59 lines
2.0 KiB
Markdown
59 lines
2.0 KiB
Markdown
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# Sieve
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Use the Sieve of Eratosthenes to find all the primes from 2 up to a given
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number.
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The Sieve of Eratosthenes is a simple, ancient algorithm for finding all
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prime numbers up to any given limit. It does so by iteratively marking as
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composite (i.e. not prime) the multiples of each prime,
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starting with the multiples of 2.
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Create your range, starting at two and continuing up to and including the given limit. (i.e. [2, limit])
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The algorithm consists of repeating the following over and over:
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- take the next available unmarked number in your list (it is prime)
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- mark all the multiples of that number (they are not prime)
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Repeat until you have processed each number in your range.
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When the algorithm terminates, all the numbers in the list that have not
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been marked are prime.
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The wikipedia article has a useful graphic that explains the algorithm:
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https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
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Notice that this is a very specific algorithm, and the tests don't check
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that you've implemented the algorithm, only that you've come up with the
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correct list of primes.
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* * * *
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For installation and learning resources, refer to the
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[exercism help page](http://exercism.io/languages/ruby).
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For running the tests provided, you will need the Minitest gem. Open a
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terminal window and run the following command to install minitest:
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gem install minitest
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If you would like color output, you can `require 'minitest/pride'` in
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the test file, or note the alternative instruction, below, for running
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the test file.
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Run the tests from the exercise directory using the following command:
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ruby sieve_test.rb
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To include color from the command line:
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ruby -r minitest/pride sieve_test.rb
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## Source
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Sieve of Eratosthenes at Wikipedia [http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes](http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes)
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## Submitting Incomplete Solutions
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It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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