Hacker News Reader

Negative indices aren't magic. They follow the obvious wraparound (modular) pattern: Negative `i` indexes the element in array `a` located at `i % len(a)`. If they worked any other way it'd be madness.

View on HN

True, they're not magic, it just looks ugly for the most common case of fetching to the end of the array. The new syntax is much nicer than -1.

I don't think you got that index calculation right. -1 in array of 5 elements is located at 5-1 == 4, not at 1%5 == 1.

It depends on how you define the modulo operator [0]--there are definitions where (-1)%5 == 4 and definitions where (-1)%5 == -1. Presumably the GP is referring to the former case.

[0]: https://en.wikipedia.org/wiki/Modulo_operation#Remainder_cal...

I don't follow your point. We're concerned with the value of -1 % 5, not 1 % 5. There's no definition of the modulus operator on which -1 % 5 = 1.

Phrasing issue, I think. I understood "Negative `i` indexes the element" as `i` itself being positive. Like you'd say "negative 1 indexes" (i is 1), not "negative -1 indexes" (i is -1)

Reply on news.ycombinator.com