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Sat, 04 Mar 2006

Structured BASIC
Aristotle Pagaltzis reminisces about programming microcomputers in BASIC in the 1980s:

That's what I started with, on the Acorn Electron. And I remember being excited about finding and understanding DEF FN. I also remember my disappointment about how limited it was. I remember my frustration whenever BASIC forced me into writing messy code.

I remember my frustration with this too. I realized fairly early on that it was important to organize one's code in a modular fashion. My clearest memory of this was in developing an Adventure-style program. Each of the locations in the world was assigned a sequence number. Location #23 was handled by lines 2300--2399 of the program. Lines 2300--2319 would print the description of the location. Line 2320 would set the variables that recorded the player's location, and called the subroutine to print the descriptions of the other objects at that location. Line 2380 would call the subroutine that prompted the user for their next command. Other lines in between would provide the implementation of whatever special effects were required for that location.

All the important utility subroutines were at mnemonic line numbers; the main loop was at line 50000, and the command processing was at 51000. Special handling for objects was in the 40000 range, with one hundred statement numbers reserved for each object.

After each user command was processed, control was dispatched back to the appropriate part of the program, depending on where the player was now. Microsoft BASIC didn't have a computed GOTO, so the dispatch was performed by a jump table. I was unhappy with the jump table, recognizing that it didn't scale well.

Object sizes and descriptions were stored in a table. I don't know why I didn't store the location descriptions in the table in the same way, but I suspect that I tried and found that my microcomputer didn't have enough string memory. I also discovered that the algorithm that mapped statement numbers to code did not scale well to programs with a lot of numbered statements; editing the program grew intolerably slow once the world contained more than about fifty locations.

Still, I was pleased with the outcome. My goal (at the tender age of sixteen, or whatever) had been to adopt conventions that made it easy to extend or modify the world and to add new locations or objects, and I felt at the time that I had achieved that.

M. Pagaltzis says:

I guess I have a natural penchant for structured code. Penchant? Instinct.

I think anyone who is really interested in writing programs in BASIC and who reflects on the results of his projects is going to come to the conclusion that BASIC is a very poor tool for the job. These problems force themselves on everyone, and if you are thoughtful you will see the problems and try to come up with some techniques to solve them.

I really wish I could see those old programs again. I'm sure I would learn a lot from them.

I do have some code I wrote in C as long ago as 1987. I remember that shortly after that I got sick of programming and took a vacation from it for a year.

One day the following year I was reading netnews, and I overheard a colleague complaining about his CS homework. He had to write a program in C to count the number of occurrences of each word in its input, using a binary tree to store the words. I said he was complaining about nothing and that I, a math major, could turn out such a program in two hours. I don't know why I said this, since I hadn't done any C programming in a year, and I didn't have any significant experience with C, but I was inspired, and I did finish it quickly, and it worked. I have been programming regularly ever since. I still have the source code for that program.

Here's the funny thing about the programs from that time: when I look at the pre-vacation programs, they look to me as though they were written by someone else. When I look at the tree-sort program or any other program I have written since then, I recognize it as my own code.

I don't know what happened in my brain during my one-year vacation, but my current programming style first emerged in that tree-sort program, and the code from after the break has all been a lot better than the code I wrote before.

I'd like to take another vacation, but I can't now, because I have to earn a living.


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