Bc (programming language)

bc, for basic calculator, is "an arbitrary-precision calculator language" with syntax similar to the C programming language. bc is typically used as either a mathematical scripting language or as an interactive mathematical shell.

Overview
A typical interactive usage is typing the command  on a Unix command prompt and entering a mathematical expression, such as (1 + 3) * 2, whereupon 8 will be output. While bc can work with arbitrary precision, it actually defaults to zero digits after the decimal point, so the expression 2/3 yields 0 (results are truncated, not rounded). This can surprise new bc users unaware of this fact. The -l option to bc sets the default scale (digits after the decimal point) to 20 and adds several additional mathematical functions to the language.

History
bc first appeared in Version 6 Unix in 1975. It was written by Lorinda Cherry of Bell Labs as a front end to dc, an arbitrary-precision calculator written by Robert Morris and Cherry. dc performed arbitrary-precision computations specified in reverse Polish notation. bc provided a conventional programming-language interface to the same capability via a simple compiler (a single yacc source file comprising a few hundred lines of code), which converted a C-like syntax into dc notation and piped the results through dc.

In 1991, POSIX rigorously defined and standardized bc. Four implementations of this standard survive today: The first is the traditional Unix implementation, a front-end to dc, which survives in Unix and Plan 9 systems. The second is the free software GNU bc, first released in 1991 by Philip A. Nelson. The GNU implementation has numerous extensions beyond the POSIX standard and is no longer a front-end to dc (it is a bytecode interpreter). The third is a re-implementation by OpenBSD in 2003. The fourth is an independent implementation by Gavin Howard that is included in Android (operating system),  FreeBSD as of 13.3-RELEASE,   and macOS as of 13.0.

POSIX bc
The POSIX standardized bc language is traditionally written as a program in the dc programming language to provide a higher level of access to the features of the dc language without the complexities of dc's terse syntax.

In this form, the bc language contains single-letter variable, array and function names and most standard arithmetic operators, as well as the familiar control-flow constructs (,  and  ) from C. Unlike C, an   clause may not be followed by an  .

Functions are defined using a   keyword, and values are returned from them using a   followed by the return value in parentheses. The   keyword (optional in C) is used to declare a variable as local to a function.

All numbers and variable contents are arbitrary-precision numbers whose precision (in decimal places) is determined by the global   variable.

The numeric base of input (in interactive mode), output and program constants may be specified by setting the reserved   (input base) and   (output base) variables.

Output is generated by deliberately not assigning the result of a calculation to a variable.

Comments may be added to bc code by use of the C   and   (start and end comment) symbols.

Exactly as C
The following POSIX bc operators behave exactly like their C counterparts:

+    -     *     / +=    -=    *=    /= ++    --    <     > ==    !=    <=    >=     [ ]   { }

Similar to C
The modulus operators,  and   behave exactly like their C counterparts only when the global   variable is set to 0, i.e. all calculations are integer-only. Otherwise the computation is done with the appropriate scale. is defined as. Examples:

Conflicting with C
The operators

^    ^=

superficially resemble the C bitwise exclusive-or operators, but are in fact the bc integer exponentiation operators.

Of particular note, the use of the  operator with negative numbers does not follow the C operator precedence. gives the answer of 4 under bc rather than −4.

"Missing" operators relative to C
The bitwise, boolean and conditional operators:

&    |     ^     &&    || &=    |=    ^=    &&=   ||= <<    >> <<=   >>= ?:

are not available in POSIX bc.

Built-in functions
The   function for calculating square roots is POSIX bc's only built-in mathematical function. Other functions are available in an external standard library.

The   function for determining the precision (as with the   variable) of its argument and the   function for determining the number of significant decimal digits in its argument are also built-in.

Standard library functions
bc's standard math library (defined with the -l option) contains functions for calculating sine, cosine, arctangent, natural logarithm, the exponential function and the two parameter Bessel function J. Most standard mathematical functions (including the other inverse trigonometric functions) can be constructed using these. See external links for implementations of many other functions.

The -l option changes the scale to 20, so things such as modulo may work unexpectedly. For example, writing  and then the command   outputs 0. But writing  after   and then the command   will output 1.

Plan 9 bc
Plan 9 bc is identical to POSIX bc but for an additional   statement.

GNU bc
GNU bc derives from the POSIX standard and includes many enhancements. It is entirely separate from dc-based implementations of the POSIX standard and is instead written in C. Nevertheless, it is fully backwards compatible as all POSIX bc programs will run unmodified as GNU bc programs.

GNU bc variables, arrays and function names may contain more than one character, some more operators have been included from C, and notably, an   clause may be followed by an  .

Output is achieved either by deliberately not assigning a result of a calculation to a variable (the POSIX way) or by using the added   statement.

Furthermore, a   statement allows the interactive input of a number into a running calculation.

In addition to C-style comments, a   character will cause everything after it until the next new-line to be ignored.

The value of the last calculation is always stored within the additional built-in   variable.

Extra operators
The following logical operators are additional to those in POSIX bc:

&&    ||      !

They are available for use in conditional statements (such as within an   statement). Note, however, that there are still no equivalent bitwise or assignment operations.

Functions
All functions available in GNU bc are inherited from POSIX. No further functions are provided as standard with the GNU distribution.

Example code
Since the bc  operator only allows an integer power to its right, one of the first functions a bc user might write is a power function with a floating-point exponent. Both of the below assume the standard library has been included:

Calculating π to 10000 digits
Calculate pi using the builtin arctangent function, a:

A translated C function
Because the syntax of bc is similar to that of C, published numerical functions written in C can often be translated into bc quite easily, which immediately provides the arbitrary precision of bc. For example, in the Journal of Statistical Software (July 2004, Volume 11, Issue 5), George Marsaglia published the following C code for the cumulative normal distribution:

With some necessary changes to accommodate bc's different syntax, and noting that the constant "0.9189..." is actually log(2*PI)/2, this can be translated to the following GNU bc code:

Using bc in shell scripts
bc can be used non-interactively, with input through a pipe. This is useful inside shell scripts. For example: In contrast, note that the bash shell only performs integer arithmetic, e.g.: One can also use the here-string idiom (in bash, ksh, csh):