how to generate random number between -n and n

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Ricardo - 15 Jul 2006 01:05 GMT

Hi wizards , I need to know how to generate random numbers between -n
and n with matlab , n is a integer .
Thanks in advance .

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Furious George - 15 Jul 2006 06:01 GMT

> Hi wizards , I need to know how to generate random numbers between -n
> and n with matlab , n is a integer .
> Thanks in advance .

If n>=1 or n<=-1 then java.lang.Math.random() will generate a pseudo
random number between -n and n. (But not from a uniform (-n,n)
distribution.)

If n=0, then it is degenerate. Just choose 0.

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Red Orchid - 15 Jul 2006 10:50 GMT

"Ricardo" <ricrio@gmail.com> wrote or quoted in
Message-ID: <1152921907.595376.304880@s13g2000cwa.googlegroups.com>:

> Hi wizards , I need to know how to generate random numbers between -n
> and n with matlab , n is a integer .

For x,
if 0 < x < 2n,

0 - n < x < 2n - n
and then,
-n < x < n

Besides,
'Random.nextInt(2*n)' produces '0 <= x < 2n'.

Java documentation says about 'Random.nextInt(int n)'

"... All n possible int values are produced with (approximately)
equal probability ... "

Therefore,
if 'Random.nextInt(2*n)' were a perfect source,

'Random.nextInt(2*n) - n' would produce
-n < x < n
with perfect uniformity.

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Red Orchid - 15 Jul 2006 12:03 GMT

"Red Orchid" <windfollowcloud@yahoo.com> wrote or quoted in
Message-ID: <e9adp3$m7k$1@news2.kornet.net>:

> 'Random.nextInt(2*n) - n' would produce
> -n < x < n
> with perfect uniformity.

Correct.

-n <= x < n

If '-n < x < n' is required,

<code>
Random ran = new Random();
...
int n = ...
int value = ran.nextInt(n);
int sign = ran.nextInt(2);

.... = (sign == 0) ? value : -value;

</code>

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Red Orchid - 15 Jul 2006 12:43 GMT

"Red Orchid" <windfollowcloud@yahoo.com> wrote or quoted in
Message-ID: <e9ai1o$noa$1@news2.kornet.net>:

> If '-n < x < n' is required,
>
[quoted text clipped - 8 lines]
>
> </code>

Sorry.

The above code do not have uniformity because 0 can
be chosen as +0 and -0.

If -n < x < n, the solution will be

Random.nextInt( 2 * (n -1) + 1 ) - ( n - 1 );

Because, ..

'Random.nextInt( 2 * (n -1) + 1 ) ' produces

0 <= x < 2 * (n -1) + 1

Then,

0 - (n -1) <= x < 2 * (n - 1) + 1 - (n - 1)

-(n - 1) <= x < (n - 1) + 1

-(n - 1) <= x < n

-n < x < n (here, n is integer).

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Chris Uppal - 15 Jul 2006 11:09 GMT

> Hi wizards , I need to know how to generate random numbers between -n
> and n with matlab , n is a integer .

"with matlab" -- I think you should ask in a different forum.
comp.soft-sys.matlab seems quite active at a casual glance.

-- chris

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Ricardo - 15 Jul 2006 16:24 GMT

n is a positive integer , I need to know it in Java.

> Hi wizards , I need to know how to generate random numbers between -n
> and n with matlab , n is a integer .
> Thanks in advance .

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Patricia Shanahan - 16 Jul 2006 00:45 GMT

> n is a positive integer , I need to know it in Java.
>
>> Hi wizards , I need to know how to generate random numbers between -n
>> and n with matlab , n is a integer .
>> Thanks in advance .

Do you want an integer or a double? Are the range limits inclusive, or
exclusive?

Generally, to do this sort of thing in Java you use an instance of
java.util.Random.

Patricia

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Thomas Schodt - 16 Jul 2006 06:44 GMT

>> Hi wizards , I need to know how to generate random numbers between -n
>> and n with matlab , n is a integer .
>> Thanks in advance .
>
> n is a positive integer , I need to know it in Java.

You need to understand the math.

You wish to generate random scalars in a predetermined range
between -k and k such that
-k <= x < k

now, if you add k to that you get
0 <= k+x < 2k

use java.util.Random to generate an integer (k+x) < 2k
then, to get the desired value x just subtract k.

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Ricardo - 16 Jul 2006 15:13 GMT

Thanks I understand this issue

> >> Hi wizards , I need to know how to generate random numbers between -n
> >> and n with matlab , n is a integer .
[quoted text clipped - 14 lines]
> use java.util.Random to generate an integer (k+x) < 2k
> then, to get the desired value x just subtract k.

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how to generate random number between -n and n

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