DEF FN

Purpose

To define and name a function that is written by the user.

Syntax

DEF FN<name>[(<parameter list>)]=<function definition>

Alternative forms (all valid):

DEF FN<name> [(<parameter list>)] = <function definition>  ' With spaces
DEF FN<name>(<parameter list>)=<function definition>       ' Without spaces

Parameters

• name - A legal variable name that becomes the function name after FN.
• Original MBASIC 5.21: Single character only (e.g., A, B, X$)
• This implementation: Can be one or more characters (e.g., A, ABC, UCASE$, DISTANCE)
• parameter list - Variable names in the function definition that are to be replaced when the function is called. Items are separated by commas.
• function definition - An expression that performs the operation of the function. Limited to one line.

Syntax Notes

Function Names

Original MBASIC 5.21: Function names were limited to a single character after FN: - ✓ FNA - single character - ✓ FNB$ - single character with type suffix

This implementation (extension): Function names can be multiple characters: - ✓ FNA - single character (compatible with original) - ✓ FNABC - multiple characters - ✓ FNUCASE$ - multi-character with type suffix - ✓ FNDIST - descriptive multi-character name - ✓ FNAREA% - multi-character with integer type

Spacing

Space after FN is optional. Both styles are valid: - DEF FN A(X) = X * 2 - with space after FN (FN and A are separate) - DEF FNA(X) = X * 2 - without space after FN (FNA is one token)

Choose the style that matches your coding preference. Both are equally supported.

Remarks

Function Definition

Variable names that appear in the function definition serve only to define the function; they do not affect program variables that have the same name.

A variable name used in a function definition may or may not appear in the parameter list: - If it does, the value of the parameter is supplied when the function is called - Otherwise, the current value of the variable is used

The variables in the parameter list represent, on a one-to-one basis, the argument variables or values that will be given in the function call.

Note: In 8K BASIC, only one argument is allowed in a function call, therefore the DEF FN statement will contain only one variable.

Function Types

• Extended and Disk BASIC: User-defined functions may be numeric or string
• 8K BASIC: User-defined string functions are not allowed

Type Specification

If a type is specified in the function name: - The value of the expression is forced to that type before it is returned to the calling statement - If the argument type does not match, a "Type mismatch" error occurs

Execution Requirements

• A DEF FN statement must be executed before the function it defines may be called
• If a function is called before it has been defined, an "Undefined user function" error occurs
• DEF FN is illegal in direct mode

Example

Example 1: Single-Character Name (Traditional Style)

10 DEF FND(X) = X * 2
20 FOR I = 1 TO 5
30   PRINT FND(I);
40 NEXT I
50 END

Output:

 2  4  6  8  10

Example 2: Multi-Character Name (More Descriptive)

10 DEF FNAREA(R) = 3.14159 * R^2
20 RADIUS = 5
30 PRINT "Area of circle with radius"; RADIUS; "is"; FNAREA(RADIUS)
40 END

Output:

Area of circle with radius 5 is 78.53975

Example 3: Compact Style (No Spaces)

10 DEF FNMAX(A,B)=A*(A>=B)+B*(B>A)
20 PRINT FNMAX(10, 20)
30 PRINT FNMAX(30, 15)
40 END

Output:

 20
 30

Explanation: - Uses compact syntax without spaces - Returns the maximum of two numbers - Uses boolean expressions: (A>=B) is -1 if true, 0 if false

Example 4: String Function - Uppercase Conversion

10 DEF FNUCASE$(Z$,N)=CHR$(ASC(MID$(Z$,N,1)) AND &H5F)
20 A$ = "hello world"
30 FOR I = 1 TO LEN(A$)
40   PRINT FNUCASE$(A$, I);
50 NEXT I
60 PRINT
70 END

Output:

HELLO WORLD

Explanation: - Multi-character function name with type suffix - Converts a single character to uppercase using bit manipulation - &H5F is hexadecimal notation (hex 5F = decimal 95 = binary 01011111) - AND &H5F clears bit 5 (the lowercase bit in ASCII), converting lowercase to uppercase - For more on hexadecimal constants, see Constants

Example 5: Distance Calculation (Real-World Example)

10 DEF FNDIST(X1,Y1,X2,Y2) = SQR((X2-X1)^2 + (Y2-Y1)^2)
20 ' Calculate distance between two points
30 PRINT "Distance from (0,0) to (3,4):"; FNDIST(0,0,3,4)
40 PRINT "Distance from (1,1) to (4,5):"; FNDIST(1,1,4,5)
50 END

Output:

Distance from (0,0) to (3,4): 5
Distance from (1,1) to (4,5): 5

Example 6: Using Current Variable Values

10 A = 10
20 DEF FNX(Y) = Y + A
30 PRINT "With A=10: FNX(5) ="; FNX(5)
40 A = 20
50 PRINT "With A=20: FNX(5) ="; FNX(5)
60 END

Output:

With A=10: FNX(5) = 15
With A=20: FNX(5) = 25

Explanation: - The function FNX uses the current value of variable A - Variable A is not in the parameter list, so its current value is used - First call: Y=5, A=10, result = 15 - Second call: Y=5, A=20, result = 25

Example 7: Integer Function with Type Suffix

10 DEF FNHALF%(N) = N \ 2
20 PRINT FNHALF%(7)
30 PRINT FNHALF%(10)
40 PRINT FNHALF%(15)
50 END

Output:

 3
 5
 7

Explanation: - Function returns integer division (truncates, doesn't round) - Type suffix % forces integer result

Example 8: Temperature Conversion Functions

10 DEF FNFTOC(F) = (F - 32) * 5 / 9
20 DEF FNCTOF(C) = C * 9 / 5 + 32
30 PRINT "100°F ="; FNFTOC(100); "°C"
40 PRINT "0°C ="; FNCTOF(0); "°F"
50 PRINT "32°F ="; FNFTOC(32); "°C"
60 END

Output:

100°F = 37.77778 °C
0°C = 32 °F
32°F = 0 °C

Common Errors

Undefined User Function

10 X = FNA(5)    ' Error: FNA not defined yet
20 DEF FNA(Y) = Y * 2

Error: "Undefined user function"

Fix: Define the function before calling it:

10 DEF FNA(Y) = Y * 2
20 X = FNA(5)

Type Mismatch

10 DEF FNA%(X) = X * 2    ' Integer function
20 Y = FNA%(3.5)          ' Error: passing float to integer function

Error: "Type mismatch"

Fix: Match argument types or remove type suffix

See Also

• DEF USR - Define assembly subroutine address
• USR - Call assembly language subroutine
• GOSUB-RETURN - Branch to and return from a subroutine