Ruby: find left truncatable primes between 10 to 9999(efficiently) -

as title says. want find left truncatable primes between 10 , 9999 using ruby language. following tried , gives me result.

require 'prime' require 'benchmark'  trunc_primes = []  benchmark.bmbm |bm|   bm.report{     prime.each(10000).each |num|       is_tunc_prime = true       (1..(num.to_s.length)).to_a.reverse.each |i|         factor = 10 **         unless num.divmod(factor)[1].prime?           is_tunc_prime = false           break         end       end       trunc_primes << num if is_tunc_prime     end;nil   } end 

question: can script optimized further(lesser loops, use formula rather factor division technique) ?

---

benchmark stats:

➜  truncatable git:(master) ✗ ruby truncatable_primes.rb rehearsal ------------------------------------    0.000000   0.000000   0.000000 (  0.000026) --------------------------- total: 0.000000sec         user     system      total        real    0.000000   0.000000   0.000000 (  0.000020) ➜  truncatable git:(master) ✗ ruby truncatable_primes.rb rehearsal ------------------------------------    0.000000   0.000000   0.000000 (  0.000024) --------------------------- total: 0.000000sec         user     system      total        real    0.000000   0.000000   0.000000 (  0.000017) ➜  truncatable git:(master) ✗ ruby truncatable_primes.rb rehearsal ------------------------------------    0.000000   0.000000   0.000000 (  0.000028) --------------------------- total: 0.000000sec         user     system      total        real    0.000000   0.000000   0.000000 (  0.000044) ➜  truncatable git:(master) ✗ ruby truncatable_primes.rb rehearsal ------------------------------------    0.000000   0.000000   0.000000 (  0.000030) --------------------------- total: 0.000000sec         user     system      total        real    0.000000   0.000000   0.000000 (  0.000019) 

---

benchmark above script , amadan's script

rehearsal ------------------------------------------ mine     0.020000   0.000000   0.020000 (  0.022804) amadan   0.000000   0.000000   0.000000 (  0.001559) --------------------------------- total: 0.020000sec             user     system      total        real mine     0.020000   0.000000   0.020000 (  0.019625) amadan   0.000000   0.000000   0.000000 (  0.001540) 

since you're using prime, why invent hot water?

prime.each(10000).reject { |p| p < 10 }.select { |p|   s = p.to_s   1.upto(s.length - 1).all? { |i|     s[i] != '0' && prime.prime?(s[i .. -1].to_i)   } } 

edit: can optimised further though - if abcd turns out not truncatable prime, zabcd not need tested:

require 'set' require 'prime' set.new.tap { |f|   prime.each(10000) { |p|     s = p.to_s     f << p if p < 10 || (s[1] != '0' && f.include?(s[1 .. -1].to_i))   } }.reject { |p| p < 10 } 

edit: forgot no zeroes requirement. added. being on mobile now, can't check if still running ok.