Question

I’m trying to generate a bunch of random numbers using rand() % (range).
Here’s how my code is setup :

srand(time(NULL));
someClass *obj = new someClass(rand()%range1,rand()%range2... etc ); // i.e. a number of random numbers one after the other

Whenever I run this, it seems all the calls to rand() generate the same number. I tried doing it without the : (edit : all rand() do not generate the same number it seems , read edit at the end)

srand(time(NULL));

then , every execution of the program yields the same results.

Also, since all calls to rand() are in a constructor , I cant really reseed it all the time. I guess I can create all objects sent to the constructor beforehand and reseed the random number generator in between, but it seems like an inelegant solution.

How can I generate a bunch of different random numbers ?

edit: It seems because I was creating a lot of objects in a loop, so every time the loop iterated srand(time(NULL)) was reseeded and the sequence got reset ( as time(NULL) has a resolution of a second) , that’s why all subsequent objects had very similar properties.

Answer 1

If you call srand once, then all subsequent rand calls will return (different) pseudorandom numbers. If they don’t, you’re doing it wrong. 🙂

Apart from this, rand is pretty useless. Boost.Random (or the C++11 standard library <random> header) provides much more powerful random number generators, with nicer, more modern interfaces as well (for example allowing you to have multiple independent generators, unlike rand which uses a single global seed)