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Re: vector-last / vector-ref with negative indices?

From: Christopher Lam
Subject: Re: vector-last / vector-ref with negative indices?
Date: Sun, 20 Dec 2020 10:05:51 +0000

Easy - vector-ref with a negative index is not defined in r[5|7]rs.

On Sun, 20 Dec 2020 at 06:59, Aleix Conchillo Flaqué <>

> Hi,
> This month I'm trying to go through Advent Of Code one more year
> (previous years I didn't get too far) and I've been finding myself
> writing the same patterns multiple times that could be avoided by just
> having a helper function.
> One of them is getting the last element of a vector. It is a quite
> common operation when solving these types of problems. For example
> Python as you might know uses negative indices.
> I have looked around and haven't seen it (unless I completely missed
> it which would be shameful) but wouldn't it be good to have this
> built-in by default?
> Instead of having to write:
> (vector-ref v (- (vector-length v) 1))
> you would write:
> (vector-last v) or even better (vector-ref v -1).
> Interestingly Racket doesn't offer those functions either as far as I can
> tell.
> Basic use case? Get the maximum element of a sorted vector which would
> be done in constant time.
> I'm trying to solve the problems idiomatically, to the best of my
> limited knowledge, with Scheme. It is possible that the times I'm
> using vectors I should think about it differently and not use them,
> but I'm not 100% sure.
> What am I missing?
> Thank you in advance,
> Aleix

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