# Euler problems/31 to 40

## Contents

## Problem 31

Investigating combinations of English currency denominations.

Solution:

This is the naive doubly recursive solution. Speed would be greatly improved by use of memoization, dynamic programming, or the closed form.

```
problem_31 = pence 200 [1,2,5,10,20,50,100,200]
where pence 0 _ = 1
pence n [] = 0
pence n denominations@(d:ds)
| n < d = 0
| otherwise = pence (n - d) denominations
+ pence n ds
```

A beautiful solution, making usage of laziness and recursion to implement a dynamic programming scheme, blazingly fast despite actually generating the combinations and not only counting them :

```
coins = [1,2,5,10,20,50,100,200]
combinations = foldl (\without p ->
let (poor,rich) = splitAt p without
with = poor ++
zipWith (++) (map (map (p:)) with)
rich
in with
) ([[]] : repeat [])
problem_31 = length $ combinations coins !! 200
```

## Problem 32

Find the sum of all numbers that can be written as pandigital products.

Solution:

```
problem_32 = sum $ nub $ map (\(a, b) -> a * b) multiplicands
where
multiplicands =
[(a,b)| a <- [2..5000], b <- [a..(9999 `div` a)], check a b]
check a b =
no_zero s
&& (length ss) == 9
&& foldr (\x y -> length x == 1 && y) True ss
where
s = show a ++ show b ++ show (a*b)
ss = group $ sort s
no_zero (x:xs)
| x == '0' = False
| null xs = True
| otherwise = no_zero xs
```

## Problem 33

Discover all the fractions with an unorthodox cancelling method.

Solution:

```
import Ratio
problem_33 = denominator (product $ rs ++ rs')
rs = [(x%y) | a <- [0..9], b <- [1..9], c <- [1..9], let x = 10*a + c, let y = 10*c + b, x /= y, x%y < 1, x%y == a%b]
rs' = filter (<1) $ map (\x -> denominator x % numerator x) rs
```

## Problem 34

Find the sum of all numbers which are equal to the sum of the factorial of their digits.

Solution:

```
import Data.Char
problem_34 = sum [ x | x <- [3..100000], x == facsum x ]
where facsum = sum . map (product . enumFromTo 1 . digitToInt) . show
```

## Problem 35

How many circular primes are there below one million?

Solution:

```
import Data.List (tails, (\\))
primes :: [Integer]
primes = 2 : filter ((==1) . length . primeFactors) [3,5..]
primeFactors :: Integer -> [Integer]
primeFactors n = factor n primes
where
factor _ [] = []
factor m (p:ps) | p*p > m = [m]
| m `mod` p == 0 = p : factor (m `div` p) (p:ps)
| otherwise = factor m ps
isPrime :: Integer -> Bool
isPrime 1 = False
isPrime n = case (primeFactors n) of
(_:_:_) -> False
_ -> True
permutations :: Integer -> [Integer]
permutations n = take l $ map (read . take l) $ tails $ take (2*l -1) $ cycle s
where
s = show n
l = length s
circular_primes :: [Integer] -> [Integer]
circular_primes [] = []
circular_primes (x:xs)
| all isPrime p = x : circular_primes xs
| otherwise = circular_primes xs
where
p = permutations x
problem_35 :: Int
problem_35 = length $ circular_primes $ takeWhile (<1000000) primes
```

## Problem 36

Find the sum of all numbers less than one million, which are palindromic in base 10 and base 2.

Solution:

```
import Numeric
import Data.Char
showBin = flip (showIntAtBase 2 intToDigit) ""
isPalindrome x = x == reverse x
problem_36 = sum [x | x <- [1,3..1000000], isPalindrome (show x), isPalindrome (showBin x)]
```

## Problem 37

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

Solution:

```
import Data.List (tails, inits, nub)
primes :: [Integer]
primes = 2 : filter ((==1) . length . primeFactors) [3,5..]
primeFactors :: Integer -> [Integer]
primeFactors n = factor n primes
where
factor _ [] = []
factor m (p:ps) | p*p > m = [m]
| m `mod` p == 0 = p : factor (m `div` p) (p:ps)
| otherwise = factor m ps
isPrime :: Integer -> Bool
isPrime 1 = False
isPrime n = case (primeFactors n) of
(_:_:_) -> False
_ -> True
truncs :: Integer -> [Integer]
truncs n = nub . map read $ (take l . tail . tails) s ++ (take l . tail . inits) s
where
l = length s - 1
s = show n
problem_37 = sum $ take 11 [x | x <- dropWhile (<=9) primes, all isPrime (truncs x)]
```

## Problem 38

What is the largest 1 to 9 pandigital that can be formed by multiplying a fixed number by 1, 2, 3, ... ?

Solution:

```
problem_38 = maximum $ catMaybes [result | j <- [1..9999],
let p2 = show j ++ show (2*j),
let p3 = p2 ++ show (3*j),
let p4 = p3 ++ show (4*j),
let p5 = p4 ++ show (5*j),
let result
| isPan p2 = Just p2
| isPan p3 = Just p3
| isPan p4 = Just p4
| isPan p5 = Just p5
| otherwise = Nothing]
where isPan s = sort s == "123456789"
```

Other solution:

```
import Data.List
mult n i vs | length (concat vs) >= 9 = concat vs
| otherwise = mult n (i+1) (vs ++ [show (n * i)])
problem_38 :: Int
problem_38 = maximum $ map read $ filter ((['1'..'9'] ==) .sort) $ [ mult n 1 [] | n <- [2..9999] ]
```

## Problem 39

If p is the perimeter of a right angle triangle, {a, b, c}, which value, for p ≤ 1000, has the most solutions?

Solution: We use the well known formula to generate primitive Pythagorean triples. All we need are the perimeters, and they have to be scaled to produce all triples in the problem space.

```
problem_39 = head $ perims !! indexMax
where perims = group
$ sort [n*p | p <- pTriples, n <- [1..1000 `div` p]]
counts = map length perims
Just indexMax = findIndex (== (maximum counts)) $ counts
pTriples = [p |
n <- [1..floor (sqrt 1000)],
m <- [n+1..floor (sqrt 1000)],
even n || even m,
gcd n m == 1,
let a = m^2 - n^2,
let b = 2*m*n,
let c = m^2 + n^2,
let p = a + b + c,
p < 1000]
```

## Problem 40

Finding the nth digit of the fractional part of the irrational number.

Solution:

```
problem_40 = (d 1)*(d 10)*(d 100)*(d 1000)*(d 10000)*(d 100000)*(d 1000000)
where n = concat [show n | n <- [1..]]
d j = Data.Char.digitToInt (n !! (j-1))
```