rand() returns same values when called within a single function

Question

I'm a C++ newbie and I'm stumped on this. I need to call this function in my main function three times but each time it gives me the same result, i.e. pull_1, pull_2, pull_3 are the same. What do I need to do to make them actually random?

string PullOne()
{
    string pick;
    string choices[3] = {"BAR", "7", "cherries"};

    std::srand(time(0));
    pick = choices[(std::rand() % 3)];
    return pick;
}

From my main function:

string pull_1, pull_2, pull_3;
pull_1 = PullOne();
pull_2 = PullOne();
pull_3 = PullOne();

BTW, if you use `rand() % 3`, you introduce a bias in the output: eg. values 0, 1, 2 don't have the same probability. See https://stackoverflow.com/questions/10984974/why-do-people-say-there-is-modulo-bias-when-using-a-random-number-generator — Yann Droneaud, Jul 17 '18 at 17:20

score 17 · Accepted Answer · edited Feb 21 '19 at 10:27

17

You shouldn't call srand() before each call to rand(). Call it once – somewhere at the start of your program.

The problem is you restart the random generator so it starts to produce the very same pseudorandom sequence from the very same point.

edited Feb 21 '19 at 10:27

Baum mit Augen

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answered Jul 18 '11 at 06:22

sharptooth

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Only if the function is called multiple times during the same second you get the same results. – Yann Droneaud Jul 17 '18 at 17:18

Tugrul Ates · Answer 2 · 2011-07-18T06:25:39.997

14

The random number generator is reset to an initial state, which is dictated by the seed value, every time you call srand. Time value may be the same between successive calls to time, hence the same seed and the same number generated.

Call seeding function (srand) only once in your main function before generating random samples.

edited Jul 18 '11 at 06:25

answered Jul 18 '11 at 06:20

Tugrul Ates

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score 7 · Answer 3 · answered Jul 18 '11 at 06:21

Why do you keep calling std::srand(time(0));? That re-seeds the PRNG.... and because this all happens within the same second, you're always re-seeding it with the same sequence.

Call srand once in your program, and once only.

Also, I would recommend, at least on POSIX-compliant systems, something like std::srand(time(0) ^ getpid()), so that you can run your program twice within the same "second" and still get a new PRNG sequence.

vsekhar · Answer 4 · 2011-07-18T06:27:40.840

2

The time(0) function may not have 'ticked' between function calls. So you are seeding the random number generator with the same value each time, leading to identical values for rand()

edited Jul 18 '11 at 06:27

answered Jul 18 '11 at 06:21

vsekhar

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I don't see what clock granularity has to do with it. The "second" is a pretty universal measure of time... – Lightness Races in Orbit Jul 18 '11 at 06:22
Corrected. (some libraries provide sub-second resolution, but the std library does not). – vsekhar Jul 18 '11 at 06:28

score 1 · Answer 5 · answered Jul 18 '11 at 06:22

Your problem is that you seed the RNG each time you call the function. You should only seed this once in a program for best results. If you want the result to vary from one execution of the program to the next, srand based on the result of the time() function.

rand() returns same values when called within a single function

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