Project Euler

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# [[Problem 10]]

## Summation of Primes

The sum of the primes below $10$ is $2 + 3 + 5 + 7 = 17$.

Find the sum of all the primes below two million.

## 素数の和

10以下の素数の和は 2 + 3 + 5 + 7 = 17 である.

200万以下の全ての素数の和を求めよ.

```scheme

#!r6rs

#!chezscheme

(import (chezscheme))

(define (eratosthenes-sub1! table erase-num)

  (assert (vector-ref table erase-num))

  (let loop ([current (* erase-num 2)])

    (cond

      [(<= (vector-length table) current) table]

      [else

        (vector-set! table current #f)

        (loop (+ current erase-num))])))

(define (eratosthenes-sub2 table)

  (let-values ([(primes falses) (partition

                                  (lambda (num-or-f)

                                    (if num-or-f

                                      num-or-f

                                      #f))

                                  (vector->list table))])

    primes))

(define (eratosthenes num)

  (let ([table (list->vector (iota num))]

        [count (isqrt (sub1 num))])

    (vector-set! table 0 #f)

    (vector-set! table 1 #f)

    (let loop ([index 2] [table table])

      (cond

        [(< count index) (eratosthenes-sub2 table)]

        [(vector-ref table index)

         (loop (add1 index) (eratosthenes-sub1! table index))]

        [else (loop (add1 index) table)]))))

(define answer-10

  (apply + (eratosthenes 2000001)))

(printf "10: ~D~%" answer-10)

```