Rebol3 Code Examplex

Rebol [
    title: "Rosetta code: Additive primes"
    file:  %Additive_primes.r3
    url:   https://rosettacode.org/wiki/Additive_primes
    note:  "Based on Red language solution"
]
primes: function [n [integer!]][
    ;; See: https://rosettacode.org/wiki/Sieve_of_Eratosthenes#Rebol
    poke prim: make bitset! n 1 true
    r: 2 while [r * r <= n][repeat q n / r - 1 [poke prim q + 1 * r true]
    until [not pick prim r: r + 1]]
    collect [repeat i n [unless prim/:i [keep i]]]
]
;; Compute the digit sum (cross-sum) of an integer n
cross-sum: function [n][
    out: 0                 ;; accumulator for sum of digits
    foreach m form n [     ;; iterate over the characters of n's string form
        out: out - 48 + m  ;; convert char to integer, add to out
    ]
    out
]
;; Return all additive primes <= n:
;; A prime p is "additive" if its digit-sum is also prime.
additive-primes: function [n][
    collect [
        foreach p ps: primes n [       ;; generate primes up to n and iterate over them
            if find ps cross-sum p [   ;; check if digit-sum of p is itself in the prime set
                keep p                 ;; keep p if digit-sum is prime
            ]
        ]
    ]
]

;; Generate additive primes up to 500
result: additive-primes 500
;; Format nicely by rows of 10 (with newlines)
print [length? result "additive primes < 500:"]
forall result [
    prin pad result/1 -4
    if zero? ((index? result) % 10) [print ""]
]
print ""

54 additive primes < 500:
   2   3   5   7  11  23  29  41  43  47
  61  67  83  89 101 113 131 137 139 151
 157 173 179 191 193 197 199 223 227 229
 241 263 269 281 283 311 313 317 331 337
 353 359 373 379 397 401 409 421 443 449
 461 463 467 487