Quick C++ srand() question!!! - Overclock.net

Gnarly · 06-22-09

So I have a function that returns a random float, but it only works once. As you can see, I try calling this function 100 times, and it returns the same random number each time. I figure there is an easy explanation for this, but after playing with it, I can't figure it out.

Code:

#include <cstdlib>
#include <iostream>
#include <ctime> 

using namespace std;

class DataStream
{
      public:
      int randnum();
	  void mstimer();

};
	  
int DataStream::randnum()
{
srand(time(0));
float rnd_val = ((float)rand() / RAND_MAX * 20.0f) + 170;
cout << scientific << rnd_val << endl;
};

int main() 
{ 

DataStream p;
for (int i = 100; i>1; i--)
{
p.randnum();
}




int q;
cin >> q;
return 0;
};

DuckieHo · 06-22-09

You need a seed value. You can use current time: srand(time(0))

srand() creates a psuedorandom number for a seed value. i.e. srand(100) will always produce the same random number. srand of the ever changing current time will fill it with different seeds and hence different values every millisecond.

Gnarly · 06-22-09

I have it in there, and it works if I put a loop in the function. However, I need the function to a return a unique value every time I call it in my int main().

Code:

int DataStream::randnum()
{
srand(time(0));
float rnd_val = ((float)rand() / RAND_MAX * 20.0f) + 170;
cout << scientific << rnd_val << endl;
};

Gnarly · 06-22-09

Part of it has to do with my program running too fast. time(0) isn't changing values. When I slow down my program (making it go though an arbitrarly large loop before calling time(0) again) I get different random numbers, but only slightly e.g. 173.25677 then 173.25679.

rabidgnome229 · 06-22-09

You only need to seed the generator once, not every time you generate a number

Gnarly · 06-22-09

*slaps his own face*

Thank you!

Reps all around.

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Quick C++ srand() question!!!

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