[Coco] DEFFN in Extended BASIC

[Allen Huffman]

James Jones replied to my “Optimizing BASIC” blog post and mentioned a few more things. One of them was the DEFFN function, which I recalled but don’t think I ever used it. It allows creating custom functions. I tried to look it up in the Getting Started with Extended Color Basic, but the appendix referred me to page 193 and it was not mentioned that. I did find it demonstrated in the Super Extended Color Basic manual for the CoCo 3.

You can make a function called “A” that multiplies an integer by 1000:
DEF FNA(X)=X*1000

FOR A=1 to 10
PRINT FNA(A)
NEXT A

If you use X*1000 more times in your program than the overhead of the DEF FN line, you save code space.

It said you could pass multiple parameters, but there was no example. Using a command like C didn’t work, but a space did. Here is a function I will be using in my Pac-Man demo to convert an X and Y to a POKE location on the VDG text screen:

DEFFNP(X Y)=1024+Y*32+X

Now instead of doing:

P=1024+Y*32+X:POKEP,42

P=FNP(X Y):POKEP,42

Or,

POKE1024+Y*32+X,42
vs
POKEFNP(X Y),42 — four bytes less

Defining the function took 25 bytes, so I would have to use this at least 7 times in my code before I would save any time.

It does appear to be a tradeoff in speed. Doing a FOR/NEXT of X/Y to POKE every byte on the screen took more time.

Cool.
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