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From: Stephane Chazelas <stephane.chazelas@gmail.com>
To: zsh-workers@zsh.org
Subject: Re: shuffle array
Date: Mon, 2 Dec 2019 15:16:52 +0000
Message-ID: <20191202151652.yke22w7mign4i26q@chaz.gmail.com> (raw)
In-Reply-To: <CAN=4vMpt+hQw2zxUC1LH+3bPymBX_5qqexFw-vqkN6ye+dG1ZQ__45487.0572009654$1575297047$gmane$org@mail.gmail.com>

2019-12-02 15:29:17 +0100, Roman Perepelitsa:
> Another alternative is to implement the standard inplace shuffle
> algorithm from scratch:
> 
>     local -i i
>     for ((i = 2; i <= $#l; ++i)); do
>       local j=$((RANDOM % i + 1))

shouldn't that be "RANDOM % $#l + 1"?

>       # swap l[i] and l[j]
>       local tmp=$l[i]
>       l[i]=$l[j]
>       l[j]=$tmp
>     done
> 
> Due to RANDOM having a rather narrow range, this will introduce bias
> on large arrays and won't work at all on arrays with more than 32k
> elements. These issues can be mitigated by replacing RANDOM with
> (RANDOM << 15 | RANDOM) or even with (RANDOM << 30 | RANDOM << 15 |
> RANDOM).
[...]

Or use rand48() in zsh/mathfunc

((j = 1 + rand48() * $#l))

-- 
Stephane


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Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <867e3jxtao.fsf@zoho.eu>
     [not found] ` <20191129064215.GA14095@prometheus.u-strasbg.fr>
     [not found]   ` <8636e7w0w9.fsf__36104.0529723809$1575015640$gmane$org@zoho.eu>
     [not found]     ` <20191201200719.anzbs27c7phgdnmm@chaz.gmail.com>
     [not found]       ` <20191201233848.7selcpkvenlu65px__33142.5388505281$1575243619$gmane$org@tarpaulin.shahaf.local2>
     [not found]         ` <20191202141421.sj7nlhsdrpqxpr42@chaz.gmail.com>
     [not found]           ` <CAN=4vMpt+hQw2zxUC1LH+3bPymBX_5qqexFw-vqkN6ye+dG1ZQ__45487.0572009654$1575297047$gmane$org@mail.gmail.com>
2019-12-02 15:16             ` Stephane Chazelas [this message]
2019-12-02 15:27               ` Roman Perepelitsa
     [not found] <867e3jxtao.fsf__7557.99096530012$1574997710$gmane$org@zoho.eu>
2019-12-01 19:57 ` Stephane Chazelas

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