## The Sieve of Eratosthenes

### Sieve of what??

The Sieve of Eratosthenes is an ancient algorithm for finding all prime numbers up to a given limit. It is one of the most efficient ways of finding small prime numbers, however above a certain threshold other sieves like the Sieve of Atkin or Sieve of Sundaram should be considered instead.

The sieve itself is quite simple to understand, and translating it to code is very straightforward, however since writing a blog post about current exercises is highly encouraged by a course that I'm taking right now, I took the opportunity to start writing again.

### How it works

The Sieve of Eratosthenes works by identifying a prime within the range of numbers you're considering (ex. 1 to 25), then crossing out all multiples of that prime (since it's a multiple it obviously can't be a prime...).

$$\left[\begin{matrix} 1 & 2 & 3 & 4 & 5 \\ 6 & 7 & 8 & 9 & 10 \\ 11 & 12 & 13 & 14 & 15 \\ 16 & 17 & 18 & 19 & 20 \\ 21 & 22 & 23 & 24 & 25 \\ \end{matrix}\right]$$

Starting with 2, the smallest prime, you then eliminate all numbers that are multiples of 2.

$$\require{cancel} \left[\begin{matrix} 1 & 2 & 3 & \cancel{4} & 5 \\ \cancel{6} & 7 & \cancel{8} & 9 & \cancel{10} \\ 11 & \cancel{12} & 13 & \cancel{14} & 15 \\ \cancel{16} & 17 & \cancel{18} & 19 & \cancel{20} \\ 21 & \cancel{22} & 23 & \cancel{24} & 25 \\ \end{matrix}\right]$$

Which leaves us with:

$$\left[\begin{matrix} 1 & 2 & 3 & & 5 \\ & 7 & & 9 & \\ 11 & & 13 & & 15 \\ & 17 & & 19 & \\ 21 & & 23 & & 25 \\ \end{matrix}\right]$$

Then you take the next number after that prime, in this case 3, and do it all over again.

$$\left[\begin{matrix} 1 & 2 & 3 & & 5 \\ & 7 & & \cancel{9} & \\ 11 & & 13 & & \cancel{15} \\ & 17 & & 19 & \\ \cancel{21} & & 23 & & 25 \\ \end{matrix}\right]$$

Leaving us:

$$\left[\begin{matrix} 1 & 2 & 3 & & 5 \\ & 7 & & & \\ 11 & & 13 & & \\ & 17 & & 19 & \\ & & 23 & & 25 \\ \end{matrix}\right]$$

And again take the next prime, 5, and eliminate all multiples of 5 (25 seems to be the only victim here). Eventually the numbers remaining will be:

$$\left[ \begin{matrix} 2 & 3 & 5 & 7 & 11 & 13 & 17 & 19 & 23 \end{matrix} \right ]$$

$$\ast$$ 1 is not considered a prime number.

So as you can see, the technique is quite simple and effective, albeit a little time-consuming, especially for large ranges. This is where computers enter the stage.

### The Essence of Programming

...is to make the computer do the repetitive, computational-demanding work. The course I'm taking uses Ruby so I've coded this in Ruby as well.

#### Breaking it down

First, we need to create the sequence of numbers, given a limit as input.

Then we have the part of the program that removes all the multiples of the given prime. Here we use the select method to filter out all numbers that DO NOT leave a remainder (hence a multiple) when divided with the prime.

Then we just iterate over the range, methodically removing numbers until we are only left with the primes.

### Testing it

Here are a few runs to make sure that it works:

Numbers 1 - 10.

2.0.0-p647 :561 >   Sieve.new(10).primes
=> [2, 3, 5, 7]


Numbers 1 - 25

2.0.0-p647 :524 > Sieve.new(25).primes
=> [2, 3, 5, 7, 11, 13, 17, 19, 23]


Numbers 1-100

2.0.0-p647 :576 > Sieve.new(100).primes
=> [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97]


The complete source (I had to use a class since the test-suite provided assumed a class, although for these types of problems I would have just used functions normally):