Your own functions

DEF FN names a formula. Write it once, use it everywhere.

The shape of it

10 DEF FN F(C) = C * 9 / 5 + 32
20 PRINT FN F(100)

RUN
 212

OK

DEF FN F(C) = … defines a function called F whose argument is called C. FN F(100) calls it with 100. The FN is part of the call and you can't leave it out.

The rules

• One expression. No loops, no IF, no multiple lines. If you need those, use a subroutine.
• One argument.
• Define it before you use it. DEF lines usually sit at the top of the program, and a FN called before its DEF has run gives you ?UNDEF'D STATEMENT ERROR.
• The name follows variable rules — first two characters significant, and no keyword hiding inside it. FN BILL and FN BIKE are both BI, so the second definition quietly replaces the first. FN COST won't define at all, because COS is a keyword.

Why bother

Because the formula ends up in one place. This:

10 DEF FN R(X) = INT(X * 100 + .5) / 100

rounds to two decimal places, and now every price in your program can be FN R(P) instead of that expression written out eight times with a typo in the sixth one.

Some that earn their keep:

Definition	What it gives you
DEF FN R(X) = INT(X * 100 + .5) / 100	round to 2 decimals
DEF FN M(X) = X - INT(X / 7) * 7	remainder after dividing by 7
DEF FN D(X) = INT(RND(1) * X) + 1	a dice roll from 1 to X
DEF FN C(X) = X * 3.14159 * 2	circumference from a radius

That third one is the useful one. FN D(6) rolls a six-sided die, FN D(52) picks a card, and the awkward INT(RND(1) * n) + 1 only has to be got right once.

The argument is only borrowed

The name inside the parentheses is private to the function. This works fine even though there's already a C in the program:

10 C = 999
20 DEF FN F(C) = C * 9 / 5 + 32
30 PRINT FN F(100); C

RUN
 212 999

OK

C inside the definition means "whatever gets passed in". C outside it is still 999.

A worked example

10 DEF FN F(C) = C * 9 / 5 + 32
20 PRINT "C", "F"
30 FOR C = 0 TO 100 STEP 25
40 PRINT C, FN F(C)
50 NEXT C

RUN
C             F
 0             32
 25            77
 50            122
 75            167
 100           212

OK

One definition, five uses, and a table you can widen by changing one number in line 30.

Next: making it look and sound like something.