# Learning Competitive Programming on Retro Computers **URL:** \ **Category:** Code Reading\ **Tags:** commodore64\ **Created:** [January 19, 2024, 3:50pm UTC](https://pluralist.net/t/learning-competitive-programming-on-retro-computers/50 "2024-01-19T15:50:37Z")\ **Posts on this page:** 1\ **Page:** 1
**Author:** ![delyan](https://pluralist.net/user_avatar/pluralist.net/delyan/32/245_2.png) [@delyan](https://pluralist.net/u/delyan)\ **Post date:** [January 19, 2024, 3:50pm UTC](https://pluralist.net/t/learning-competitive-programming-on-retro-computers/50/1 "2024-01-19T15:50:37Z")
I have been researching whether retro computers are suitable for teaching (competitive) programming to elementary school children. I will go into the reasons `why` when I publish my research as an official blog post on [delyan.org](http://delyan.org). Here are the supported languages on CodeForces for example: [Codeforces Command Lines (2023-10-06)](https://codeforces.com/blog/entry/121114). Pascal may be the most “retro” language and it is [not a bad choice](https://news.ycombinator.com/item?id=34163168). Definitely won’t go as esoteric as [DJGPP](https://www.delorie.com/djgpp/). What if we added Commodore64 BASIC to the supported languages on CodeForces? Ok! I know it sounds crazy - but it [may be possible](https://forums.atariage.com/topic/278813-c64-basic-compiler-for-windows/). As part of my research I wanted to program something simple to understand how painful it would be to code in BASIC. What if we wanted to implement an algorithm to find all permutations of A, B, C, and D on a Commodore64 using BASIC? First - we are going to have to use nested loops - BASIC doesn’t have built-in functions for permutations. For a tiny number of N - something like this may be acceptable: ```BASIC 10 FOR A = 65 TO 68 20 FOR B = 65 TO 68 30 FOR C = 65 TO 68 40 FOR D = 65 TO 68 50 IF A <> B AND A <> C AND A <> D AND B <> C AND B <> D AND C <> D THEN 60 PRINT CHR$(A); CHR$(B); CHR$(C); CHR$(D) 70 END IF 80 NEXT D 90 NEXT C 100 NEXT B 110 NEXT A ``` This may be perfect and effective for teaching a 10 year old child about competitive programming: 1. We learn about `FOR` loops 2. We learn about ASCII values and how iterate over these 3. What uniqueness means and how to check for that 4. What are permutations vs combinations and the fact that permutations implies no repetitions 5. How to print/convert ASCII values to characters using CHR$ What if we try to write a recursion on a Commodore64? Overall recursion should be possible in Commodore 64 BASIC. Maybe we can use `GOSUB` and `RETURN` statements. Hopefully we won’t hit the Commodore 64 BASIC limitations around the (hardware) stack size and memory management. These limitations make deep recursion inefficient and we may run into stack overflows. But it is possible after all. Here is a simple recursion: ```BASIC 10 INPUT "Enter a number: ", N 20 GOSUB 100 30 END 100 REM Here is a recursive subroutine. It prints 100 through 1. 110 IF N = 0 THEN RETURN 120 PRINT N 130 N = N - 1 140 GOSUB 100 150 RETURN ``` Unfortunately there is no notion of stack with `GOSUB` and `RETURN`. (It turns out the [MOS Technology 6510 CPU](https://en.wikipedia.org/wiki/MOS_Technology_6510) architecture of the Commodore64 does have a hardware stack. It is used by the assembly language operations and so it is only indirectly accessed when using BASIC.) ![image](https://pluralist.net/uploads/default/original/1X/193579f255733a252b5a1110922d7e4eac0c7852.jpeg) * * * More interesting stuff on this topic: - [GitHub - Fr0sT-Brutal/awesome-pascal: A curated list of awesome Delphi/FreePascal/(any)Pascal frameworks, libraries, resources, and shiny things. Inspired by awesome-... stuff. Open source and freeware only! · GitHub](https://github.com/Fr0sT-Brutal/awesome-pascal) - [Using the Free Pascal IDE for a week ~ Cypher Phunk](https://cypherphunk.blogspot.com/2022/05/using-free-pascal-ide-for-week.html) - [Free Pascal has now a WebAssembly back end](https://lists.freepascal.org/pipermail/fpc-pascal/2019-September/056848.html)