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An OS/8 BASIC Implementation of Project Euler Problem #1
Before you begin, you may wish to start with the companion article, Getting Text In.
The BASIC implementation of a program to solve Project Euler Problem #1 turned out to be surprisingly difficult to write for this old Applesoft BASIC hand. There are a number of differences between these two dialects of BASIC. There are also some surprising limitations in the OS/8 dialect relative to Applesoft BASIC.
Here is the code I came up with:
10 FOR I = 1 TO 999 20 A = I / 3 \ B = I / 5 30 IF INT(A) = A GOTO 60 40 IF INT(B) = B GOTO 60 50 GOTO 70 60 T = T + I 70 NEXT I 80 PRINT "TOTAL: "; T 90 END
A surprising aspect of this implementation of BASIC is that it seems to be using floating-point numbers for integer calculations such as this. Since the mantissa is 6 significant digits in this implementation of BASIC, it means we effectively get full integer precision for our calculation, since the answer is less than 7 digits. (And that's all I'm going to say about the answer here.)
That feature means I didn't need the dirty subtotaling trick I used in the PAL8 assembly version.
Limitations Making the Program Longer
OS/8 BASIC only accepts a
GOTOas the "then" clause of
IF. In fact, you can also write
IF condition GOTO xxas
IF condition THEN xx; both mean the same thing.
You can't have multiple
NEXTstatements for a given
FORstatement; it doesn't happen in this program, but this limitation can require you to use a
GOTOto jump to the single line containing the
ENDstatement is not optional. OS/8 BASIC will flatly refuse to run the program without it.
You can chain expressions together in an
IFstatement, but OS/8 basic does not consider a relation as an expression. It only considers simple arithmetic as an expression. Thus, we must test the two conditions in separate
OS/8 BASIC only supports
ANDfor testing multiple conditions in a single
IFstatement; there is no
Applesoft BASIC — and in fact, several other microcomputer BASICs from the late 1970s — didn't have any of these limitations, allowing the program to be a lot shorter and clearer:
10 FOR I = 1 TO 999 20 A = I / 3 : B = I / 5 30 IF INT(A) = A OR INT(B) = B THEN T = T + I 40 NEXT I 50 PRINT "TOTAL: "; T
We're down to 5 lines, a length I feel is exactly right for this problem's proper solution.
While it isn't exactly structured programming, it at least has no
GOTOs and only a single
Further Limitations Not Affecting Program Length
Error reportage is typical of its era, which is to say quite primitive, even by mid-1970s microcomputer standards. I tried to structure the above program with two
NEXT statements, one for the
B code paths, which yielded the super-helpful
NF error, which you are meant to understand means
NEXT without FOR. And never mind that it's misleading, since of course there is a
FOR statement, right there on line 10.
Just as with the PAL8 assembly language version, OS/8 BASIC has no modulo operator, so I had to resort to floating-point arithmetic with truncation to detect an even division. This makes me uneasy, but it gives the right answer, so clearly I have enough precision to pull this off, at least for the 1..999 case.
I also noticed that OS/8 BASIC uses the backslash character instead of a colon for multiple statements on a line, whereas microcomputer BASICs of the late 1970s typically used the colon instead.
Other Programs in the Series
I've solved this same problem in many other languages available for the PiDP-8/I:
You may find it interesting to compare their solutions.
Copyright © 2017 by Warren Young. This document is licensed under the terms of the SIMH license.