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Generate random numbers without repetitions

up vote 14 down vote

I want to generate a list of N different random numbers:

public static List<int> GetRandomNumbers(int count)
{
    List<int> randomNumbers = new List<int>(); 

    for (int i=0; i<count; i++) 
    {    
        int number;

        do number = random.Next();
        while (randomNumbers.Contains(number));

        randomNumbers.Add(number);
    }

    return randomNumbers;
}

But I feel there is a better way. This do...while loop especially makes this ugly. Any suggestions on how to improve this?

2 Answers

up vote 4 down vote accept

The algorithm looks fine to me. It shows no clear flaws, and will work fine in common use cases.
For extreme cases of selecting most of the numbers in a very large interval, it will be very inefficient. But unless that is an expected use case, I would not bother changing the code to more complicated approaches.

But I suggest you always use explicit braces around blocks even when they have a single statement:

do
{
    bar();
}
while (foo);

I would also rename the method to GetNRandomNumbers, to make the method's name match its purpose better.

up vote 3 down vote

Another solution is to use a Format Preserving Encryption cipher (such as AES using the FFX mode), configured to map from 32 bit integers to 32 bit integers.

Seed it randomly, then simply encrypt the first 'count' integers.

These encrypted numbers will be as random as the seed, and won't repeat.

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Generate random numbers without repetitions

up vote 14 down vote

I want to generate a list of N different random numbers:

public static List<int> GetRandomNumbers(int count)
{
    List<int> randomNumbers = new List<int>(); 

    for (int i=0; i<count; i++) 
    {    
        int number;

        do number = random.Next();
        while (randomNumbers.Contains(number));

        randomNumbers.Add(number);
    }

    return randomNumbers;
}

But I feel there is a better way. This do...while loop especially makes this ugly. Any suggestions on how to improve this?

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up vote 9 down vote

The algorithm looks fine to me. It shows no clear flaws, and will work fine in common use cases.
For extreme cases of selecting most of the numbers in a very large interval, it will be very inefficient. But unless that is an expected use case, I would not bother changing the code to more complicated approaches.

But I suggest you always use explicit braces around blocks even when they have a single statement:

do
{
    bar();
}
while (foo);

I would also rename the method to GetNRandomNumbers, to make the method's name match its purpose better.

edit

@FlorianF this is Code Review, not Stack Overflow. Reviewers are always free to comment on any aspect of the code. Always. - Mathieu Guindon Sep 4 '14 at 23:44

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