Comment by taeric
1 day ago
I'm not clear that jumping backwards is that tough to reason with. Notably, Knuth's algorithms do that quite commonly, right?
I do think they need to be somewhat constrained to not jump to places that need new things initialized. Which, it is truly mind blowing to know folks used to just jump straight into other functions. Mid function. Because why not.
Knuth's algorithms do that because they are written in assembly language.
In assembly language you must use backwards jumps to implement loops.
However, good assembly language programmers do not use arbitrary backwards jumps, but they use only a certain number of patterns, which correspond to the various kinds of loops that are also encountered in high-level programming languages.
Many programming languages are somewhat incomplete, because they do not have all the kinds of loops that exist in other programming languages. When programming in an assembly language, a good programmer will not restrict the loops to only the kinds of loops that are available in C/C++, but the non-nested loops that are possible with arbitrary GOTO will not be used.
The best practice in assembly programming is to not use explicit backwards jumps, but to define macros for different kinds of loops, then use the macros, which make the code look exactly like in a high-level programming language.
Knuth's algorithms do not use macros, like in real assembly programming, because their purpose is to show you an actual implementation, not a higher-level abstraction.
I am not talking about the implementations in assembly, I'm talking about the way he lists them in prose.
I can accept that he lists them the way he does because of familiarity to a style of assembly, but that doesn't change the fact that his prose can be easier to read than some of the alternative schemes people have used. In particular, I have found them to be far more illuminating to what is happening. With some odd challenges on finding ways to convert them to the standard control structure of modern languages.
You don't have to make a larger explanation of the argument for the more common control structures we use today. I am largely bought off on them and I am not trying to argue for a return to "only using jumps."
jumping backward creates all the non-linear issues I assume
Fair that it can create some. But just allowing of nested loops already creates some of these. And, I know folks have tried to disallow loops, but that feels extreme.
Again, I would point to many of Knuth's descriptions as already allowing jumps forward and backward in steps as evidence that they can be useful.
When backward jumps are allowed you can create loops that are much more tangled and incomprehensible than when you are nesting the loop structures of modern languages.
With backward jumps, you can make multiple loops that are not nested, but you could visualize them as a complex graph that has sequences of instructions in the nodes and which has multiple cycles through which the execution may or may not pass and which intersect each other. Good luck to understand how the code works, because you cannot separate parts of it that can be understood independently, like when using the "structured programming" that is ubiquitous in modern programming languages.
Such indecomposable complex multiple loops were not uncommon before 1970 in languages like FORTRAN or COBOL, and precisely this kind of control structures were the reason why the use of GOTO was criticized and considered harmful.
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