[Zimwy]

Hi,
In java there is lovely old math.random(). Does anyone know if there's anything close in C++?

Thanks!
Gabe

[Chuckit]

Originally posted by Zimwy:
Hi,
In java there is lovely old math.random(). Does anyone know if there's anything close in C++?

There's srand() and rand().

[Zimwy]

That's great! Thanks. Where exactly does rand() live? Also, it seems that it gives rather large numbers. I was wondering if you had any advice as to how to get a random number between 0 and 1. Thanks again,

Gabe

[Gul Banana]

rand returns longs, right? So try something along the lines of

Code:
double randomNumber = rand() / sizeof(long);

[Chuckit]

Originally posted by Zimwy:
That's great! Thanks. Where exactly does rand() live?

It lives in cstdlib.

Also, it seems that it gives rather large numbers. I was wondering if you had any advice as to how to get a random number between 0 and 1.

The formula I've learned for getting a random number in the range [0, x) is this:
rand() / (RAND_MAX / x)

So to get a number between 0 and 1, with 0 being the minimum and the number always being less than 1, it should be something along these lines:

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

int main (void)
{
    srand(time(NULL));
    cout << (double)rand() / RAND_MAX << endl;
    return 0;
}

RAND_MAX is a constant defined in stdlib.h.

[EDIT:] Real dumb typo in my code. D'oh.

[Chuckit]

Originally posted by Gul Banana:
rand returns longs, right? So try something along the lines of

Code:
double randomNumber = rand() / sizeof(long);

This won't give you a small number, because sizeof(long) = 4, and 3000000/4 is still a hella big number.

[Gul Banana]

Geh. I meant MAX_LONG, of course, not sizeof(long).

[Zimwy]

Hey,
Thanks for the great responses. I'm having a little trouble with the seed though. (I'm assuming srand(int) is what controls the seed). If I do:

srand(time(NULL));

then rand() just returns 0 all the time. If I don't ever call srand(), then it will give me random numbers, but always the same order. Does anyone know what to do about this? I've read that it's a good idea to use the time for the random number seed, but it seems that time(NULL) doesn't really cut it.

Thanks a lot!!

Gabe

[Chuckit]

Originally posted by Zimwy:
Hey,
Thanks for the great responses. I'm having a little trouble with the seed though. (I'm assuming srand(int) is what controls the seed). If I do:

srand(time(NULL));

then rand() just returns 0 all the time. If I don't ever call srand(), then it will give me random numbers, but always the same order. Does anyone know what to do about this? I've read that it's a good idea to use the time for the random number seed, but it seems that time(NULL) doesn't really cut it.

That's very strange. srand(time(NULL)) is a time-honored tradition.
Here's some simple test code I just wrote and compiled that works fine over here; see if it works on your machine.

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

int main (void)
{
        srand(time(NULL));
        for (int i = 0; i < 20; i++)
                cout << rand();
        return 0;
}

I just ran it five times, each time getting a random sequence of numbers like you'd expect, so I'm sure the code is fine. So if that doesn't work for you, it sounds like there's something a little funny with your computer.

[Zimwy]

Hi,
Yeah, that gives me a sequence of random numbers too. But, every time I run it, I get the same sequence. That's my problem. Any ideas for why that's happening?
You're only supposed to call srand(time(NULL)) once per program run, right?

Thanks,
Gabe

[Zimwy]

Never mind. I wasn't actually calling srand(time(NULL)) where I thought I was. Thanks againf or all of your various helps.

Gabe

[Wevah]

There's also random and srandom ... according to their manpages they're slower but "better".

*shrug*

[Angus_D]

Yeah, rand() isn't terribly random.