rand() not generating random numbers even after srand(time(NULL))

Question

I'm trying to call a class function using a loop

for (int i = 0; i < Basket.getLemonNum(); i++)
{
    lemonWeights[i] = Fruit.generateWeight(fruit, fruitWeight);
    cout << lemonWeights[i] << " ";
}

This goes to the Fruit class to it's member function generateWeight():

   int fruitClass::generateWeight(char fruitN, int& fruitW)
    {
    srand(time(NULL));

    int weight = 0;

    switch (fruitName)
    {
    case 'a':
        weight = rand() % 500 + 100;
        return fruitW = weight;
        break;
    case 'l':
        weight = rand() % 400 + 300; 
        return fruitW = weight;
        break;
    case 'w':
        weight = rand() % 1000 + 800; 
        return fruitW = weight;
        break;
    }
} 



Output:
128 128 128 128

but it's generating the same number all the time, even when I use a different function to call it:

for (int i = 0; i < Basket.getWatermelonNum(); i++)
{
    watermelonWeights[i] = Fruit.generateWeight(fruit, fruitWeight);
    cout << watermelonWeights[i] << " ";

}

Output:
128 128 128

As you can see, I did seed srand(). Also, the header files and are included. What is going on?

[See this](http://stackoverflow.com/questions/7343833/srand-why-call-it-only-once) — Rakete1111, Jun 15 '16 at 15:57
Using `srand()` in loop might not good, and in this case it *is* wrong. — MikeCAT, Jun 15 '16 at 15:57
Although not critical here, it is worth noting that rand() is not a particularly good pseudorandom number generator. Take a look at things like std::mt19937 — James Elderfield, Jun 15 '16 at 16:00

score 5 · Accepted Answer · answered Jun 15 '16 at 15:57

5

You must call srand() once, whereas you call it on every entry into generateWeight(). Since nowadays computers are fast and time() returns the time in seconds, this mostly restarts the random number generator from the same seed.

answered Jun 15 '16 at 15:57

Leon

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score 0 · Answer 2 · answered Jun 15 '16 at 21:00

The problem is that you initialize the pseudo random number generator seed again and again with the same seed (the time which has not changed since the last call since CPUs are very fast today) and, thus, you always get the same "random" number.

Generelly, 1) don't use srand() in a loop and 2) rand() has several defects as it does not generate nicely distributed random numbers (nice video about this rand() considered harmful)

Instead of rand() you should use std::uniform_int_distribution (requires C++11):

#include <random>
#include <iostream>

int main()
{
    std::random_device rd;
    std::mt19937 gen(rd());
    std::uniform_int_distribution<> dis(1, 6);

    for (int n=0; n<10; ++n)
        std::cout << dis(gen) << ' ';
    std::cout << '\n';
}

rand() not generating random numbers even after srand(time(NULL))

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