bc.1

Unix操作系统minix 2.0源码

.RS
a = 3 < 5
.RE
.PP
Most C programmers would assume this would assign the result of "3 <
5" (the value 1) to the variable "a".  What this does in \fBbc\fR is
assign the value 3 to the variable "a" and then compare 3 to 5.  It is
best to use parenthesis when using relational and logical operators
with the assignment operators.
.PP
There are a few more special expressions that are provided in \fBbc\fR.
These have to do with user defined functions and standard
functions.  They all appear as "\fIname\fB(\fIparameters\fB)\fR".
See the section on functions for user defined functions.  The standard
functions are:
.IP "length ( expression )"
The value of the length function is the number of significant digits in the
expression.
.IP "read ( )"
The read function (an extension) will read a number from the standard
input, regardless of where the function occurs.   Beware, this can
cause problems with the mixing of data and program in the standard input.
The best use for this function is in a previously written program that
needs input from the user, but never allows program code to be input
from the user.  The value of the read function is the number read from
the standard input using the current value of the variable 
\fBibase\fR for the conversion base.
.IP "scale ( expression )"
The value of the scale function is the number of digits after the decimal
point in the expression.
.IP "sqrt ( expression )"
The value of the sqrt function is the square root of the expression.  If
the expression is negative, a run time error is generated.
.SS STATEMENTS
Statements (as in most algebraic languages) provide the sequencing of
expression evaluation.  In \fBbc\fR statements are executed "as soon
as possible."  Execution happens when a newline in encountered and
there is one or more complete statements.  Due to this immediate
execution, newlines are very important in \fBbc\fR. In fact, both a
semicolon and a newline are used as statement separators.  An
improperly placed newline will cause a syntax error.  Because newlines
are statement separators, it is possible to hide a newline by using
the backslash character.  The sequence "\e<nl>", where <nl> is the
newline appears to \fBbc\fR as whitespace instead of a newline.  A
statement list is a series of statements separated by semicolons and
newlines.  The following is a list of \fBbc\fR statements and what
they do: (Things enclosed in brackets ([]) are optional parts of the
statement.)
.IP "expression"
This statement does one of two things.  If the expression starts with
"<variable> <assignment> ...", it is considered to be an assignment
statement.  If the expression is not an assignment statement, the
expression is evaluated and printed to the output.  After the number
is printed, a newline is printed.  For example, "a=1" is an assignment
statement and "(a=1)" is an expression that has an embedded
assignment.  All numbers that are printed are printed in the base
specified by the variable \fBobase\fR. The legal values for \fB
obase\fR are 2 through BC_BASE_MAX.  (See the section LIMITS.)  For
bases 2 through 16, the usual method of writing numbers is used.  For
bases greater than 16, \fBbc\fR uses a multi-character digit method
of printing the numbers where each higher base digit is printed as a
base 10 number.  The multi-character digits are separated by spaces.
Each digit contains the number of characters required to represent the
base ten value of "obase-1".  Since numbers are of arbitrary
precision, some numbers may not be printable on a single output line.
These long numbers will be split across lines using the "\e" as the
last character on a line.  The maximum number of characters printed
per line is 70.  Due to the interactive nature of \fBbc\fR printing
a number cause the side effect of assigning the printed value the the
special variable \fBlast\fR. This allows the user to recover the
last value printed without having to retype the expression that
printed the number.  Assigning to \fBlast\fR is legal and will
overwrite the last printed value with the assigned value.  The newly
assigned value will remain until the next number is printed or another
value is assigned to \fBlast\fR.
.IP "string"
The string is printed to the output.  Strings start with a double quote
character and contain all characters until the next double quote character.
All characters are take literally, including any newline.  No newline
character is printed after the string.
.IP "\fBprint\fR list"
The print statement (an extension) provides another method of output.
The "list" is a list of strings and expressions separated by commas.
Each string or expression is printed in the order of the list.  No
terminating newline is printed.  Expressions are evaluated and their
value is printed and assigned the the variable \fBlast\fR. Strings
in the print statement are printed to the output and may contain
special characters.  Special characters start with the backslash
character (\e).  The special characters recognized by \fBbc\fR are
"b" (bell), "f" (form feed), "n" (newline), "r" (carriage return), "t"
(tab), and "\e" (backslash).  Any other character following the
backslash will be ignored.  This still does not allow the double quote
character to be part of any string.
.IP "{ statement_list }"
This is the compound statement.  It allows multiple statements to be
grouped together for execution.
.IP "\fBif\fR ( expression ) \fBthen\fR statement1 [\fBelse\fR statement2]"
The if statement evaluates the expression and executes statement1 or
statement2 depending on the value of the expression.  If the expression
is non-zero, statement1 is executed.  If statement2 is present and
the value of the expression is 0, then statement2 is executed.  (The
else clause is an extension.)
.IP "\fBwhile\fR ( expression ) statement"
The while statement will execute the statement while the expression
is non-zero.  It evaluates the expression before each execution of
the statement.   Termination of the loop is caused by a zero
expression value or the execution of a break statement.
.IP "\fBfor\fR ( [expression1] ; [expression2] ; [expression3] ) statement"
The for statement controls repeated execution of the statement.  
Expression1 is evaluated before the loop.  Expression2 is evaluated
before each execution of the statement.  If it is non-zero, the statement
is evaluated.  If it is zero, the loop is terminated.  After each
execution of the statement, expression3 is evaluated before the reevaluation
of expression2.  If expression1 or expression3 are missing, nothing is
evaluated at the point they would be evaluated.
If expression2 is missing, it is the same as substituting
the value 1 for expression2.  (The optional expressions are an
extension. POSIX \fBbc\fR requires all three expressions.)
The following is equivalent code for the for statement:
.nf
.RS
expression1;
while (expression2) {
   statement;
   expression3;
}
.RE
.fi
.IP "\fBbreak\fR"
This statement causes a forced exit of the most recent enclosing while
statement or for statement.
.IP "\fBcontinue\fR"
The continue statement (an extension)  causes the most recent enclosing
for statement to start the next iteration.
.IP "\fBhalt\fR"
The halt statement (an extension) is an executed statement that causes
the \fBbc\fR processor to quit only when it is executed.  For example,
"if (0 == 1) halt" will not cause \fBbc\fR to terminate because the halt is
not executed.
.IP "\fBreturn\fR"
Return the value 0 from a function.  (See the section on functions.)
.IP "\fBreturn\fR ( expression )"
Return the value of the expression from a function.  (See the section on 
functions.)
.SS PSEUDO STATEMENTS
These statements are not statements in the traditional sense.  They are
not executed statements.  Their function is performed at "compile" time.
.IP "\fBlimits\fR"
Print the local limits enforced by the local version of \fBbc\fR.  This
is an extension.
.IP "\fBquit\fR"
When the quit statement is read, the \fBbc\fR processor
is terminated, regardless of where the quit statement is found.  For
example, "if (0 == 1) quit" will cause \fBbc\fR to terminate.
.IP "\fBwarranty\fR"
Print a longer warranty notice.  This is an extension.
.SS FUNCTIONS
Functions provide a method of defining a computation that can be executed
later.  Functions in 
.B bc
always compute a value and return it to the caller.  Function definitions
are "dynamic" in the sense that a function is undefined until a definition
is encountered in the input.  That definition is then used until another
definition function for the same name is encountered.  The new definition
then replaces the older definition.  A function is defined as follows:
.nf
.RS
\fBdefine \fIname \fB( \fIparameters \fB) { \fInewline
\fI    auto_list   statement_list \fB}\fR
.RE
.fi
A function call is just an expression of the form
"\fIname\fB(\fIparameters\fB)\fR".
.PP
Parameters are numbers or arrays (an extension).  In the function definition,
zero or more parameters are defined by listing their names separated by
commas.  Numbers are only call by value parameters.  Arrays are only
call by variable.  Arrays are specified in the parameter definition by
the notation "\fIname\fB[]\fR".   In the function call, actual parameters
are full expressions for number parameters.  The same notation is used
for passing arrays as for defining array parameters.  The named array is
passed by variable to the function.  Since function definitions are dynamic,
parameter numbers and types are checked when a function is called.  Any
mismatch in number or types of parameters will cause a runtime error.
A runtime error will also occur for the call to an undefined function.
.PP
The \fIauto_list\fR is an optional list of variables that are for
"local" use.  The syntax of the auto list (if present) is "\fBauto
\fIname\fR, ... ;".  (The semicolon is optional.)  Each \fIname\fR is
the name of an auto variable.  Arrays may be specified by using the
same notation as used in parameters.  These variables have their
values pushed onto a stack at the start of the function.  The
variables are then initialized to zero and used throughout the
execution of the function.  At function exit, these variables are
popped so that the original value (at the time of the function call)
of these variables are restored.  The parameters are really auto
variables that are initialized to a value provided in the function
call.  Auto variables are different than traditional local variables
in the fact that if function A calls function B, B may access function
A's auto variables by just using the same name, unless function B has
called them auto variables.  Due to the fact that auto variables and
parameters are pushed onto a stack, \fBbc\fR supports recursive functions.
.PP
The function body is a list of \fBbc\fR statements.  Again, statements
are separated by semicolons or newlines.  Return statements cause the
termination of a function and the return of a value.  There are two
versions of the return statement.  The first form, "\fBreturn\fR", returns
the value 0 to the calling expression.  The second form, 
"\fBreturn ( \fIexpression \fB)\fR", computes the value of the expression
and returns that value to the calling expression.  There is an implied
"\fBreturn (0)\fR" at the end of every function.  This allows a function
to terminate and return 0 without an explicit return statement.
.PP
Functions also change the usage of the variable \fBibase\fR.  All
constants in the function body will be converted using the value of
\fBibase\fR at the time of the function call.  Changes of \fBibase\fR
will be ignored during the execution of the function except for the
standard function \fBread\fR, which will always use the current value
of \fBibase\fR for conversion of numbers.
.SS MATH LIBRARY
If \fBbc\fR is invoked with the \fB-l\fR option, a math library is preloaded
and the default scale is set to 20.   The math functions will calculate their
results to the scale set at the time of their call.  
The math library defines the following functions:
.IP "s (\fIx\fR)"
The sine of x in radians.
.IP "c (\fIx\fR)"
The cosine of x in radians.
.IP "a (\fIx\fR)"
The arctangent of x.
.IP "l (\fIx\fR)"
The natural logarithm of x.
.IP "e (\fIx\fR)"
The exponential function of raising e to the value x.
.IP "j (\fIn,x\fR)"
The bessel function of integer order n of x.
.SS EXAMPLES
In /bin/sh,  the following will assign the value of "pi" to the shell
variable \fBpi\fR.
.RS
\fB
pi=$(echo "scale=10; 4*a(1)" | bc -l)
\fR
.RE