C/C++ C++ Random Function: Generate Uniformly Distributed Numbers

[omri3012]

Hallo,

I'm looking for a Random function in c++. i tried to use in the function rand()% but
it does not generate a truly uniformly distributed random number in the span (since my span is large. 100000*100000). in other word it dosent generate a random number in equal probabilty in lage dimentions.
if you know a function or a way that i could generate random number in large span it would
be very helpful.

Thanks,

Omri

 

[Borek]

That's because rand() generates numbers from 0..MAX_RAND range (and I think MAX_RAND is by default 32767). I remember seeing libraries for better pseudorandom number generators, shouldn't be difficult to google.

 

[harborsparrow]

you could divide your range into 32767 zones and use rand() first to get a zone, then again to get a number within the zone. etc.

 

[Borek]

That may not work correctly for pseudorandom numbers, as bits in two consecutive numbers can be correlated.

 

[mgb_phys]

you could divide your range into 32767 zones and use rand() first to get a zone, then again to get a number within the zone. etc.

Which is roughly equivalent to calling rand() twice, multiplying the first one by RAND_MAX and then adding them.

If you need super random numbers you might want to look at a better rand library such as boost rand
Otherwise calling rand twice and shift+add might be ok

 

[Codexus]

The standard rand() function is really bad and you'll run into problems very quickly (even for non-scientific applications like games).

An easy solution is to use a Mersenne twister generator instead. (There are some ready to use implementations linked at the bottom of the wiki page).

 

[shoehorn]

rand() is terrible and should not be used for anything even halfway serious. If you need a robust, fast, and accurate C++ random number generator you don't need to look any further than boost::random. You'll need to spend half an hour reading the docs in order to use it, but once you do it's a snap.