Boundary Checks with gforth

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Boundary Checks with gforth

updated by rck, 2004-05-21

Forth is a fascinating language. I've been hunting a bug in definition of mere 5 words for quite some weeks now. Now that I know, why things are the way the are, it seems quite trivial. I'm documenting the definition of check-range ( u lower-bound upper-bound -- ), while I still know about it.

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The big BUT
So far, so good. This word was defined in the MIPS-Assembler already. So where was my problem? Well, to be honest, the understanding of that function wasn't my primal goal. It didn't work the way I supposed it to! I got the invalid argument error all the time. what result would you expect from `0 0 0 within .`? I'd expect -1, that would be true. But it's 0!! To get -1, you'd have to check `0 0 1 within .`. That's because the check within performs isn't lower <= value <= upper, but lower <= value < upper. The upper-value always has to be one more, than the highest possible value.

The big BUT

So far, so good. This word was defined in the MIPS-Assembler already. So where was my problem? Well, to be honest, the understanding of that function wasn't my primal goal. It didn't work the way I supposed it to! I got the invalid argument error all the time.

what result would you expect from 0 0 0 within .? I'd expect -1, that would be true. But it's 0!!

To get -1, you'd have to check 0 0 1 within .. That's because the check within performs isn't lower <= value <= upper, but lower <= value < upper. The upper-value always has to be one more, than the highest possible value.

Conclusion
Programming is for masochists. Especially with forth. On the other hand, it's kind of a cool language, if you have figured it out. With the right ammount of documentation, problems like this one wouldn't exist. Well, try to search the web for "forth within definition word". You'll get everything but useful results.

Conclusion

Programming is for masochists. Especially with forth. On the other hand, it's kind of a cool language, if you have figured it out.

With the right ammount of documentation, problems like this one wouldn't exist. Well, try to search the web for "forth within definition word". You'll get everything but useful results.

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Preface
Checking Bounds in Forth
The big BUT