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Generate random numbers without repetitions
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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2 Answers
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.
2,85213 silver badges29 bronze badges
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.
1911 bronze badge
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Generate random numbers without repetitions
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?
2,3844 gold badges18 silver badges35 bronze badges
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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.
2,85213 silver badges29 bronze badges
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