Project Euler

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# [[Problem 34]]

## Digit Factorials

$145$ is a curious number, as $1! + 4! + 5! = 1 + 24 + 120 = 145$.

Find the sum of all numbers which are equal to the sum of the factorial of their digits.

Note: As $1! = 1$ and $2! = 2$ are not sums they are not included.

## 桁の階乗

145は面白い数である. 1! + 4! + 5! = 1 + 24 + 120 = 145となる.

各桁の数の階乗の和が自分自身と一致するような数の和を求めよ.

**注:** 1! = 1 と 2! = 2 は総和に含めてはならない.

```scheme

(import (scheme base)

        (gauche base))

(define (digit-factorial-num n)

  (fold (^[n result]

          (+ result

             (apply * (iota n 1))))

        0

        (map digit->integer

             (string->list (number->string n)))))

(define digit-factorials

  (fold (^[n lis]

          (if (= n (digit-factorial-num n))

            (cons n lis)

            lis))

        '()

        ; 9! * 7 = 2540160

        ; 9! * 8 = 2903040

        ; のため、3 ～ 2540160 まで求めれば良い

        (iota 2540158 3)))

(define answer-34

  (apply + digit-factorials))

(format #t "34: ~d~%" answer-34)

```