Unique Random Numbers

[Xoomer]

Hi,

Can some post a code with simple Unique Random Numbers.

Random numbers stored in arr[20] but unique.

[=?ISO-8859-1?Q?Erik_Wikstr=F6m?=]

On 2007-03-25 16:31, Xoomer wrote:

Hi,

Can some post a code with simple Unique Random Numbers.

Random numbers stored in arr[20] but unique.

From the top of my head:

#include <cstdlib>
#include <ctime>

int main()
{
srand(time(0));
int arr[20];

for (int i = 0; i < 20; ++i)
arr[i] = rand();

return 0;
}

--
Erik Wikström

[Xoomer]

On Mar 25, 3:53 pm, Erik Wikström <[e-mail]:

On 2007-03-25 16:31, Xoomer wrote:

Hi,

Can some post a code with simpleUniqueRandomNumbers.

Randomnumbersstored in arr[20] butunique.

From the top of my head:

#include <cstdlib>
#include <ctime>

int main()
{
srand(time(0));
int arr[20];

for (int i = 0; i < 20; ++i)
arr[i] = rand();

return 0;

}

--
Erik Wikström

Thanks. But the number may repeat the same.. its not Unique.

[Xoomer]

On Mar 25, 3:53 pm, Erik Wikström <[e-mail]:

On 2007-03-25 16:31, Xoomer wrote:

Hi,

Can some post a code with simpleUniqueRandomNumbers.

Randomnumbersstored in arr[20] butunique.

From the top of my head:

#include <cstdlib>
#include <ctime>

int main()
{
srand(time(0));
int arr[20];

for (int i = 0; i < 20; ++i)
arr[i] = rand();

return 0;

}

--
Erik Wikström

Thanks. But the random number may repeat the same.. its not Unique.

[red floyd]

Xoomer wrote:

Hi,

Can some post a code with simple Unique Random Numbers.

Random numbers stored in arr[20] but unique.

Your answer may be found at
http://www.parashift.com/c++-faq-lit...t.html#faq-5.2

[osmium]

"Xoomer" writes:

Can some post a code with simple Unique Random Numbers.

Random numbers stored in arr[20] but unique.

The magic word is shuffle. See what you can find on the net.

[Jacek Dziedzic]

Xoomer wrote:

Hi,

Can some post a code with simple Unique Random Numbers.

Random numbers stored in arr[20] but unique.

Well, if the array is only 20 elements long, why not
generate a random number for every entry i, and then
compare it with all previous numbers arr[0..i-1], if
it compares, throw it away and generate a new one?

Not very efficient, but for 20 elements it would be fine.

HTH,
- J.

[rossum]

On 25 Mar 2007 07:59:37 -0700, "Xoomer" <sh*****@gmail.comwrote:

>On Mar 25, 3:53 pm, Erik Wikström <[e-mail]:

>On 2007-03-25 16:31, Xoomer wrote:

Hi,

Can some post a code with simpleUniqueRandomNumbers.

>Randomnumbersstored in arr[20] butunique.

From the top of my head:

#include <cstdlib>
#include <ctime>

int main()
{
srand(time(0));
int arr[20];

for (int i = 0; i < 20; ++i)
arr[i] = rand();

return 0;

}

--
Erik Wikström

Thanks. But the number may repeat the same.. its not Unique.

Highly unlikely. The repeat length of any halfway decent RNG is a lot
longer than 20. Knuth Chapter Three refers.

rossum

[Pete Becker]

rossum wrote:

Highly unlikely. The repeat length of any halfway decent RNG is a lot
longer than 20. Knuth Chapter Three refers.

That's repetition of the sequence, not of individual values.

--

-- Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com)
Author of "The Standard C++ Library Extensions: a Tutorial and
Reference." (www.petebecker.com/tr1book)

[Jim Langston]

"Xoomer" <sh*****@gmail.comwrote in message
news:11*********************@d57g2000hsg.googlegro ups.com...

Hi,

Can some post a code with simple Unique Random Numbers.

Random numbers stored in arr[20] but unique.

Unique, but sorted.

#include <iostream>
#include <string>
#include <set>
#include <ctime>

int main()
{
std::set<intRandomNumbers;
std::srand( std::clock() );

while ( RandomNumbers.size() < 20 )
RandomNumbers.insert( RandomNumbers.end(), rand() );

for ( std::set<int>::iterator it = RandomNumbers.begin(); it !=
RandomNumbers.end(); ++it )
std::cout << *it << " ";
std::cout << std::endl;

std::string wait;
std::getline( std::cin, wait );

}

[Alexander D. B. Kim]

How about just make the array first and then mixing the numbers by
using the random function?

For example...

#include <cstdlib>
#include <ctime>

int main()
{
srand(time(0));
int arr[20] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19 };
int repeat = rand() % HOW_MANY_TIMES_DO_YOU_WANT;
int pos = 0;

for (int i = 0; i < repeat; ++i)
{
pos = rand() % 20;

if(pos == 19)
swap(0, 19); // Swapping the first and the last
else
swap(pos, pos+1); // Swapping with the next
}
return 0;
}

I just made the source code without thinking deeply and using IDE so
if there is a silly mistake then please forgive me =)

Cheers,

On Mar 26, 10:29 am, "Jim Langston" <[e-mail]:

"Xoomer" <[e-mail] in message

news:11*********************@d57g2000hsg.googlegro ups.com...

Hi,

Can some post a code with simple Unique Random Numbers.

Random numbers stored in arr[20] but unique.

Unique, but sorted.

#include <iostream>
#include <string>
#include <set>
#include <ctime>

int main()
{
std::set<intRandomNumbers;
std::srand( std::clock() );

while ( RandomNumbers.size() < 20 )
RandomNumbers.insert( RandomNumbers.end(), rand() );

for ( std::set<int>::iterator it = RandomNumbers.begin(); it !=
RandomNumbers.end(); ++it )
std::cout << *it << " ";
std::cout << std::endl;

std::string wait;
std::getline( std::cin, wait );

}

[Pete Becker]

Alexander D. B. Kim wrote:

How about just make the array first and then mixing the numbers by
using the random function?

Use std::random_shuffle.

--

-- Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com)
Author of "The Standard C++ Library Extensions: a Tutorial and
Reference." (www.petebecker.com/tr1book)

[rossum]

On Sun, 25 Mar 2007 16:14:23 -0400, Pete Becker
<pe**@versatilecoding.comwrote:

>rossum wrote:

>Highly unlikely. The repeat length of any halfway decent RNG is a lot
longer than 20. Knuth Chapter Three refers.

That's repetition of the sequence, not of individual values.

Correct, but with a reasonable linear congruential generator
individual values do not repeat until the whole sequence repeats. If
the OP limits responses to a given range then repeats do become
possible.

rossum

[Alexander D. B. Kim]

Thanks =)

On Mar 26, 9:47 pm, Pete Becker <[e-mail]:

Alexander D. B. Kim wrote:

How about just make the array first and then mixing the numbers by
using the random function?

Use std::random_shuffle.

--

-- Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com)
Author of "The Standard C++ Library Extensions: a Tutorial and
Reference." (www.petebecker.com/tr1book)

[rmpowell77]

On Mar 25, 7:31 am, "Xoomer" <[e-mail]:

Hi,

Can some post a code with simple Unique Random Numbers.

Random numbers stored in arr[20] but unique.

Jon Bentley's book Programming Pearls Second Edition has this exact
question and discussion. Check chapter 1 problem 4.

[Kai-Uwe Bux]

rossum wrote:

On Sun, 25 Mar 2007 16:14:23 -0400, Pete Becker
<pe**@versatilecoding.comwrote:

>>rossum wrote:

>>Highly unlikely. The repeat length of any halfway decent RNG is a lot
longer than 20. Knuth Chapter Three refers.

That's repetition of the sequence, not of individual values.

Correct, but with a reasonable linear congruential generator
individual values do not repeat until the whole sequence repeats.

That's one of their weaknesses: the Birthday paradox shows that repetitions
_should_ happen much earlier.

If
the OP limits responses to a given range then repeats do become
possible.

They also become possible if the OP uses a different RNG.
Best

Kai-Uwe Bux

[Seemanta Dutta]

Xoomer wrote:

On Mar 25, 3:53 pm, Erik Wikström <[e-mail]:

>On 2007-03-25 16:31, Xoomer wrote:

>>Hi,
Can some post a code with simpleUniqueRandomNumbers.
Randomnumbersstored in arr[20] butunique.

From the top of my head:

#include <cstdlib>
#include <ctime>

int main()
{
srand(time(0));
int arr[20];

for (int i = 0; i < 20; ++i)
arr[i] = rand();

return 0;

}

--
Erik Wikström

Thanks. But the random number may repeat the same.. its not Unique.

Yes, this might generate repeated random numbers.
You can try generating the index randomly and assign
monotonically increasing count value to that array element,
after checking the presence of a flag.

Here's how:

int main()
{
srand(time(0));
int arr[20] = {-1}; //init the array with a flag

for (int i = 0; i < 20; ++i)
{
if(arr[i] == -1) //assign counter only if flag is found
{
arr[rand()%20] = i;
}
}
return 0;

}

Hope this helps...

regards,
Seemanta

[Alexander D. B. Kim]

On Mar 26, 9:53 pm, Seemanta Dutta <[e-mail]:

Xoomer wrote:

On Mar 25, 3:53 pm, Erik Wikström <[e-mail]:

On 2007-03-25 16:31, Xoomer wrote:

>Hi,
Can some post a code with simpleUniqueRandomNumbers.
Randomnumbersstored in arr[20] butunique.
From the top of my head:

#include <cstdlib>
#include <ctime>

int main()
{
srand(time(0));
int arr[20];

for (int i = 0; i < 20; ++i)
arr[i] = rand();

return 0;

}

--
Erik Wikström

Thanks. But the random number may repeat the same.. its not Unique.

Yes, this might generate repeated random numbers.
You can try generating the index randomly and assign
monotonically increasing count value to that array element,
after checking the presence of a flag.

Here's how:

int main()
{
srand(time(0));
int arr[20] = {-1}; //init the array with a flag

for (int i = 0; i < 20; ++i)
{
if(arr[i] == -1) //assign counter only if flag is found
{
arr[rand()%20] = i;
}
}
return 0;

}

Hope this helps...

regards,
Seemanta

what if "rand()%20" keeps generating the same number (what a
coincidence!!!) =P