class

# Prime

v2_1_10 - Show latest stable - 1 note - Superclass: Object

The set of all prime numbers.

## Example

```Prime.each(100) do |prime|
p prime  #=> 2, 3, 5, 7, 11, ...., 97
end
```
```Prime.first 5 # => [2, 3, 5, 7, 11]
```

## Retrieving the instance

Prime.new is obsolete. Now Prime has the default instance and you can access it as Prime.instance.

For convenience, each instance method of Prime.instance can be accessed as a class method of Prime.

e.g.

```Prime.instance.prime?(2)  #=> true
Prime.prime?(2)           #=> true
```

## Generators

A “generator” provides an implementation of enumerating pseudo-prime numbers and it remembers the position of enumeration and upper bound. Furthermore, it is an external iterator of prime enumeration which is compatible with an Enumerator.

Prime::PseudoPrimeGenerator is the base class for generators. There are few implementations of generator.

Prime::EratosthenesGenerator

Uses eratosthenes’ sieve.

Prime::TrialDivisionGenerator

Uses the trial division method.

Prime::Generator23

Generates all positive integers which are not divisible by either 2 or 3. This sequence is very bad as a pseudo-prime sequence. But this is faster and uses much less memory than the other generators. So, it is suitable for factorizing an integer which is not large but has many prime factors. e.g. for Prime#prime? .

Show files where this class is defined (1 file)
August 15, 2008
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#### Cheking if a number is prime?

It’s a class for generating an enumerator for prime numbers and traversing over them.

It’s really slow and will be replaced in ruby 1.9 with a faster one.

Note: if you just want to test whether a number is prime or not, you can use this piece of code:

```class Fixnum
def prime?
('1' * self) !~ /^1?\$|^(11+?)\1+\$/
end
end

10.prime?```