Project Euler

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# [[Problem 17]]

## Number Letter Counts

If the numbers $1$ to $5$ are written out in words: one, two, three, four, five, then there are $3 + 3 + 5 + 4 + 4 = 19$ letters used in total.

If all the numbers from $1$ to $1000$ (one thousand) inclusive were written out in words, how many letters would be used?

**NOTE:** Do not count spaces or hyphens. For example, $342$ (three hundred and forty-two) contains $23$ letters and $115$ (one hundred and fifteen) contains $20$ letters. The use of "and" when writing out numbers is in compliance with British usage.

## 数字の文字数

1 から 5 までの数字を英単語で書けば one, two, three, four, five であり, 全部で 3 + 3 + 5 + 4 + 4 = 19 の文字が使われている.

では 1 から 1000 (one thousand) までの数字をすべて英単語で書けば, 全部で何文字になるか.

注: 空白文字やハイフンを数えないこと. 例えば, 342 (three hundred and forty-two) は 23 文字, 115 (one hundred and fifteen) は20文字と数える. なお, "and" を使用するのは英国の慣習.

```scheme

#!r6rs

#!chezscheme

(import (chezscheme))

(define (number-letter num)

  (assert (and (<= 1 num) (<= num 1000)))

  (case num

    [1 "one"]

    [2 "two"]

    [3 "three"]

    [4 "four"]

    [5 "five"]

    [6 "six"]

    [7 "seven"]

    [8 "eight"]

    [9 "nine"]

    [10 "ten"]

    [11 "eleven"]

    [12 "twelve"]

    [13 "thirteen"]

    [14 "fourteen"]

    [15 "fifteen"]

    [16 "sixteen"]

    [17 "seventeen"]

    [18 "eighteen"]

    [19 "nineteen"]

    [20 "twenty"]

    [30 "thirty"]

    [40 "forty"]

    [50 "fifty"]

    [60 "sixty"]

    [70 "seventy"]

    [80 "eighty"]

    [90 "ninety"]

    [1000 "one thousand"]

    [else

      (if (< num 100)

        ; 2桁（10の位と1の位に分けて処理）

        (let-values ([(upper lower) (div-and-mod num 10)])

          (format "~D-~D" (number-letter (* upper 10)) (number-letter lower)))

        ; 3桁（100の位と残り2桁に分けて処理）

        (let-values ([(upper lower) (div-and-mod num 100)])

          (if (zero? lower)

            (format "~D hundred" (number-letter upper))

            (format "~D hundred and ~D"

                    (number-letter upper)

                    (number-letter lower)))))]))

(define all-number-letters

  (map number-letter (cdr (iota 1001))))

(define concatenate-all-number-letters

  (fold-left

    (lambda (result letter)

      (string-append result letter))

    ""

    all-number-letters))

(define (erase-space-hyphen str)

  (list->string

    (filter

      (lambda (char)

        (case char

          [(#\space #\-) #f]

          [else #t]))

      (string->list str))))

(define answer-17

  (string-length (erase-space-hyphen concatenate-all-number-letters)))

(printf "17: ~D~%" answer-17)

```