# Scala | Sieve of Eratosthenes

Eratosthenes of Cyrene was a Greek mathematician, who discovered an amazing algorithm to find prime numbers.

This article performs this algorithm in Scala.

**Step 1** : Creating an Int Stream

`def` `numberStream(n` `:` `Int)` `:` ` ` `Stream[Int] ` `=` `Stream.from(n) ` ` ` `println(numberStream(` `10` `)) ` |

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Output of above step is **Stream(10, ?).**

**Step 2 :** Sieve of Eratosthenes function

`// Defining Sieve of Eratosthenes ` `def` `sieve` `_` `of` `_` `Eratosthenes(stream` `:` `Stream[Int])` `:` ` ` `Stream[Int] ` `=` `stream.head ` `#::` `sieve` `_` `of` `_` `Eratosthenes( ` ` ` `(stream.tail) ` ` ` `filter (x ` `=>` `x ` `%` `stream.head !` `=` `0` `) ` ` ` `) ` ` ` `println(sieve` `_` `of` `_` `Eratosthenes(numberStream(` `10` `))) ` |

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Output of above step is **Stream(10, ?)**.

**Step 3 : **Extracting “N” number of primes

`val` `no` `_` `of` `_` `primes ` `=` `sieve` `_` `of` `_` `Eratosthenes(numberStream(` `2` `)) ` ` ` `// Selecting number of primes ` `println(no` `_` `of` `_` `primes) ` `(no` `_` `of` `_` `primes take ` `7` `) foreach { ` ` ` `println(` `_` `) ` ` ` `} ` |

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Output of above step is **Stream(2, ?)
2
3
5
7
11
13
17**.

**Below is the complete program**

`def` `numberStream(n` `:` `Int)` `:` ` ` `Stream[Int] ` `=` `Stream.from(n) ` ` ` `println(numberStream(` `10` `)) ` ` ` `// Defining Sieve of Eratosthenes ` `def` `sieve` `_` `of` `_` `Eratosthenes(stream` `:` `Stream[Int])` `:` ` ` `Stream[Int] ` `=` `stream.head ` `#::` `sieve` `_` `of` `_` `Eratosthenes( ` ` ` `(stream.tail) ` ` ` `filter (x ` `=>` `x ` `%` `stream.head!` `=` `0` `) ` ` ` `) ` ` ` `println(sieve` `_` `of` `_` `Eratosthenes(numberStream(` `10` `))) ` ` ` ` ` `val` `no` `_` `of` `_` `primes ` `=` `sieve` `_` `of` `_` `Eratosthenes(numberStream(` `2` `)) ` ` ` `// Selecting number of primes ` `println(no` `_` `of` `_` `primes) ` `(no` `_` `of` `_` `primes take ` `7` `) foreach { ` ` ` `println(` `_` `) ` ` ` `} ` |

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**Output:**

Stream(10, ?) Stream(10, ?) Stream(2, ?) 2 3 5 7 11 13 17

**Insights from the code**

- Using
**stream.form(), a stream is created which is generating successive numbers. And this number starts off from the argument.** - A number stream is given to the
**“sieve_of_Eratosthenes”**method. This method by filtering out the elements, lazily generates the successive elements.

**Below is the complete working code with explanation: **

**Working:** *abc()* method inserts the debug statement in the **filter()** method. If an element is not evenly divisible by the head, the stream treats it as a good element. The code prints it and return true. Otherwise the filtered out sequence is printed and finally the stream is returned.

Some modification is done in **sieve_of_Eratosthenes** method so as to use the stream creation – **abc()** method. Elements are taken out from the recursive stream and is printed.

`object` `Sieve ` `extends` `App { ` ` ` `def` `abc(s` `:` `Stream[Int], head` `:` `Int) ` `=` `{ ` ` ` `val` `r ` `=` `s filter { ` ` ` `x ` `=>` `{ ` ` ` `if` `(x ` `%` `head !` `=` `0` `) { ` ` ` `println() ` ` ` `println(s` `"${x} is not evenly divisible by ${head}"` `) ` ` ` ` ` `true` ` ` `} ` ` ` `else` `{ ` ` ` `println() ` ` ` `println(s` `"${x} is evenly divisible by ${head}. So Discard ${x}"` `) ` ` ` `false` ` ` `} ` ` ` `} ` ` ` `} ` ` ` `r ` ` ` `} ` ` ` `def` `numberStream(g` `:` `Int)` `:` `Stream[Int] ` `=` `Stream.from(g) ` ` ` ` ` `def` `sieve` `_` `of` `_` `Eratosthenes(stream` `:` `Stream[Int])` `:` ` ` `Stream[Int] ` `=` `stream.head ` `#::` `sieve` `_` `of` `_` `Eratosthenes( ` ` ` `abc(stream.tail, stream.head)) ` ` ` ` ` `val` `no` `_` `of` `_` `primes ` `=` `sieve` `_` `of` `_` `Eratosthenes(numberStream(` `2` `)) ` ` ` `(no` `_` `of` `_` `primes take ` `7` `) foreach { ` ` ` `println(` `_` `) ` ` ` `} ` `} ` |

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**Output :**

2 3 is not evenly divisible by 2 3 4 is evenly divisible by 2. So Discard 4 5 is not evenly divisible by 2 5 is not evenly divisible by 3 5 6 is evenly divisible by 2. So Discard 6 7 is not evenly divisible by 2 7 is not evenly divisible by 3 7 is not evenly divisible by 5 7 8 is evenly divisible by 2. So Discard 8 9 is not evenly divisible by 2 9 is evenly divisible by 3. So Discard 9 10 is evenly divisible by 2. So Discard 10 11 is not evenly divisible by 2 11 is not evenly divisible by 3 11 is not evenly divisible by 5 11 is not evenly divisible by 7 11 12 is evenly divisible by 2. So Discard 12 13 is not evenly divisible by 2 13 is not evenly divisible by 3 13 is not evenly divisible by 5 13 is not evenly divisible by 7 13 is not evenly divisible by 11 13 14 is evenly divisible by 2. So Discard 14 15 is not evenly divisible by 2 15 is evenly divisible by 3. So Discard 15 16 is evenly divisible by 2. So Discard 16 17 is not evenly divisible by 2 17 is not evenly divisible by 3 17 is not evenly divisible by 5 17 is not evenly divisible by 7 17 is not evenly divisible by 11 17 is not evenly divisible by 13 17

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