用随机数填充向量

Question

I have a function in a class I'm writing that is just supposed to fill a vector with random numbers. For some reason, it only generates the same random number and fills the array with that number, despite using a loop where rand() is called over and over. I seeded rand with time(0). I tried placing the seed inside of the loop and it made no difference. Here's my function. 'array' is actually a vector.

void fillArray()
   {
      srand(time(0));

      for (unsigned int i = 0; i < array.size(); i++)
      {
         array.at(i) = rand() % 200;
      }
   }

Answer 1

You could check the state of array. Perhaps, size () is zero when the call of for is done, in which case nothing is written into array. I suggest that you check that first.

Moreover, to perform module 200 is not good for randomness. I copy a snippet from man page for rand():

In  Numerical Recipes in C: The Art of Scientific Computing (William H.
       Press, Brian P. Flannery, Saul A. Teukolsky, William T. Vetterling; New
       York:  Cambridge University Press, 1992 (2nd ed., p. 277)), the follow-
       ing comments are made:
          "If you want to generate a random integer between 1 and 10,  you
          should always do it by using high-order bits, as in

             j = 1 + (int) (10.0 * (rand() / (RAND_MAX + 1.0)));

          and never by anything resembling

             j = 1 + (rand() % 10);

          (which uses lower-order bits)."

Answer 2

The code above is apparently correct to fill a vector with random numbers. The error was coming from this function in my class:

int at(int index)
   {
      return array.at(index);
   }

It was created to use the vector class's at function, but I originally declared it const which made it return the same index for every loop iteration.