#truncatable

[talkingnumbers]

Stop That!

A mathcomic for Sophia Wood’s #mathober theme “truncate.” There must be a name for these primes. 

Derek Orr says these are a thing! “Yes these are things!!! Right-truncatable primes: http://oeis.org/A024770″

[fuckyeahterminals]

Problem 37

The number 3797 has an interesting property. Being prime itself, it is possible to continuously remove digits from left to right, and remain prime at each stage: 3797, 797, 97, and 7. Similarly we can work from right to left: 3797, 379, 37, and 3.

Find the sum of the only eleven primes that are both truncatable from left to right and right to left.

NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.

import Data.Char truncationL xs | length xs > 0 = xs:(truncationL $ tail xs) | otherwise = [] truncationR xs | length xs > 0 = xs:(truncationR $ take ((length xs)-1) xs) | otherwise = [] fromDigits = foldl addDigit 0 where addDigit num d = 10*num + d isPrime :: Int->Bool isPrime x = if x == 1 then False else null [y | y<-[2..floor (sqrt (fromIntegral x))], x `mod` y == 0] isTruncatablePrime n = all (\e -> isPrime $ fromDigits e) (truncationL $ map digitToInt $ show n) && all (\e -> isPrime $ fromDigits e) (truncationR $ map digitToInt $ show n) truncatablePrimes :: [Int] truncatablePrimes = [ x | x<-[11..(10^6)], isTruncatablePrime x] main = print(foldl (+) 0 truncatablePrimes)

List Operations so by definition will be slow, not too slow though...maybe this would be faster in CLISP, someone try it..