From ab80499b45fd080d94b2d083dd3b7de740273782 Mon Sep 17 00:00:00 2001 From: Correl Roush Date: Tue, 11 Oct 2011 17:15:05 -0400 Subject: [PATCH] Haskell 004 --- haskell/e004.hs | 32 ++++++++++++++++++++++++++++++++ 1 file changed, 32 insertions(+) create mode 100644 haskell/e004.hs diff --git a/haskell/e004.hs b/haskell/e004.hs new file mode 100644 index 0000000..490575b --- /dev/null +++ b/haskell/e004.hs @@ -0,0 +1,32 @@ +{- Find the largest palindrome made from the product of two 3-digit numbers. + +A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 99. +Find the largest palindrome made from the product of two 3-digit numbers. +-} + +import Data.List (nub, sort) +import Text.Printf + +is_palindrome :: (Integral a) => a -> Bool +is_palindrome n = do + let s = (show n) + s == (reverse s) + +largest_palindrome :: (Integral a) => a -> a +largest_palindrome digits = do + let min = 10 ^ (digits - 1) + let max = 10 ^ digits - 1 + largest_palindrome_ min max max max 0 + +largest_palindrome_ :: (Integral a) => a -> a -> a -> a -> a -> a +largest_palindrome_ min max a b p + | a < min = p + | a * b < p || b < min = largest_palindrome_ min max (a - 1) max p + | otherwise = + if is_palindrome (a * b) + then largest_palindrome_ min max a (b - 1) (a * b) + else largest_palindrome_ min max a (b - 1) p + +main = do + printf "Largest palindrome product of 2-digit numbers: %d\n" (largest_palindrome 2 :: Integer) + printf "Largest palindrome product of 3-digit numbers: %d\n" (largest_palindrome 3 :: Integer)