bc(1) bc(1)
NAME
bc - An arbitrary precision calculator language
SYNTAX
bc [ -lws ] [ file ... ]
DESCRIPTION
bc 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. 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 proces-
sor, bc will never read from the standard input.)
This version of bc contains several extensions beyond tra-
ditional bc 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.
OPTIONS
-l Define the standard math library.
-w Give warnings for extensions to POSIX bc.
-s Process exactly the POSIX bc language.
NUMBERS
The most basic element in bc 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:
.000001 has a length of 6 and scale of 6.
1935.000 has a length of 7 and a scale of 3.
VARIABLES
Numbers are stored in two types of variables, simple vari-
ables and arrays. Both simple variables and array vari-
ables are named. Names begin with a letter followed by
any number of letters, digits and underscores. All let-
ters must be lower case. (Full alpha-numeric names are an
extension. In POSIX bc all names are a single lower case
. 1
bc(1) bc(1)
letter.) The type of variable is clear by the context
because all array variable names will be followed by
brackets ([]).
There are four special variables, scale, ibase, obase, and
last. scale defines how some operations use digits after
the decimal point. The default value of scale is 0. ibase
and obase define the conversion base 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 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.
EXPRESSIONS
The numbers are manipulated by expressions and statements.
Since the language was designed to be interactive, state-
ments 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 con-
stants 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 (F). 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 letters 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 lan-
guages. 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
. 2
bc(1) bc(1)
to the maximum number representable by a C integer.
In the following descriptions of legal expressions, "expr"
refers to a complete expression and "var" refers to a sim-
ple or an array variable. A simple variable is just a
name
and an array variable is specified as
name[expr]
Unless specifically mentioned the scale of the result is
the maximum scale of the expressions involved.
- expr The result is the negation of the expression.
++ var The variable is incremented by one and the new
value is the result of the expression.
-- var The variable is decremented by one and the new
value is the result of the expression.
var ++ The result of the expression is the value of the
variable and then the variable is incremented by
one.
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 expressions.
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 scale.
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 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 inte-
gers this expression is the integer remainder func-
tion.
. 3
bc(1) bc(1)
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 exponent 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 expres-
sion.
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.
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
expr1 < expr2
The result is 1 if expr1 is strictly less than
expr2.
expr1 <= expr2
The result is 1 if expr1 is less than or equal to
expr2.
expr1 > expr2
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.
. 4
bc(1) bc(1)
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 opera-
tions 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 high-
est)
|| 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 pro-
grams 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:
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:
length ( expression )
The value of the length function is the number of
. 5
bc(1) bc(1)
significant digits in the expression.
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 pre-
viously 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 ibase for the conver-
sion 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 expression. If the expression is negative,
a run time error is generated.
STATEMENTS
Statements (as in most algebraic languages) provide the
sequencing of expression evaluation. In bc 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 bc. In fact, both a semicolon and a new-
line are used as statement separators. An improperly
placed newline will cause a syntax error. Because new-
lines 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 separated by semicolons and newlines.
The following is a list of bc statements 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 state-
ment. If the expression is not an assignment
statement, the expression is evaluated and printed
to the output. After the number is printed, a new-
line is printed. For example, "a=1" is an assign-
ment 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
. 6
bc(1) bc(1)
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 num-
ber of characters required to represent the base
ten value of "obase-1". Since numbers are of arbi-
trary precision, 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 charac-
ters printed per line is 70. Due to the interac-
tive nature of bc printing a number cause the side
effect of assigning the printed value the the spe-
cial 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 value. The newly assigned
value will remain until the next number is printed
or another value is assigned to last.
string The string is printed to the output. Strings start
with a double quote character and contain all char-
acters until the next double quote character. All
characters are take literally, including any new-
line. No newline character is printed after the
string.
print 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 last. Strings in the print state-
ment are printed to the output and may contain spe-
cial characters. Special characters start with the
backslash character (\). The special characters
recognized by bc are "b" (bell), "f" (form feed),
"n" (newline), "r" (carriage return), "t" (tab),
and "\" (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.
{ statement_list }
This is the compound statement. It allows multiple
statements to be grouped together for execution.
. 7
bc(1) bc(1)
if ( expression ) then statement1 [else statement2]
The if statement evaluates the expression and exe-
cutes 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.)
while ( 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 expres-
sion 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 evaluated before each execu-
tion 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 expres-
sion2. (The optional expressions are an extension.
POSIX bc requires all three expressions.) The fol-
lowing 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 enclosing for statement to start the
next iteration.
halt The halt statement (an extension) is an executed
statement that causes the bc processor to quit only
when it is executed. For example, "if (0 == 1)
halt" will not cause bc to terminate because the
halt is not executed.
return Return the value 0 from a function. (See the sec-
tion on functions.)
. 8
bc(1) bc(1)
return ( expression )
Return the value of the expression from a function.
(See the section on functions.)
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 ver-
sion of bc. This is an extension.
quit When the quit statement is read, the bc processor
is terminated, regardless of where the quit state-
ment is found. For example, "if (0 == 1) quit"
will cause bc to terminate.
warranty
Print a longer warranty notice. This is an exten-
sion.
FUNCTIONS
Functions provide a method of defining a computation that
can be executed 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 def-
inition 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
autolist statementlist }
A function call is just an expression of the form
"name(parameters)".
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 defini-
tion by the notation "name[]". In the function call,
actual parameters are full expressions for number parame-
ters. 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.
The autolistis 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
. 9
bc(1) bc(1)
name is the name of an auto variable. Arrays may be spec-
ified 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 ini-
tialized 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 pro-
vided in the function call. Auto variables are different
than traditional local variables in the fact that if func-
tion 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, bc sup-
ports recursive functions.
The function body is a list of bc 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, "return", returns the
value 0 to the calling expression. The second form,
"return ( expression )", computes the value of the expres-
sion and returns that value to the calling expression.
There is an implied "return (0)" at the end of every func-
tion. This allows a function to terminate and return 0
without an explicit return statement.
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.
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 fol-
lowing functions:
s (x) The sine of x in radians.
c (x) The cosine of x in radians.
a (x) The arctangent of x.
l (x) The natural logarithm of x.
e (x) The exponential function of raising e to the value
x.
. 10
bc(1) bc(1)
j (n,x)
The bessel function of integer order n of x.
EXAMPLES
In /bin/sh, the following will assign the value of "pi"
to the shell variable pi.
pi=$(echo "scale=10; 4*a(1)" | bc -l)
The following is the definition of the exponential func-
tion 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 implement a simple program for calculating checkbook
. 11
bc(1) bc(1)
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"
print " Exit by a 0 transaction.\n\n"
print "Initial balance? "; bal = read()
bal /= 1
print "\n"
while (1) {
"current balance = "; bal
"transaction? "; trans = read()
if (trans == 0) break;
bal -= trans
bal /= 1
}
quit
The following is the definition of the recursive factorial
function.
define f (x) {
if (x <= 1) return (1);
return (f(x-1) * x);
}
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 implemen-
tations. 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 "undocumented" 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 fea-
ture is extended to add more functionality and
additions, where new features are added. The following
is the list of differences and extensions.
LANG This version does not conform to the POSIX standard
in the processing 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-character names that
start with a letter and may contain letters, num-
bers and the underscore character.
. 12
bc(1) bc(1)
last POSIX bc does not have a last variable. Some
implementations of bc use the period (.) in a simi-
lar way.
comparisons
POSIX bc allows comparisons only in the if state-
ment, the while statement, and the second expres-
sion of the for statement. Also, only one rela-
tional operation is allowed in each of those state-
ments.
if statement, else clause
POSIX bc does not have an else clause.
for statement
POSIX bc requires all expressions to be present in
the for statement.
&&, ||, !
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.
array parameters
POSIX bc does not have array parameters. Other
implementations of bc may have call by value array
parameters.
=+, =-, =*, =/, =%, =^
POSIX bc does not require these "old style" assign-
ment operators to be defined. This version may
allow these "old style" assignments. Use the lim-
its statement to see if the installed version sup-
ports 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 num-
bers. For example, "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 implementa-
tions in terms of what code will be executed when
. 13
bc(1) bc(1)
syntax and other errors are found in the program.
If a syntax error is found in a function defini-
tion, 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 unde-
fined. Syntax errors in the interactive 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 state-
ments. For example,
a = 1
b = 2
has two execution blocks and
{ a = 1
b = 2 }
has one execution block. Any runtime error will terminate
the execution of the current execution block. A runtime
warning will not terminate the current execution block.
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.
multiply
The multiply routine may yield incorrect results if
a number has more than LONG_MAX / 90 total digits.
For 32 bit longs, this number is 23,860,929 digits.
. 14
bc(1) bc(1)
code size
Each function and the "main" program are limited to
10240 bytes of compiled byte code each. This limit
(BC_MAX_SEGS) can be easily changed to have more
than 10 segments of 1024 bytes.
variable names
The current limit on the number of unique names is
32767 for each of simple variables, arrays and
functions.
FILES
In most installations, bc is completely self-contained.
Where executable size is of importance or the C compiler
does not deal with very long strings, bc will read the
standard math library from the file
/usr/local/lib/libmath.b. (The actual location may vary.
It may be /lib/libmath.b.)
DIAGNOSTICS
If any file on the command line can not be opened, bc will
report that the file is unavailable and terminate. Also,
there are compile and run time diagnostics that should be
self-explanatory.
BUGS
Error recovery is not very good yet.
AUTHOR
Philip A. Nelson
phil@cs.wwu.edu
ACKNOWLEDGEMENTS
The author would like to thank Steve Sommars
(sesv@iwtsf.att.com) for his extensive help in testing the
implementation. Many great suggestions were given. This
is a much better product due to his involvement.
. 15