[RESOLVED] question: how to generate random numbers

[Harsh Gupta]

hello,

i know to use srand, rand funcs but they do not generate random numbers.

basically i have 2 questions-

1: does Dev C++ has randomize() and random() functions??

2: do i have to loop again to check if the newly generated random number have been generated before, or is there any function present to do this?

thank you.

[Jmacp]

#include <iostream>
#include <conio.h>
using namespace std;

int main(){

	srand(time(NULL));
    for(int i= 0; i<10; i ++){
	cout<<rand()%10<<endl;
}
    getch();
	return 0;
}

[Harsh Gupta]

Jmacp,

thank you for your response. i already know this. but have you tried your code?? it may not generate different random numbers. thats what my query is: do i have to store these numbers somewhere (like array) and loop to check if the newly generated number was also generated before.

thank you.

[CornedBee]

rand() has no guarantee that it doesn't create the same number twice or more times in a row. The only guarantee it has is that, over time, the distribution of the numbers it generates is uniform over the range 0 <= n <= RAND_MAX. Doing a modulo 10 on this range will make repeats a lot more likely (and since 10 is neither a prime number nor a power of 2, it will also make the distribution non-uniform, which is when quality numbers are needed, a different method is used.)

But if you want high-quality pseudorandom numbers, you should look into Boost.Random, which is also part of C++ TR1.

[Harsh Gupta]

rand() has no guarantee that it doesn't create the same number twice or more times in a row. The only guarantee it has is that, over time, the distribution of the numbers it generates is uniform over the range 0 <= n <= RAND_MAX. Doing a modulo 10 on this range will make repeats a lot more likely (and since 10 is neither a prime number nor a power of 2, it will also make the distribution non-uniform, which is when quality numbers are needed, a different method is used.)

But if you want high-quality pseudorandom numbers, you should look into Boost.Random, which is also part of C++ TR1.

thanks, thats look promising.

could you please explain how to use them?

[CornedBee]

Nope, never used them myself, so I don't know. Read the documentation.

But there's a very simple example right at the page I linked.

  boost::mt19937 rng;                 // produces randomness out of thin air
                                      // see pseudo-random number generators
  boost::uniform_int<> six(1,6)       // distribution that maps to 1..6
                                      // see random number distributions
  boost::variate_generator<boost::mt19937&, boost::uniform_int<> >
           die(rng, six);             // glues randomness with mapping
  int x = die();                      // simulate rolling a die

All you need to do is replace the range with whatever range you want (that would be (0,9) in Jmacp's example).

[SpS]

#include <iostream>
#include <conio.h>
using namespace std;

int main(){

	srand(time(NULL));
    for(int i= 0; i<10; i ++){
	cout<<rand()%10<<endl;
}
    getch();
	return 0;
}

<conio.h> non-standard header
getch() non-standard function
and you need <ctime> for your code to compile

[Harsh Gupta]

<conio.h> non-standard header
getch() non-standard function
and you need <ctime> for your code to compile

thats 1 headache in Dev C++, it will compile the code even if <ctime> header file is not included!! (atleast Jmacp's code is running on my pc without <ctime>, but of course, not solving my problem)

i wonder how and why??

[CornedBee]

Because standard headers can and do internally include each other. Thus, your particular standard library might include ctime from, say, iostream, and everytime you grab iostream you also get ctime.
The problem is that you can't rely on this behaviour, because other standard libraries might do things differently.

Another issue is that GCC is, due to legacy code still compiled with it, very forgiving regarding not declaring functions. You can call any function that returns int or void without ever declaring it, and GCC will merely emit a warning, and that only if you've got all warnings enabled.

[Harsh Gupta]

thats what i thought of CB, but just wanted to confirm. thank you.

all right then, i believe that either i have to use files from boost.org (as suggested by CB) or using the standard method of srand with loop checking all the previous generated random numbers.

and still one question left: does Dev C++ has randomize() and random() functions??

thank you.

[CornedBee]

No, I'm pretty sure it doesn't.

[Harsh Gupta]

No, I'm pretty sure it doesn't.

yeah, it seems so.

thanks to all.