How to avoid random numbers duplication?

Amit Ziv

ive created 2 seperate randoms:

private Random r1 = new Random();
private Random r2 = new Random();

then i did the random.next method:

maxGreenDelay = r1.Next(10, 40);
maxRedDelay = r2.Next(10, 40);

and the same numbers appeared.
how can i avoid that? (get 2 different randoms withing this min - max values)

4 Replies

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Vulpes12y ago
Accepted answer
You need to give the Random objects different seeds.

One way to do this is to make the seeds time dependent:

private Random r1 = new Random((int) DateTime.Now.Ticks & 0x0000FFFF);
private Random r2 = new Random((int) DateTime.Now.Ticks & 0x0000FFFF);
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Munesh Sharma12y ago
You should not create a new Random instance in a loop. Try something like:

var rnd = new Random();

for(int i = 0; i < 100; ++i)

Console.WriteLine(rnd.Next(1, 100));

The sequence of random numbers generated by a single Random instance is supposed to be uniformly distributed. By creating a new Random instance for every random number in quick successions, you are likely to seed them with identical values and have them generate identical random numbers. Of course, in this case, the generated sequence will be far from uniform distribution.
For the sake of completeness, if you really need to reseed a Random, you'll create a new instance of Random with the new seed:

rnd = new Random(newSeed);
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Vulpes12y ago
The numbers produced by the Random class aren't truly random.

They're actually generated by an algorithm which is such that the sequence will eventually repeat after a very large number of numbers have been generated. Nevertheless, the sequence is sufficiently random for most practical purposes.

The sequence is generated from a starting number known as the seed. If you instantiate two Random objects with the same seed, then they will produce exactly the same sequence of numbers.

To avoid this problem, the Random constructor which takes no parameters generates a time-based seed using the system clock.

However, as a rough rule of thumb, if you create two Random objects within 10 ms of each other (quite a long time in modern computing) they will have the same seed and hence generate the same sequence of numbers.

You can improve matters by using your own time-based seed and the method I used above works on my machine as the number of ticks in DateTime.Now is slightly more when the second object is created than it was when the first object was created.

However, this might not be the case on all machines and, if it doesn't work on yours, you could increase the seed for the second object by 100 say.

Incidentally, it's not usually necessary to create two Random objects - one will suffice for most purposes - and, if you only have one object, then it avoids this particular problem.

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Amit Ziv12y ago
Hi, first of all thanks for the quick response. second, i didnt quiet understand what is the seed, can you explain a bit about it?
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