5 Functions ***********
· Math Library 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 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 definition, zero or more parameters are defined by listing their names separated 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 defining array parameters. The named array is passed by value 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. 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 function name. For example, consider the following session. define py (y) { print "--->", y, "<---", "\n"; } define void px (x) { print "--->", x, "<---", "\n"; } 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.