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 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:
definename(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 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 "name[ ]". 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.
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 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
because 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, bc supports 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 expression
and returns that value to the calling expression. There is an implied
"return (0)" at the end of every function. 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.
As an extension, 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); }
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)
c (x)
a (x)
l (x)
e (x)
j (n,x)
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