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Comment by dataflow | Hacker News Reader

> I really don't see what's supposedly awful about that loop

The stopping condition is incredibly confusing and non-obvious. Misleading at first glance, in fact. The whole thing is so unidiomatic that I don't think I've even seen it once in my life. It's a better contender for an underhanded C++ code contest than production code.

> but if you want to count down to x instead of 0 you just do i >= x

No you can't. That fails if x == 0. Which perfectly illustrates why using unsigned everywhere isn't so great. And I say this as someone who likes unsigned types and uses them more than average!

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Thinking of it as a "stopping condition" is backwards, that part of the loop is called the invariant:

https://en.wikipedia.org/wiki/Loop_invariant

You should think of it as the condition that's true for all iterations, not a one-time event that halts the loop. The loop is short for this:

  for(size_t i = size - 1; 0 <= i && i < size; i--){
  
  }

Which works for both signed and unsigned numbers. It just so happens that for unsigned numbers you can omit the left-hand side of the &&, and for signed numbers you can omit the right-hand side. To support arbitrary lower bounds, you omit neither.

> You should think of it as the condition that's true for all iterations, not a one-time event that halts the loop.

(a) It's "a" condition that holds true for all iterations, not "the" condition. Plenty of other conditions can hold true across the iterations of any given loop too. In fact the one interesting thing about the loop invariant compared to any other conditions is the very fact that it is guaranteed to cease to hold immediately after the loop, assuming you don't break in the loop. Other conditions can still continue to hold. i.e. The stopping condition is the entire point of the loop invariant.

(b) I'm well aware what a loop invariant is; I've worked on compilers. I am also a human. Humans care about when a loop starts and stops. There's nothing backwards about it, it's the most straightforward way people think of loops. And it's literally why more modern languages have introduced better syntaxes that merely spell the boundaries and/or values, and skip spelling the invariants entirely.

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