of original
numbers, the operation performed and in many cases, the value of the
variable \fBscale\fR. Legal values of the variable \fBscale\fR are
0 to the maximum number representable by a C integer.
.PP
In the following descriptions of legal expressions, "expr" refers to a
complete expression and "var" refers to a simple or an array variable.
A simple variable is just a
.RS
\fIname\fR
.RE
and an array variable is specified as
.RS
\fIname\fR[\fIexpr\fR]
.RE
Unless specifically
mentioned the scale of the result is the maximum scale of the
expressions involved.
.IP "- expr"
The result is the negation of the expression.
.IP "++ var"
The variable is incremented by one and the new value is the result of
the expression.
.IP "-- var"
The variable
is decremented by one and the new value is the result of the
expression.
.IP "var ++"
 The result of the expression is the value of
the variable and then the variable is incremented by one.
.IP "var --"
The result of the expression is the value of the variable and then
the variable is decremented by one.
.IP "expr + expr"
The result of the expression is the sum of the two expressions.
.IP "expr - expr"
The result of the expression is the difference of the two expressions.
.IP "expr * expr"
The result of the expression is the product of the two expressions.
.IP "expr / expr"
The result of the expression is the quotient of the two expressions.
The scale of the result is the value of the variable \fBscale\fR.
.IP "expr % expr"
The result of the expression is the "remainder" and it is computed in the
following way.  To compute a%b, first a/b is computed to \fBscale\fR
digits.  That result is used to compute a-(a/b)*b to the scale of the
maximum of \fBscale\fR+scale(b) and scale(a).  If \fBscale\fR is set
to zero and both expressions are integers this expression is the
integer remainder function.
.IP "expr ^ expr"
The result of the expression is the value of the first raised to the
second. The second expression must be an integer.  (If the second
expression is not an integer, a warning is generated and the
expression is truncated to get an integer value.)  The scale of the
result is \fBscale\fR if the exponent is negative.  If the exponent
is positive the scale of the result is the minimum of the scale of the
first expression times the value of the exponent and the maximum of
\fBscale\fR and the scale of the first expression.  (e.g. scale(a^b)
= min(scale(a)*b, max( \fBscale,\fR scale(a))).)  It should be noted
that expr^0 will always return the value of 1.
.IP "( expr )"
This alters the standard precedence to force the evaluation of the
expression.
.IP "var = expr"
The variable is assigned the value of the expression.
.IP "var <op>= expr"
This is equivalent to "var = var <op> expr" with the exception that
the "var" part is evaluated only once.  This can make a difference if
"var" is an array.
.PP
 Relational expressions are a special kind of expression
that always evaluate to 0 or 1, 0 if the relation is false and 1 if
the relation is true.  These may appear in any legal expression.
(POSIX bc requires that relational expressions are used only in if,
while, and for statements and that only one relational test may be
done in them.)  The relational operators are
.IP "expr1 < expr2"
The result is 1 if expr1 is strictly less than expr2.
.IP "expr1 <= expr2"
The result is 1 if expr1 is less than or equal to expr2.
.IP "expr1 > expr2"
The result is 1 if expr1 is strictly greater than expr2.
.IP "expr1 >= expr2"
The result is 1 if expr1 is greater than or equal to expr2.
.IP "expr1 == expr2"
The result is 1 if expr1 is equal to expr2.
.IP "expr1 != expr2"
The result is 1 if expr1 is not equal to expr2.
.PP
Boolean operations are also legal.  (POSIX \fBbc\fR does NOT have
boolean operations). The result of all boolean operations are 0 and 1
(for false and true) as in relational expressions.  The boolean
operators are:
.IP "!expr"
The result is 1 if expr is 0.
.IP "expr && expr"
The result is 1 if both expressions are non-zero.
.IP "expr || expr"
The result is 1 if either expression is non-zero.
.PP
The expression precedence is as follows: (lowest to highest)
.nf
.RS
|| operator, left associative
&& operator, left associative
! operator, nonassociative
Relational operators, left associative
Assignment operator, right associative
+ and - operators, left associative
*, / and % operators, left associative
^ operator, right associative
unary - operator, nonassociative
++ and -- operators, nonassociative
.RE
.fi
.PP
This precedence was chosen so that POSIX compliant \fBbc\fR programs
will run correctly. This will cause the use of the relational and
logical operators to have some unusual behavior when used with
assignment expressions.  Consider the expression: