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digitalmars.D.learn - Random Numbers

reply "Ralph Main" <rmain1972 live.com> writes:
Gotchas!

The module std.random documentation doesn't work as per the 
examples.
The example shows getting a random number by the following code 
does not work:

<code>
     // Generate a uniformly-distributed integer in the range [0, 
14]
     auto i = uniform(0, 15);
     // Generate a uniformly-distributed real in the range [0, 100)
     // using a specific random generator
     Random gen;
     auto r = uniform(0.0L, 100.0L, gen);
</code>

<code>
     // Gets a random number
     int get_random() {
         auto rng = new Random(unpredictableSeed);
         auto rn = uniform(0, m_files.length, rng);
         return rn;
     }
</code>

The new keyword was not in the example, and the original example 
code would not work.  When looking at the source code of the std 
libraries, a struct can contain a constructor, so therefore it is 
similar to a class; and on a whim I tried the new keyword.  So I 
thought I would pass this information along.  I looked at other 
posts in the forum, but didn't see anyone using the new keyword.  
Is this a bug, or a change to the D language implementation?
Aug 04 2012
next sibling parent reply Philippe Sigaud <philippe.sigaud gmail.com> writes:
On Sat, Aug 4, 2012 at 6:40 PM, Ralph Main <rmain1972 live.com> wrote:

Hello Ralph,


 The module std.random documentation doesn't work as per the examples.
 The example shows getting a random number by the following code does not
 work:


(snip)

Both code samples you gave work perfectly for me (DMD 2.060, Linux).
Which version are you using?


 The new keyword was not in the example, and the original example code would
 not work.  When looking at the source code of the std libraries, a struct
 can contain a constructor, so therefore it is similar to a class; and on a
 whim I tried the new keyword.  So I thought I would pass this information
 along.  I looked at other posts in the forum, but didn't see anyone using
 the new keyword.  Is this a bug, or a change to the D language
 implementation?


The 'new' keyword has been in D from the beginning, but it's used to
create classes (which are reference types in D) or values on the heap.
Most of the time, you create a struct like this:

MyStruct s = MyStruct(arguments);

For a class:

MyClass c = new MyClass(arguments);

See: http://dlang.org/expression.html#NewExpression
Aug 04 2012
parent reply "Ralph Main" <rmain1972 live.com> writes:
On Saturday, 4 August 2012 at 16:59:09 UTC, Philippe Sigaud wrote:

 On Sat, Aug 4, 2012 at 6:40 PM, Ralph Main <rmain1972 live.com> 
 wrote:

 Hello Ralph,


 The module std.random documentation doesn't work as per the 
 examples.
 The example shows getting a random number by the following 
 code does not
 work:


 (snip)

 Both code samples you gave work perfectly for me (DMD 2.060, 
 Linux).
 Which version are you using?


DMD32 D Compiler v2.059 on Linux (Fedora 17)

All I know, is that it wasn't working, and I finally got it to 
work when I used the new keyword.

I tried the code in just main() like bearophile's post, and it 
worked just fine.  I also removed the new keyword from my program 
and it compiled.

The error message that I got yesterday with the example code 
returned an error message that had something do with void and 0 
arguments.

<shrug>

Thanks to both of you for the information.
Aug 04 2012
parent reply Philippe Sigaud <philippe.sigaud gmail.com> writes:
On Sat, Aug 4, 2012 at 7:27 PM, Ralph Main <rmain1972 live.com> wrote:


 I tried the code in just main() like bearophile's post, and it worked just
 fine.  I also removed the new keyword from my program and it compiled.

 The error message that I got yesterday with the example code returned an
 error message that had something do with void and 0 arguments.


In your previous code, did you put it in the module scope or in a
function? (main() is a function)
Aug 04 2012
parent "Ralph Main" <rmain1972 live.com> writes:
On Saturday, 4 August 2012 at 20:22:23 UTC, Philippe Sigaud wrote:

 On Sat, Aug 4, 2012 at 7:27 PM, Ralph Main <rmain1972 live.com> 
 wrote:


 I tried the code in just main() like bearophile's post, and it 
 worked just
 fine.  I also removed the new keyword from my program and it 
 compiled.

 The error message that I got yesterday with the example code 
 returned an
 error message that had something do with void and 0 arguments.


 In your previous code, did you put it in the module scope or in 
 a
 function? (main() is a function)


It was in the scope of a public function in a class.  I was 
experimenting with the D language.  Trying it out.  I like it, 
but there are some things that I don't like (but that is true of 
any programming language.)

class A {
private:
      //Code
public:
     int get_random() {
     //Code
     }
}
Aug 04 2012
prev sibling parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Ralph Main:


 The module std.random documentation doesn't work as per the 
 examples.
 The example shows getting a random number by the following code 
 does not work:

 <code>
     // Generate a uniformly-distributed integer in the range 
 [0, 14]
     auto i = uniform(0, 15);
     // Generate a uniformly-distributed real in the range [0, 
 100)
     // using a specific random generator
     Random gen;
     auto r = uniform(0.0L, 100.0L, gen);
 </code>

 <code>
     // Gets a random number
     int get_random() {
         auto rng = new Random(unpredictableSeed);
         auto rn = uniform(0, m_files.length, rng);
         return rn;
     }
 </code>


This code works, but it's a bad idea to create a new generator 
inside getRandom():


import std.stdio, std.random;

// Gets a random number, badly
int getRandom(int m) {
     auto rng = new Random(unpredictableSeed);
     return uniform(0, m, rng);
}

void main() {
     // Generate a uniformly-distributed integer in the range [0, 
14]
     auto i = uniform(0, 15);

     // Generate a uniformly-distributed real in the range [0, 100)
     // using a specific random generator
     Random gen;
     auto r = uniform(0.0L, 100.0L, gen);

     writeln(r);
     writeln(getRandom(10));
}



 The new keyword was not in the example, and the original 
 example code would not work.  When looking at the source code 
 of the std libraries, a struct can contain a constructor, so 
 therefore it is similar to a class; and on a whim I tried the 
 new keyword.  So I thought I would pass this information along.
  I looked at other posts in the forum, but didn't see anyone 
 using the new keyword.  Is this a bug, or a change to the D 
 language implementation?


In D you instantiate a class with new, it generally gets 
allocated on the heap, and what you obtain is a class reference, 
that is a kind of pointer.

Structs can be allocated with new, usually on the heap, and you 
get a pointer to a struct. Or they can be created locally without 
"new", often on the stack or inside another struct/class 
instance, and what you obtain is a struct value.

std.random.Random is a struct.

std.random.uniform() as third optional value seems to accept both 
a struct pointer and a struct (that it takes by reference, so 
using a pointer adds another indirection level, and this is not 
good. I don't know if the D compiler is able to remove this extra 
indirection level).

Are my answers enough?

Bye,
bearophile
Aug 04 2012
parent reply "Ralph Main" <rmain1972 live.com> writes:
On Saturday, 4 August 2012 at 17:02:19 UTC, bearophile wrote:

 Ralph Main:


 The module std.random documentation doesn't work as per the 
 examples.
 The example shows getting a random number by the following 
 code does not work:

 <code>
    // Generate a uniformly-distributed integer in the range 
 [0, 14]
    auto i = uniform(0, 15);
    // Generate a uniformly-distributed real in the range [0, 
 100)
    // using a specific random generator
    Random gen;
    auto r = uniform(0.0L, 100.0L, gen);
 </code>

 <code>
    // Gets a random number
    int get_random() {
        auto rng = Random(unpredictableSeed);
        auto rn = uniform(0, m_files.length, rng);
        return rn;
    }
 </code>


 This code works, but it's a bad idea to create a new generator 
 inside getRandom():


get_random is inside of a class.  Because of scope problems, I 
don't think I can put it anywhere else.  It would be better if 
you could create rng as

int rng;
rng = Random(unpredictableSeed);

then you would be able to put it elsewhere.

Ralph
Aug 04 2012
parent "Ralph Main" <rmain1972 live.com> writes:
On Saturday, 4 August 2012 at 17:45:19 UTC, Ralph Main wrote:

 On Saturday, 4 August 2012 at 17:02:19 UTC, bearophile wrote:

 Ralph Main:


 The module std.random documentation doesn't work as per the 
 examples.
 The example shows getting a random number by the following 
 code does not work:

 <code>
   // Generate a uniformly-distributed integer in the range 
 [0, 14]
   auto i = uniform(0, 15);
   // Generate a uniformly-distributed real in the range [0, 
 100)
   // using a specific random generator
   Random gen;
   auto r = uniform(0.0L, 100.0L, gen);
 </code>

 <code>
   // Gets a random number
   int get_random() {
       auto rng = Random(unpredictableSeed);
       auto rn = uniform(0, m_files.length, rng);
       return rn;
   }
 </code>


 This code works, but it's a bad idea to create a new generator 
 inside getRandom():


 get_random is inside of a class.  Because of scope problems, I 
 don't think I can put it anywhere else.  It would be better if 
 you could create rng as

 int rng;
 rng = Random(unpredictableSeed);

 then you would be able to put it elsewhere.

 Ralph


I did take out the new keyword. So I guess the garbage collector 
will be happier. ;)
Aug 04 2012