Truncatable primes: Difference between revisions

=={{header|Racket}}==

<lang racket>

#lang racket

(require math/number-theory)

(define (truncate-right n)

(quotient n 10))

(define (truncate-left n)

(define s (number->string n))

(string->number (substring s 1 (string-length s))))

(define (contains-zero? n)

(member #\0 (string->list (number->string n))))

(define (truncatable? truncate n)

(and (prime? n)

(not (contains-zero? n))

(or (< n 10)

(truncatable? truncate (truncate n)))))

; largest left truncatable prime

(for/first ([n (in-range 1000000 1 -1)]

#:when (truncatable? truncate-left n))

n)

; largest right truncatable prime

(for/first ([n (in-range 1000000 1 -1)]

#:when (truncatable? truncate-right n))

n)

; Output:

998443

739399

</lang>