rand() 在每次函数调用时产生相同的结果（使用 srand(time(0)）

Question

I have a member function of a class that is supposed to generate a random number in a range. To do so, I am using the rand() function. The function generates a random number like this:

    unsigned seed;
    seed = time(0);
    srand(seed);
    std::cout << "Random Number: "<< rand() << std::endl;

The function is called on two different objects. The result is:

Random Number: 1321638448
Random Number: 1321638448

This is consistent every-time I call it. What am i doing wrong?

Answer 1

(Converting my comment to an answer).

For most applications, you'll only really want to seed rand once in the course of running a program. Seeding it multiple times requires you to get different random seeds, and it's easy to mess that up.

In your case, the time function usually returns something with resolution on the level of seconds (though this isn't actually required by the standard ). As a result, if you call time twice within the same second, you might get back the same value. That would explain why you're getting duplicate values: you're seeding the randomizer with the same value twice and then immediately querying it for a random number.

The best solution to this is to just seed the randomizer once. Typically, you'd do that in main .

If you really do want to seed the randomizer multiple times, make sure that you're doing so using a seed that is going to be pretty much random. Otherwise, you risk something like this happening.

Answer 2

Pseudorandom number generators basically have to pass a set of statistical tests to make sure they're "random enough" as a set of numbers. But of course, it's not actually random. Calling srand(seed) with some seed basically generates a set of numbers which, if passed through those tests, will seem "random enough".

By calling srand(seed) with the same seed multiple times, you're effectively generating the same set over and over again and getting the first value in it.

You call srand(seed) ONCE, and then you call rand() to get the next values in the random number set. Or you need to call srand(seed) with a different (random) seed each time.

If you're on linux, you can also use /dev/urandom to get a random number- the kernel has been taking signal/noise from the environment to generate "entropy" for it, supposedly making it even better than an algorithm psuedorandom number generator.

Answer 3

srand function should be called only once in program(most cases, not all cases). If you want reseed, you should use different seed number. Because rand() function is pseudo-random number generator. In other words, rand() gives you a calculated number.

You can use much for powerful random number generating library after C++11. See: http://en.cppreference.com/w/cpp/numeric/random