How to use System::Random in Visual C++ 2005 Express?

[Juha Nieminen]

I'm trying to use a random number generator and rand() is just not
up to the task (RAND_MAX is way too small).

I searched in the help system and found the System::Random class
which seems to be just perfect.
However, for some odd reason the help files simply refuse to tell
what should be #included in order to use the class. I can't find this
info anywhere! Even the example program given in the help lacks this
information (and thus of course doesn't compile).
I have tried google-searching, to no avail. I can't find this info
anywhere.

[Cupu]

On Apr 4, 12:32 pm, Juha Nieminen <nos...@thanks. invalidwrote:

I'm trying to use a random number generator and rand() is just not
up to the task (RAND_MAX is way too small).

I searched in the help system and found the System::Random class
which seems to be just perfect.
However, for some odd reason the help files simply refuse to tell
what should be #included in order to use the class. I can't find this
info anywhere! Even the example program given in the help lacks this
information (and thus of course doesn't compile).
I have tried google-searching, to no avail. I can't find this info
anywhere.

That's pretty much because System::Random is in .NET land (e.g. C++/
CLI) thus not really c++ or related to this group (you could try
microsoft.publi c.dotnet.langua ges.vc). Of course in case I'm wrong and
someone defined some Random class in a System namespace .. post the
example and we'll know for sure.

[=?iso-8859-1?q?Erik_Wikstr=F6m?=]

On 4 Apr, 11:32, Juha Nieminen <nos...@thanks. invalidwrote:

I'm trying to use a random number generator and rand() is just not
up to the task (RAND_MAX is way too small).

I searched in the help system and found the System::Random class
which seems to be just perfect.
However, for some odd reason the help files simply refuse to tell
what should be #included in order to use the class. I can't find this
info anywhere! Even the example program given in the help lacks this
information (and thus of course doesn't compile).
I have tried google-searching, to no avail. I can't find this info
anywhere.

System::Random is part of the .Net framework and you must create a
managed project to use it. Notice however that using such things means
that your code is no longer pure C++, but rather a vendor-specific
"extension" .

There are ways to make do with rand(), you can call rand() twice and
multiply the numbers, run it many times and add the results until you
get a result big enough, multiply the result of rand() with some
factor to scale it, and so on. Depending on what you plan to use the
random number for some approaches are better than others.

--
Erik Wikström

[P.J. Plauger]

"Erik Wikström" <er****@student .chalmers.sewro te in message
news:11******** *************@o 5g2000hsb.googl egroups.com...

On 4 Apr, 11:32, Juha Nieminen <nos...@thanks. invalidwrote:

I'm trying to use a random number generator and rand() is just not
up to the task (RAND_MAX is way too small).

I searched in the help system and found the System::Random class
which seems to be just perfect.
However, for some odd reason the help files simply refuse to tell
what should be #included in order to use the class. I can't find this
info anywhere! Even the example program given in the help lacks this
information (and thus of course doesn't compile).
I have tried google-searching, to no avail. I can't find this info
anywhere.

System::Random is part of the .Net framework and you must create a
managed project to use it. Notice however that using such things means
that your code is no longer pure C++, but rather a vendor-specific
"extension" .

There are ways to make do with rand(), you can call rand() twice and
multiply the numbers, run it many times and add the results until you
get a result big enough, multiply the result of rand() with some
factor to scale it, and so on. Depending on what you plan to use the
random number for some approaches are better than others.

[pjp] Or, if you need higher quality random distributions, see the
random package in Boost. It's also a part of TR1, which means you
get it as part of our Compleat Libraries.

P.J. Plauger
Dinkumware, Ltd.
http://www.dinkumware.com

[Juha Nieminen]

Erik Wikström wrote:

There are ways to make do with rand(), you can call rand() twice and
multiply the numbers, run it many times and add the results until you
get a result big enough, multiply the result of rand() with some
factor to scale it, and so on.

But how many values will the standard rand() give before it starts
repeating? I certainly need more than 32768 values.

[Kai-Uwe Bux]

Juha Nieminen wrote:

Erik Wikström wrote:

>There are ways to make do with rand(), you can call rand() twice and
multiply the numbers, run it many times and add the results until you
get a result big enough, multiply the result of rand() with some
factor to scale it, and so on.

But how many values will the standard rand() give before it starts
repeating? I certainly need more than 32768 values.

That's a quality of implementation issue. However, I doubt that if RAND_MAX
is that small it would also give the period.

If you need to be certain about the quality of the random number generator,
you should not use std::rand() but a known random number generator of your
choice. There are random number generators is Boost and in tr1.
Best

Kai-Uwe Bux

[Pete Becker]

Juha Nieminen wrote:

Erik Wikström wrote:

>There are ways to make do with rand(), you can call rand() twice and
multiply the numbers, run it many times and add the results until you
get a result big enough, multiply the result of rand() with some
factor to scale it, and so on.

But how many values will the standard rand() give before it starts
repeating? I certainly need more than 32768 values.

It's not specified. But note that RAND_MAX tells you the maximum value
that rand() will return. The length of a cycle (i.e. how many values it
will give before it starts repeating) is another matter. A truly bad
implementation could simply provide alternating values of 0 and
RAND_MAX, for a cycle length of 2. Another one could run through all
possible permutations of the values from 0 to RAND_MAX, for a cycle
length of RAND_MAX factorial. In practice, the length will usually be
less than RAND_MAX.

When you combine values from multiple calls to a random number
generator, the cycle length of the result depends on how you combine the
values. It's unpredictable without more information.

For good generators with well-defined characteristics (rand doesn't
qualify here: it's largely up to the implementation to determine what it
does), look at TR1's random number generators (they're also in Boost,
although that may be a bit different from what ended up in TR1). There
are several linear congruential generators with known good properties;
there's a mersenne twister; and a couple of others that are a bit more
specialized. See chapter 13 of my book, "The Standard C++ Library
Extensions," for more details.

--

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

[James Kanze]

On Apr 4, 12:50 pm, "Erik Wikström" <eri...@student .chalmers.se>
wrote:

On 4 Apr, 11:32, Juha Nieminen <nos...@thanks. invalidwrote:

I'm trying to use a random number generator and rand() is just not
up to the task (RAND_MAX is way too small).

I searched in the help system and found the System::Random class
which seems to be just perfect.
However, for some odd reason the help files simply refuse to tell
what should be #included in order to use the class. I can't find this
info anywhere! Even the example program given in the help lacks this
information (and thus of course doesn't compile).
I have tried google-searching, to no avail. I can't find this info
anywhere.

System::Random is part of the .Net framework and you must create a
managed project to use it. Notice however that using such things means
that your code is no longer pure C++, but rather a vendor-specific
"extension" .

There are ways to make do with rand(), you can call rand() twice and
multiply the numbers, run it many times and add the results until you
get a result big enough, multiply the result of rand() with some
factor to scale it, and so on. Depending on what you plan to use the
random number for some approaches are better than others.

Several others have proposed Boost, which is doubtlessly the
best solution. I'd just like to point out that the suggestions
above do not work; the resulting distribution is anything but
linear.

--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Jim Langston]

"Juha Nieminen" <no****@thanks. invalidwrote in message
news:46******** *************** @news.song.fi.. .

I'm trying to use a random number generator and rand() is just not
up to the task (RAND_MAX is way too small).

I searched in the help system and found the System::Random class
which seems to be just perfect.
However, for some odd reason the help files simply refuse to tell
what should be #included in order to use the class. I can't find this
info anywhere! Even the example program given in the help lacks this
information (and thus of course doesn't compile).
I have tried google-searching, to no avail. I can't find this info
anywhere.

Here. A random number generator that will (on my system) produce a random
number from 0 to MAX_INT

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

unsigned int BigRand( )
{
// On my system RAND_MAX is 37267 (7 bits)
int RandNum; // I have 4 bytes on my system or 32 bits
RandNum = rand(); // Low 7 bits.
RandNum = RandNum | rand() << 7; // next 7
RandNum = RandNum | rand() << 14; // next 7
RandNum = RandNum | rand() << 21; // next 7
RandNum = RandNum | ( rand() & 0xF ) << 25; // last 4 bits
return RandNum;
}

int main()
{
srand(clock());

std::cout << RAND_MAX << "\n";

for ( int i = 0; i < 100; ++i )
std::cout << BigRand( ) << "\n";

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

Modify it to taste (sizeof int and RAND_MAX on your system).

[Alf P. Steinbach]

* Jim Langston:

// On my system RAND_MAX is 37267 (7 bits)

I count that as 15 bits.

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