How to generate a random number in C++?

[Eddie]

say if i have an integer called number, how to get a random value for it when every time i access it?
int number = ???? ;

 

[Andy Nguyen]

#include <cstdlib> 
int number;
...
number =rand();//Whenever you need a random int

 

[Eddie]

does the random number generated in this way have a bound?
... ...
and some how my rand() always give me 41 :smt102

 

[Andy Nguyen]

does the random number generated in this way have a bound?

It's compiler-dependent. rand() will give you the posite range of int i.e [0,32767]

and some how my rand() always give me 41 :smt102

You need to do a Built-All after you chance your code (Ctrl-F11)
Also, to prevent the same number generated,use srand()

//Program uses time function to seed random number generator
//and then generates random number
#include <cstdlib>
#include <ctime>
#include <iostream>

using namespace std;

int main()
{
    srand((unsigned)time(NULL)); 
    int d=rand()%12;  
    cout<<d;
}

More about random generators in C++ here
In general, you can google for much of the info you are after. It's more time effective.

 

[Eddie]

I read abou the part on this

To generate a random number we use the rand() function. This will produce a result in the range 0 to RAND_MAX, where RAND_MAX is a constant defined by the implementation.

Could not quite make sense of the part on setting the RAND_MAX, so to translate that into actual code, how to generate a rand between say 1-100, which means need to set RAND_MAX???

 

[Andy Nguyen]

Could not quite make sense of the part on setting the RAND_MAX, so to translate that into actual code, how to generate a rand between say 1-100, which means need to set RAND_MAX???

RAND_MAX is system dependent and is located within <stdlib.h> for C and <cstdlib> for C++. Determining its value is simple, just do

cout <<RAND_MAX<<endl;

Most likely, you will see 32767 as output. So, you can't change RAND_MAX.
You have the same 41 every time you run rand() because it uses the same seed (srand() initiates that seed). Put it in a for loop to get different numbers.
To generate 1-100, use this

#include <cstdlib>
#include <ctime>
#include <iostream>
using namespace std;

int main()
{

    int j;
    const int N = 100;

    // Set initial seed
    srand( (unsigned)time( NULL ) );

    for (int i = 0; i < 5; i++) {
        j = (int) N * rand() / (RAND_MAX + 1.0);
        cout << j << endl;
    }

    return 0;
}

 

[Eddie]

why is it not possible to overwrite or redefine constant RAND_MAX to other values?

 

[Andy Nguyen]

why is it not possible to overwrite or redefine constant RAND_MAX to other values?

You can. How? Rewrite the C++ compiler.
It's not how the compiler defines some system global CONST, it's how you implement your code to achieve the desired goal.

 

[RussianMike]

srand doesnt seem that random to me (I've used this with implementation of normal distrib) lots of stuff on the net. Any good advice about generating the actual random number?

 

[ExSan]

Numerical Recipes in C++ Teukolsky/Vetterling/Flannery has some good algorithms to generate random numbers

 

[John Jurcevic]

srand doesnt seem that random to me (I've used this with implementation of normal distrib) lots of stuff on the net. Any good advice about generating the actual random number?

srand is a crappy random number generator. Try the Mersenne twister.

 

[Peter]

Of course, you need context for crappy to have any meaning.

For a large number of purposes, rand() is fine.

If you want to generate RNs that are suitable for crypto, you couldn't do much better than reading bits off of /dev/random (preferred) and /dev/urandom which get their randomness from environmental noise. That's why when you generate a GPG key, the program asks you to move the mouse around and type at the keyboard. It's storing "entropy" and converting it to RNs. I would say these device files can truly be called "RNGs" in every sense of the word (acronym). However, you'll definitely run out of entropy if you try to use them with MC. See Google for more info on these two device files. Of course, they're not available on Windows (to my knowledge), so you'll have to fall back to something else, like the Mersenne twister.

Note that there are places on the net that sell random numbers.

For Monte Carlo, I've heard (but haven't used myself) that sobol sequences are the best way to go. This would be the case where best random numbers aren't random at all.

 

[Andy Nguyen]

srand is a crappy random number generator. Try the Mersenne twister.

Yes, last time I looked into this the Mesenne twister was the way to go but of course I never got around to it.

 

[Daniel Duffy]

You should avoid C rand() at all costs; RAND_MAX is a 16-bit integer and after a million RNGs all the values are the same. Even VB is better because it's max is 32 bits.

As mentioned, MT is a good one. Here is a great place to download it Boost Random Number Library


//
BTW srand() is NOT a generator; it sets the seed for the generator rand()

 

[parisjohn]

Hi, you can find here a SIMD Mersenne Twister
http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/index.html
The big advantages of Mersenne Twister is that you can generate random number by group and consequently with CUDA