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bc
SYNTAX
bc [ -hlwsqv ] [long-options] [ file ... ]
DESCRIPTION
bc is a language that supports arbitrary precision numbers with inter-
active 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. bc starts by processing code from
all the files listed on the command line in the order listed. After
all files have been processed, bc reads from the standard input. All
code is executed as it is read. (If a file contains a command to halt
the processor, bc will never read from the standard input.)
This version of bc contains several extensions beyond traditional bc
implementations and the POSIX draft standard. Command line options can
cause these extensions to print a warning or to be rejected. This doc-
ument describes the language accepted by this processor. Extensions
will be identified as such.
OPTIONS
-h, --help
Print the usage and exit.
-i, --interactive
Force interactive mode.
-l, --mathlib
Define the standard math library.
-w, --warn
Give warnings for extensions to POSIX bc.
-s, --standard
Process exactly the POSIX bc language.
-q, --quiet
Do not print the normal GNU bc welcome.
-v, --version
Print the version number and copyright and quit.
NUMBERS
The most basic element in bc is the number. Numbers are arbitrary pre-
cision 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 num-
bers, the length and the scale. The length is the total number of sig-
nificant decimal digits in a number and the scale is the total number
of decimal digits after the decimal point. For example:
.000001 has a length of 6 and scale of 6.
for input and output numbers. The default for both input and output is
base 10. last (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.
COMMENTS
Comments in bc start with the characters /* and end with the characters
*/. Comments may start anywhere and appear as a single space in the
input. (This causes comments to delimit other input items. For exam-
ple, 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.
To support the use of scripts for bc, a single line comment has been
added as an extension. A single line comment starts at a # character
and continues to the next end of the line. The end of line character
is not part of the comment and is processed normally.
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.)
A simple expression is just a constant. bc converts constants into
internal decimal numbers using the current input base, specified by the
variable ibase. (There is an exception in functions.) The legal values
of ibase are 2 through 16. Assigning a value outside this range to
ibase 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 let-
ters are variable names.) Single digit numbers always have the value
of the digit regardless of the value of ibase. (i.e. A = 10.) For
multi-digit numbers, bc changes all input digits greater or equal to
ibase to the value of ibase-1. This makes the number FFF always be the
largest 3 digit number of the input base.
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 scale.
Legal values of the variable scale are 0 to the maximum number repre-
sentable by a C integer.
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
name
and an array variable is specified as
name[expr]
var -- The result of the expression is the value of the variable and
then the variable is decremented by one.
expr + expr
The result of the expression is the sum of the two expressions.
expr - expr
The result of the expression is the difference of the two
expressions.
expr * expr
The result of the expression is the product of the two expres-
sions.
expr / expr
The result of the expression is the quotient of the two expres-
sions. The scale of the result is the value of the variable
scale.
expr % expr
The result of the expression is the "remainder" and it is com-
puted in the following way. To compute a%b, first a/b is com-
puted to scale digits. That result is used to compute a-(a/b)*b
to the scale of the maximum of scale+scale(b) and scale(a). If
scale is set to zero and both expressions are integers this
expression is the integer remainder function.
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 scale 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 expo-
nent and the maximum of scale and the scale of the first expres-
sion. (e.g. scale(a^b) = min(scale(a)*b, max( scale,
scale(a))).) It should be noted that expr^0 will always return
the value of 1.
( expr )
This alters the standard precedence to force the evaluation of
the expression.
var = expr
The variable is assigned the value of the expression.
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.
The result is 1 if expr1 is strictly greater than expr2.
expr1 >= expr2
The result is 1 if expr1 is greater than or equal to expr2.
expr1 == expr2
The result is 1 if expr1 is equal to expr2.
expr1 != expr2
The result is 1 if expr1 is not equal to expr2.
Boolean operations are also legal. (POSIX bc 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:
!expr The result is 1 if expr is 0.
expr && expr
The result is 1 if both expressions are non-zero.
expr || expr
The result is 1 if either expression is non-zero.
The expression precedence is as follows: (lowest to highest)
|| 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
This precedence was chosen so that POSIX compliant bc programs will run
correctly. This will cause the use of the relational and logical opera-
tors to have some unusual behavior when used with assignment expres-
sions. Consider the expression:
a = 3 < 5
Most C programmers would assume this would assign the result of "3 < 5"
(the value 1) to the variable "a". What this does in bc 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.
There are a few more special expressions that are provided in bc.
These have to do with user defined functions and standard functions.
They all appear as "name(parameters)". See the section on functions
for user defined functions. The standard functions are:
version base.
scale ( expression )
The value of the scale function is the number of digits after
the decimal point in the expression.
sqrt ( expression )
The value of the sqrt function is the square root of the expres-
sion. If the expression is negative, a run time error is gener-
ated.
STATEMENTS
Statements (as in most algebraic languages) provide the sequencing of
expression evaluation. In bc statements are executed "as soon as pos-
sible." Execution happens when a newline in encountered and there is
one or more complete statements. Due to this immediate execution, new-
lines are very important in bc. 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 "\<nl>", where <nl> is the newline appears to bc as whitespace
instead of a newline. A statement list is a series of statements sepa-
rated by semicolons and newlines. The following is a list of bc state-
ments and what they do: (Things enclosed in brackets ([]) are optional
parts of the statement.)
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 out-
put. 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
obase. The legal values for obase 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,
bc 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 preci-
sion, some numbers may not be printable on a single output line.
These long numbers will be split across lines using the "\" as
the last character on a line. The maximum number of characters
printed per line is 70. Due to the interactive nature of bc,
printing a number causes the side effect of assigning the
printed value to the special variable last. This allows the user
to recover the last value printed without having to retype the
expression that printed the number. Assigning to last is legal
and will overwrite the last printed value with the assigned
rated by commas. Each string or expression is printed in the
order of the list. No terminating newline is printed. Expres-
sions are evaluated and their value is printed and assigned to
the variable last. Strings in the print statement are printed to
the output and may contain special characters. Special charac-
ters start with the backslash character (\). The special char-
acters recognized by bc are "a" (alert or bell), "b"
(backspace), "f" (form feed), "n" (newline), "r" (carriage
return), "q" (double quote), "t" (tab), and "\" (backslash).
Any other character following the backslash will be ignored.
{ statement_list }
This is the compound statement. It allows multiple statements
to be grouped together for execution.
if ( expression ) statement1 [else statement2]
The if statement evaluates the expression and executes state-
ment1 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.)
while ( expression ) statement
The while statement will execute the statement while the expres-
sion is non-zero. It evaluates the expression before each exe-
cution of the statement. Termination of the loop is caused by
a zero expression value or the execution of a break statement.
for ( [expression1] ; [expression2] ; [expression3] ) statement
The for statement controls repeated execution of the statement.
Expression1 is evaluated before the loop. Expression2 is evalu-
ated before each execution of the statement. If it is non-zero,
the statement is evaluated. If it is zero, the loop is termi-
nated. After each execution of the statement, expression3 is
evaluated before the reevaluation of expression2. If expres-
sion1 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 bc requires all
three expressions.) The following is equivalent code for the
for statement:
expression1;
while (expression2) {
statement;
expression3;
}
break This statement causes a forced exit of the most recent enclosing
while statement or for statement.
continue
The continue statement (an extension) causes the most recent
PSEUDO STATEMENTS
These statements are not statements in the traditional sense. They are
not executed statements. Their function is performed at "compile"
time.
limits Print the local limits enforced by the local version of bc.
This is an extension.
quit When the quit statement is read, the bc processor is terminated,
regardless of where the quit statement is found. For example,
"if (0 == 1) quit" will cause bc to terminate.
warranty
Print a longer warranty notice. This is an extension.
FUNCTIONS
Functions provide a method of defining a computation that can be exe-
cuted later. Functions in 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:
define name ( parameters ) { newline
auto_list statement_list }
A function call is just an expression of the form "name(parameters)".
Parameters are numbers or arrays (an extension). In the function defi-
nition, zero or more parameters are defined by listing their names sep-
arated by commas. All parameters are call by value parameters. Arrays
are specified in the parameter definition by the notation "name[]".
In the function call, actual parameters are full expressions for number
parameters. The same notation is used for passing arrays as for defin-
ing array parameters. The named array is passed by value to the func-
tion. 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.
The auto_list is an optional list of variables that are for "local"
use. The syntax of the auto list (if present) is "auto name, ... ;".
(The semicolon is optional.) Each name is the name of an auto vari-
able. 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 because if function A calls function B, B may access function
Functions also change the usage of the variable ibase. All constants
in the function body will be converted using the value of ibase at the
time of the function call. Changes of ibase will be ignored during the
execution of the function except for the standard function read, which
will always use the current value of ibase for conversion of numbers.
Several extensions have been added to functions. First, the format of
the definition has been slightly relaxed. The standard requires the
opening brace be on the same line as the define keyword and all other
parts must be on following lines. This version of bc will allow any
number of newlines before and after the opening brace of the function.
For example, the following definitions are legal.
define d (n) { return (2*n); }
define d (n)
{ return (2*n); }
Functions may be defined as void. A void funtion returns no value and
thus may not be used in any place that needs a value. A void function
does not produce any output when called by itself on an input line.
The key word void is placed between the key word define and the func-
tion name. For example, consider the following session.
define py (y) { print "--->", y, "<---", "0; }
define void px (x) { print "--->", x, "<---", "0; }
py(1)
--->1<---
0
px(1)
--->1<---
Since py is not a void function, the call of py(1) prints the desired
output and then prints a second line that is the value of the function.
Since the value of a function that is not given an explicit return
statement is zero, the zero is printed. For px(1), no zero is printed
because the function is a void function.
Also, call by variable for arrays was added. To declare a call by
variable array, the declaration of the array parameter in the function
definition looks like "*name[]". The call to the function remains the
same as call by value arrays.
MATH LIBRARY
If bc is invoked with the -l 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:
s (x) The sine of x, x is in radians.
c (x) The cosine of x, x is in radians.
a (x) The arctangent of x, arctangent returns radians.
The following is the definition of the exponential function used in the
math library. This function is written in POSIX bc.
scale = 20
/* Uses the fact that e^x = (e^(x/2))^2
When x is small enough, we use the series:
e^x = 1 + x + x^2/2! + x^3/3! + ...
*/
define e(x) {
auto a, d, e, f, i, m, v, z
/* Check the sign of x. */
if (x<0) {
m = 1
x = -x
}
/* Precondition x. */
z = scale;
scale = 4 + z + .44*x;
while (x > 1) {
f += 1;
x /= 2;
}
/* Initialize the variables. */
v = 1+x
a = x
d = 1
for (i=2; 1; i++) {
e = (a *= x) / (d *= i)
if (e == 0) {
if (f>0) while (f--) v = v*v;
scale = z
if (m) return (1/v);
return (v/1);
}
v += e
}
}
The following is code that uses the extended features of bc to imple-
ment a simple program for calculating checkbook balances. This program
is best kept in a file so that it can be used many times without having
to retype it at every use.
scale=2
print "\nCheck book program!\n"
print " Remember, deposits are negative transactions.\n"
The following is the definition of the recursive factorial function.
define f (x) {
if (x <= 1) return (1);
return (f(x-1) * x);
}
READLINE AND LIBEDIT OPTIONS
GNU bc can be compiled (via a configure option) to use the GNU readline
input editor library or the BSD libedit library. This allows the user
to do editing of lines before sending them to bc. It also allows for a
history of previous lines typed. When this option is selected, bc has
one more special variable. This special variable, history is the num-
ber of lines of history retained. For readline, a value of -1 means
that an unlimited number of history lines are retained. Setting the
value of history to a positive number restricts the number of history
lines to the number given. The value of 0 disables the history fea-
ture. The default value is 100. For more information, read the user
manuals for the GNU readline, history and BSD libedit libraries. One
can not enable both readline and libedit at the same time.
DIFFERENCES
This version of bc was implemented from the POSIX P1003.2/D11 draft and
contains several differences and extensions relative to the draft and
traditional implementations. It is not implemented in the traditional
way using dc(1). This version is a single process which parses and
runs a byte code translation of the program. There is an "undocu-
mented" option (-c) that causes the program to output the byte code to
the standard output instead of running it. It was mainly used for
debugging the parser and preparing the math library.
A major source of differences is extensions, where a feature is
extended to add more functionality and additions, where new features
are added. The following is the list of differences and extensions.
LANG environment
This version does not conform to the POSIX standard in the pro-
cessing of the LANG environment variable and all environment
variables starting with LC_.
names Traditional and POSIX bc have single letter names for functions,
variables and arrays. They have been extended to be multi-char-
acter names that start with a letter and may contain letters,
numbers and the underscore character.
Strings
Strings are not allowed to contain NUL characters. POSIX says
all characters must be included in strings.
last POSIX bc does not have a last variable. Some implementations of
bc use the period (.) in a similar way.
POSIX bc does not have the logical operators.
read function
POSIX bc does not have a read function.
print statement
POSIX bc does not have a print statement .
continue statement
POSIX bc does not have a continue statement.
return statement
POSIX bc requires parentheses around the return expression.
array parameters
POSIX bc does not (currently) support array parameters in full.
The POSIX grammar allows for arrays in function definitions, but
does not provide a method to specify an array as an actual
parameter. (This is most likely an oversight in the grammar.)
Traditional implementations of bc have only call by value array
parameters.
function format
POSIX bc requires the opening brace on the same line as the
define key word and the auto statement on the next line.
=+, =-, =*, =/, =%, =^
POSIX bc does not require these "old style" assignment operators
to be defined. This version may allow these "old style" assign-
ments. Use the limits statement to see if the installed version
supports them. If it does support the "old style" assignment
operators, the statement "a =- 1" will decrement a by 1 instead
of setting a to the value -1.
spaces in numbers
Other implementations of bc allow spaces in numbers. For exam-
ple, "x=1 3" would assign the value 13 to the variable x. The
same statement would cause a syntax error in this version of bc.
errors and execution
This implementation varies from other implementations in terms
of what code will be executed when syntax and other errors are
found in the program. If a syntax error is found in a function
definition, error recovery tries to find the beginning of a
statement and continue to parse the function. Once a syntax
error is found in the function, the function will not be
callable and becomes undefined. Syntax errors in the interac-
tive execution code will invalidate the current execution block.
The execution block is terminated by an end of line that appears
after a complete sequence of statements. For example,
a = 1
b = 2
for more input. All previously defined functions remain defined
and the value of all non-auto variables are the value at the
point of interruption. All auto variables and function parame-
ters are removed during the clean up process. During a non-
interactive session, the SIGINT signal will terminate the entire
run of bc.
LIMITS
The following are the limits currently in place for this bc processor.
Some of them may have been changed by an installation. Use the limits
statement to see the actual values.
BC_BASE_MAX
The maximum output base is currently set at 999. The maximum
input base is 16.
BC_DIM_MAX
This is currently an arbitrary limit of 65535 as distributed.
Your installation may be different.
BC_SCALE_MAX
The number of digits after the decimal point is limited to
INT_MAX digits. Also, the number of digits before the decimal
point is limited to INT_MAX digits.
BC_STRING_MAX
The limit on the number of characters in a string is INT_MAX
characters.
exponent
The value of the exponent in the raise operation (^) is limited
to LONG_MAX.
variable names
The current limit on the number of unique names is 32767 for
each of simple variables, arrays and functions.
ENVIRONMENT VARIABLES
The following environment variables are processed by bc:
POSIXLY_CORRECT
This is the same as the -s option.
BC_ENV_ARGS
This is another mechanism to get arguments to bc. The format is
the same as the command line arguments. These arguments are
processed first, so any files listed in the environment argu-
ments are processed before any command line argument files.
This allows the user to set up "standard" options and files to
be processed at every invocation of bc. The files in the envi-
ronment variables would typically contain function definitions
for functions the user wants defined every time bc is run.
Error recovery is not very good yet.
Email bug reports to bug-bc@gnu.org. Be sure to include the word
''bc'' somewhere in the ''Subject:'' field.
AUTHOR
Philip A. Nelson
philnelson@acm.org
ACKNOWLEDGEMENTS
The author would like to thank Steve Sommars (Steve.Sommars@att.com)
for his extensive help in testing the implementation. Many great sug-
gestions were given. This is a much better product due to his involve-
ment.
GNU Project 2006-06-11 bc(1)