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From: Nicolas Barnier <barnier@recherche.enac.fr>
To: caml-list@inria.fr
Subject: Re: [Caml-list] Choosing a random element in a Map or a Set
Date: Thu, 18 Mar 2021 12:10:11 +0100
Message-ID: <aa6c6dc8-a1c7-f0a5-0433-c6c731451fc3@recherche.enac.fr> (raw)
In-Reply-To: <3e61c49d-8b20-378b-0c24-75b8da14b4f7@alliot.org>

Hi Jean-Marc,

Le 18/03/2021 à 10:44, jean-marc.alliot@irit.fr a écrit :
> I am trying to take a random element from a Map or a Set.
>
> Currently, I generate one random int between 1 et Card(map), and I iter
> until I reach this element. The solution is in O(n) which is not great...
>
> I was about to code the following solution: take the current Map code
> and add a function which follows the balanced binary tree from the root
> and takes randomly a left or right turn at each node until we reach a
> leaf (we only need to generate one random int and use its binary
> representation to choose the left or right direction at each node). This
> should be in O(log(n))
>
> Before I start coding like an idiot:
> 1) Is there another, more intelligent, solution?

If using a Map or a Set is not compulsory, you can design a data structure
supporting insert, delete, search and random element in constant amortized
time with an extensible array and a hash-table as explained here:

https://www.geeksforgeeks.org/design-a-data-structure-that-supports-insert-delete-search-and-getrandom-in-constant-time/

I don't know if there are any good implementation of (Python-like)
extensible arrays in a widespread OCaml library, but François Pottier
published one as part of another superb data structure (union-find):

https://gitlab.inria.fr/fpottier/unionfind/-/blob/master/src/StoreVector.ml

You may want to adjust the ratio (2 here) by which the array is resized when
necessary.

Hope this helps,

-- Nicolas


  parent reply	other threads:[~2021-03-18 11:10 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-18  9:44 jean-marc.alliot
2021-03-18 10:06 ` François Pottier
2021-03-18 14:20   ` Simon Cruanes
2021-03-18 17:06     ` jean-marc.alliot
2021-03-18 11:10 ` Nicolas Barnier [this message]
2021-03-18 11:32   ` François Pottier
2021-03-19  1:32   ` Francois Berenger
2021-03-18 12:34 ` Ivan Gotovchits
2021-03-18 12:48 ` Ivan Gotovchits

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