even random number, javascript

[Debbie Lucier]

How would I generate a even random number each time? I can generate a
random number each time but not an even one? Here is the code I use for
the random number from 1-100.

<script type="text/javascript">
no=math.random( )*100
document.write (math.round(no) )
</script>


Thanks,
Deb

*** Sent via Developersdex http://www.developersdex.com ***

[rf]

Re: even random number, javascript

Debbie Lucier
[color=blue]
> How would I generate a even random number each time? I can generate a
> random number each time but not an even one? Here is the code I use for
> the random number from 1-100.[/color]

Er, generate a random one from 1-50 and multiply it by 2.

Cheers
Richard.

[RobG]

Re: even random number, javascript

Debbie Lucier wrote:[color=blue]
> How would I generate a even random number each time? I can generate a
> random number each time but not an even one? Here is the code I use for
> the random number from 1-100.
>
> <script type="text/javascript">
> no=math.random( )*100
> document.write (math.round(no) )
> </script>
>[/color]

rf's answer should do the trick, but what about zero? Should it be
included, or is your range 2-100?


--
Rob

[Zif]

Re: even random number, javascript

Debbie Lucier wrote:[color=blue]
> How would I generate a even random number each time? I can generate a
> random number each time but not an even one? Here is the code I use for
> the random number from 1-100.
>
> <script type="text/javascript">
> no=math.random( )*100
> document.write (math.round(no) )
> </script>
>
>
> Thanks,
> Deb
>
> *** Sent via Developersdex http://www.developersdex.com ***[/color]


Math.round() has a bias toward the higher numbers (4.5 goes to 5
always and never to 4), so the starting number has a lower chance of
occurring that all the others.

A better distribution is from truncating the decimal part of numbers
in a range between min and max plus one :


<script type="text/javascript">

function genRandom( min, max ) {
return min + (max-min+1)*Math.ran dom() | 0;
}

document.write( genRandom( 0, 50 )*2 );

</script>





--
Zif

[Evertjan.]

Re: even random number, javascript

Zif wrote on 30 aug 2005 in comp.lang.javas cript:[color=blue]
> Math.round() has a bias toward the higher numbers (4.5 goes to 5
> always and never to 4),[/color]

Unimportant bias, because the chance of a pseudo random number being
exactly 4.5 should be very very small.

Other deviances between real random and pseudo random will have a much
higher effect, though also unimportant in most script applications.
[color=blue]
> so the starting number has a lower chance of
> occurring that all the others.[/color]

what do you mean by "starting number"?

--
Evertjan.
The Netherlands.
(Replace all crosses with dots in my emailaddress)

[Stephen Chalmers]

Re: even random number, javascript

Debbie Lucier <nospam@glorybo und.net> wrote in message news:QiTQe.120$ SH1.21003@news. uswest.net...[color=blue]
>
> How would I generate a even random number each time? I can generate a
> random number each time but not an even one? Here is the code I use for
> the random number from 1-100.
>
> <script type="text/javascript">
> no=math.random( )*100
> document.write (math.round(no) )
> </script>
>[/color]

n = (Math.random()* 98 +2) & 0xFE ; // 2 - 98 inclusive

n = (Math.random()* 100 +2) & 0xFE ; // 2 - 100 inclusive

n = (Math.random()* 100) & 0xFE ; // 0 - 98 inclusive

n = (Math.random()* 101) & 0xFE ; // 0 - 100 inclusive

--
S.C.

[Lasse Reichstein Nielsen]

Re: even random number, javascript

Zif <Zifud@hotmail. com> writes:
[color=blue]
> Debbie Lucier wrote:[/color]
[color=blue][color=green]
>> no=math.random( )*100
>> document.write (math.round(no) )[/color][/color]
[color=blue]
> Math.round() has a bias toward the higher numbers (4.5 goes to 5
> always and never to 4), so the starting number has a lower chance of
> occurring that all the others.[/color]

That's not the problem with using Math.round and Math.random together.
Since the range [3.5-4.5[ has the same length as [2.5-3.5[, the chance
of hitting either is close to the same.

A more serious problem is that Math.round(Math .random()*100) has 101
different possible outcomes, with 0 and 100 having only half the
chance of the others.

/L
--
Lasse Reichstein Nielsen - lrn@hotpop.com
DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleD OM.html>
'Faith without judgement merely degrades the spirit divine.'

[Evertjan.]

Re: even random number, javascript

Lasse Reichstein Nielsen wrote on 30 aug 2005 in comp.lang.javas cript:
[color=blue]
> A more serious problem is that Math.round(Math .random()*100) has 101
> different possible outcomes, with 0 and 100 having only half the
> chance of the others.
>[/color]

You are so right, Lasse.

Let's try:

Math.floor(Math .random()*100)

--
Evertjan.
The Netherlands.
(Replace all crosses with dots in my emailaddress)

[Richard Cornford]

Re: even random number, javascript

Evertjan. wrote:[color=blue]
> Lasse Reichstein Nielsen wrote:
>[color=green]
>> A more serious problem is that Math.round(Math .random()*100)
>> has 101 different possible outcomes, with 0 and 100 having
>> only half the chance of the others.
>>[/color]
>
> You are so right, Lasse.
>
> Let's try:
>
> Math.floor(Math .random()*100)[/color]

While we are at it, this seems like a good opportunity to use the
truncating side-effect of bitwise operations as the OP wants only even
numbers. Multiplying by two produces even numbers, but so does shifting
left by one bit:-

((Math.random() * 51) << 1) // even integers 0 to 100 (assuming
// a correct random implementation
// with results 0 to <1)

Richard.

[Evertjan.]

Re: even random number, javascript

Richard Cornford wrote on 31 aug 2005 in comp.lang.javas cript:
[color=blue]
> While we are at it, this seems like a good opportunity to use the
> truncating side-effect of bitwise operations as the OP wants only even
> numbers. Multiplying by two produces even numbers, but so does shifting
> left by one bit:-
>
> ((Math.random() * 51) << 1) // even integers 0 to 100 (assuming
> // a correct random implementation
> // with results 0 to <1)
>[/color]

x = 1.234
alert(x) // 1.234
x = x << 1
alert(x) // 2

Why is the truncation as it is?

x = 0.234
alert(x) // 0.234
x = x << 1
alert(x) // 0 !!!!

Wrong, in the sense of your application!

btw:

x = -1.234
alert(x) // -1.234
x = x << 1
alert(x) // -2

--
Evertjan.
The Netherlands.
(Replace all crosses with dots in my emailaddress)

[Evertjan.]

Re: even random number, javascript

Evertjan. wrote on 31 aug 2005 in comp.lang.javas cript:
[color=blue]
> x = 0.234
> alert(x) // 0.234
> x = x << 1
> alert(x) // 0 !!!!
>
> Wrong, in the sense of your application!
>[/color]

I seem not to be awake yet.

It is correct as it should be!

--
Evertjan.
The Netherlands.
(Replace all crosses with dots in my emailaddress)

[Richard Cornford]

Re: even random number, javascript

Evertjan. wrote:[color=blue]
> Richard Cornford wrote:
>[color=green]
>> While we are at it, this seems like a good opportunity to use
>> the truncating side-effect of bitwise operations as the OP wants
>> only even numbers. Multiplying by two produces even numbers,
>> but so does shifting left by one bit:-
>>
>> ((Math.random() * 51) << 1) // even integers 0 to 100 (assuming
>> // a correct random implementation
>> // with results 0 to <1)
>>[/color]
>
> x = 1.234
> alert(x) // 1.234
> x = x << 1
> alert(x) // 2
>
> Why is the truncation as it is?[/color]

The floating point number is internally converted into a signed 32 bit
integer prior to the actual shift (this happens with all bitwise
operations except - >>> -, which uses an unsigned 32 bit integer). It is
this conversion that has the truncating (towards zero) side-effect, so
the operand for the actual shift is the number one.

Richard.

[Evertjan.]

Re: even random number, javascript

Richard Cornford wrote on 31 aug 2005 in comp.lang.javas cript:
[color=blue][color=green]
>> x = 1.234
>> alert(x) // 1.234
>> x = x << 1
>> alert(x) // 2
>>
>> Why is the truncation as it is?[/color]
>
> The floating point number is internally converted into a signed 32 bit
> integer prior to the actual shift (this happens with all bitwise
> operations except - >>> -, which uses an unsigned 32 bit integer). It is
> this conversion that has the truncating (towards zero) side-effect, so
> the operand for the actual shift is the number one.
>[/color]

So this is a legal Math.floor equivalent for positive numbers?

x = x << 0

=========

different when negative:

x = -71.234
x = x << 0
alert(x) // -71
x = -71.234
x = Math.floor(x)
alert(x) // -72


--
Evertjan.
The Netherlands.
(Replace all crosses with dots in my emailaddress)

[Richard Cornford]

Re: even random number, javascript

Evertjan. wrote:[color=blue]
> Richard Cornford wrote:
>[color=green][color=darkred]
>>> x = 1.234
>>> alert(x) // 1.234
>>> x = x << 1
>>> alert(x) // 2
>>>
>>> Why is the truncation as it is?[/color]
>>
>> The floating point number is internally converted into a signed 32
>> bit integer prior to the actual shift (this happens with all bitwise
>> operations except - >>> -, which uses an unsigned 32 bit integer).
>> It is this conversion that has the truncating (towards zero)
>> side-effect, so the operand for the actual shift is the number one.[/color]
>
> So this is a legal Math.floor equivalent for positive numbers?[/color]

If the positive number can be accommodated in a 32 bit signed integer.
[color=blue]
> x = x << 0[/color]

or:-

x = x | 0;

- or any other bitwise operation that will not alter its operand. Though
OR zero and shift zero have proved the (approximately equal) fastest of
the possibilities (at between 4 and 14 times faster than Math.floor).
[color=blue]
> =========
>
> different when negative:
>
> x = -71.234
> x = x << 0
> alert(x) // -71
> x = -71.234
> x = Math.floor(x)
> alert(x) // -72[/color]

Yes, the truncating is always towards zero with bitwise operators. But
they are still a good option when you want to quickly acquire integer
values that will be non-negative and restricted in range, such as for
pixel positioning in animation.

Richard.

[Dr John Stockton]

Re: even random number, javascript

JRS: In article <df41ds$p1m$1$8 30fa7a5@news.de mon.co.uk>, dated Wed, 31
Aug 2005 11:39:54, seen in news:comp.lang. javascript, Richard Cornford
<Richard@litote s.demon.co.uk> posted :[color=blue]
>
>The floating point number is internally converted into a signed 32 bit
>integer prior to the actual shift (this happens with all bitwise
>operations except - >>> -, which uses an unsigned 32 bit integer). It is
>this conversion that has the truncating (towards zero) side-effect, so
>the operand for the actual shift is the number one.[/color]

The result of X << Y (etc.) where Y is not a small non-negative integer
look interesting, but one should check that actual results and ECMA
agree (for which it is too late tonight) before contemplating actual
use.

--
© John Stockton, Surrey, UK. ?@merlyn.demon. co.uk Turnpike v4.00 IE 4 ©
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