code review 6905049: undo CL 6845121 / 79603a5e4cda

undo CL 6845121 / 79603a5e4cda

This changes the output of

rand.Seed(0)
perm := rand.Perm(100)

When giving the same seeds to Go 1.0 and Go 1.1 programs
I would like them to generate the same random numbers.

««« original CL description
math/rand: remove noop iteration in Perm

The first iteration always do `m[0], m[0] = m[0], m[0]`, because
`rand.Intn(1)` is 0.

fun note: IIRC in TAOCP version of this algorithm, `i` goes
backward (n-1->1), meaning that the "already" shuffled part of the
array is never altered betweens iterations, while in the current
implementation the "not-yet" shuffled part of the array is
conserved between iterations.

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/6845121

Committer: Brad Fitzpatrick <bradfitz@golang.org>
»»»

[rsc, 2012-12-07 14:47:28]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://code.google.com/p/go/

[bradfitz, 2012-12-07 16:30:56]

LGTM

On Fri, Dec 7, 2012 at 6:47 AM, <rsc@golang.org> wrote:

> Reviewers: golang-dev_googlegroups.com,
>
> Message:
> Hello golang-dev@googlegroups.com,
>
> I'd like you to review this change to
> https://code.google.com/p/go/
>
>
> Description:
> undo CL 6845121 / 79603a5e4cda
>
> This changes the output of
>
> rand.Seed(0)
> perm := rand.Perm(100)
>
> When giving the same seeds to Go 1.0 and Go 1.1 programs
> I would like them to generate the same random numbers.
>
> ««« original CL description
> math/rand: remove noop iteration in Perm
>
> The first iteration always do `m[0], m[0] = m[0], m[0]`, because
> `rand.Intn(1)` is 0.
>
> fun note: IIRC in TAOCP version of this algorithm, `i` goes
> backward (n-1->1), meaning that the "already" shuffled part of the
> array is never altered betweens iterations, while in the current
> implementation the "not-yet" shuffled part of the array is
> conserved between iterations.
>
> R=golang-dev
> CC=golang-dev
>
https://codereview.appspot.**com/6845121<https://codereview.appspot.com/6845121>
>
> Committer: Brad Fitzpatrick <bradfitz@golang.org>
> »»»
>
> Please review this at
https://codereview.appspot.**com/6905049/<https://codereview.appspot.com/6905...
>
> Affected files:
>   M src/pkg/math/rand/rand.go
>
>
> Index: src/pkg/math/rand/rand.go
> ==============================**==============================**=======
> --- a/src/pkg/math/rand/rand.go
> +++ b/src/pkg/math/rand/rand.go
> @@ -100,7 +100,7 @@
>         for i := 0; i < n; i++ {
>                 m[i] = i
>         }
> -       for i := 1; i < n; i++ {
> +       for i := 0; i < n; i++ {
>                 j := r.Intn(i + 1)
>                 m[i], m[j] = m[j], m[i]
>         }
>
>
>

[rsc, 2012-12-07 16:58:52]

*** Submitted as https://code.google.com/p/go/source/detail?r=d636ee0a2861 ***

undo CL 6845121 / 79603a5e4cda

This changes the output of

rand.Seed(0)
perm := rand.Perm(100)

When giving the same seeds to Go 1.0 and Go 1.1 programs
I would like them to generate the same random numbers.

««« original CL description
math/rand: remove noop iteration in Perm

The first iteration always do `m[0], m[0] = m[0], m[0]`, because
`rand.Intn(1)` is 0.

fun note: IIRC in TAOCP version of this algorithm, `i` goes
backward (n-1->1), meaning that the "already" shuffled part of the
array is never altered betweens iterations, while in the current
implementation the "not-yet" shuffled part of the array is
conserved between iterations.

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/6845121

Committer: Brad Fitzpatrick <bradfitz@golang.org>
»»»

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/6905049