| .DEF FN Statement Details. |
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| DEF FN Statement Details |
|
| Single-Line Syntax |
| DEF FNname[(parameterlist)] = expression |
| |
| Block Syntax |
| DEF FNname[(parameterlist)] |
| [statements] |
| FNname = expression |
| [statements] |
| END DEF |
| |
| Argument Description |
| name legal variable name, up to 40 characters long. This |
| name, combined with FN, is the name of the function. |
| The name can include an explicit type-declaration |
| character to indicate the type of value returned. |
| Names that are the same except for the type- |
| declaration character are distinct names. For |
| example, the following are names of three different |
| DEF FN functions: |
| |
| FNString$ |
| FNString% |
| FNString# |
| |
| To return a value from a DEF FN function, assign the |
| value to the full function name: |
| |
| FNString$ = "No answer." |
| |
| parameterlist A list of variable names, separated by commas. The |
| syntax is explained below. When the function is |
| called, BASIC assigns the value of each argument to |
| its corresponding parameter. Function arguments are |
| passed by value. DEF FN functions do not accept |
| arrays, records, or fixed-length strings as |
| arguments. |
| |
| expression In both syntaxes, expression is evaluated and the |
| result is the function's value. In Syntax 1, |
| expression is the entire body of the function and |
| is limited to one logical line. |
| |
| When no expression is assigned to the name, the |
| default return values are zero for a numeric DEF FN |
| function, and the null string ("") for a string |
| DEF FN function. |
| |
| A parameterlist has the following syntax: |
| |
| variable [AS type] [,variable [AS type]]... |
| |
| A variable is any valid BASIC variable name. The type is INTEGER, |
| LONG, SINGLE, DOUBLE, or STRING. You may also indicate a variable's |
| type by including a type-declaration character (%, &, !, #, or $) in |
| the name. |
| |
| Note: The FUNCTION procedure offers greater flexibility and control |
| than the DEF FN function. See the FUNCTION statement for more |
| information. |
| |
| You must define a DEF FN function with a DEF FN statement before the |
| function is used. Calling a DEF FN function before it is defined |
| produces the error message "Function not defined." DEF FN function |
| definitions cannot appear inside other DEF FN definitions. In |
| addition, DEF FN functions cannot be recursive. |
| |
| You must use the EXIT DEF statement to leave a multiline DEF FN early. |
| DEF FN functions can only be used in the module in which they are |
| defined. They cannot be referenced from other modules. |
| |
| A DEF FN function may share variables with the module-level code. |
| Variables not in the parameterlist are global--their values are shared |
| with the calling program. To keep a variable value local |
| to a function definition, declare it in a STATIC statement. |
| |
| DEF FN can return either numeric or string values. DEF FN returns a |
| string value if name is a string variable name, and a numeric value |
| if name is a numeric variable name. Assigning a numeric value to a |
| string function name or assigning a string value to a numeric |
| function name produces the error message "Type mismatch." |
| |
| If the function is numeric, DEF FNname returns a value with the |
| precision specified by name. For example, if name specifies a |
| double-precision variable, then the value returned by DEF FNname is |
| double precision, regardless of the precision of expression. |
| |
| Because BASIC may rearrange arithmetic expressions for greater |
| efficiency, avoid using DEF FN functions that change program variables |
| in expressions that may be reordered. The following example may give |
| different results: |
| |
| DEF FNShort |
| I=10 |
| FNShort=1 |
| END DEF |
| I=1 : PRINT FNShort + I + I |
| |
| If BASIC reorders the expression so FNShort is called after |
| calculating (I+I), the result is 3 rather than 21. You can usually |
| avoid this problem by isolating the DEF FN function call: |
| |
| I = 1 : X = FNShort : PRINT X + I + I |
| |
| Doing I/O operations in DEF FN functions used in I/O statements |
| or doing graphics operations in DEF FN functions in graphics |
| statements may cause similar problems. |
