# Euler problems/161 to 170

(Difference between revisions)

Triominoes

## 2 Problem 162

Solution:

```import Data.Char (intToDigit)
digits n
|n<16=[n]
|otherwise= y:digits x
where
(x,y)=divMod n 16
fun k=15*16^(k-1)-15^(k)-2*14*15^(k-1)+13*14^(k-1)+2*14^k-13^k
fsum::Integer
fsum=sum \$map fun [3..16]
problem_162=map (intToDigit.fromInteger) \$reverse \$digits fsum```

## 3 Problem 163

Cross-hatched triangles

Solution:

```--http://www.math.uni-bielefeld.de/~sillke/SEQUENCES/grid-triangles
fun n=
sum[(2*n3 + 5*n2 + 2*n) `div` 8  ,
2*(n3 `div` 2- n `div` 6)  ,
6* sum[( n*(n+1)*(n+2)) `div` 6 ,
(2*n3 + 5*n2 + 2*n) `div` 8 ,
(2*n3 + 3*n2 - 3*n) `div` 18 ,
(2*n3 + 3*n2 - 3*n) `div` 10 ],
3 * ((22*n3 + 45*n2 - 4*n) `div` 48)
]
where
n3=n^3
n2=n^2
problem_163=fun 36```

## 4 Problem 164

Numbers for which no three consecutive digits have a sum greater than a given value.

Solution:

```addDigit x = [[sum [x !! b !! c | c <- [0..9-a-b]] | b <- [0..9-a]] | a<-[0..9]]
x3 = [[10-a-b | b <- [0..9-a]] | a <- [0..9]]
x20 = iterate addDigit x3 !! 17
problem_164 = sum [x20 !! a !! b | a <- [1..9], b <- [0..9-a]]```

## 5 Problem 165

Intersections

Solution:

```import Data.List (nub)

bbsGen x = (x * x) `mod` 50515093

bbsSeq = iterate bbsGen 290797

tValues = map (`mod` 500) (tail bbsSeq)

lineSeg n = take 4 (drop n tValues)

lineSegs = map lineSeg [0,4..]

implicitLine :: [Integer] -> (Integer, Integer, Integer)
implicitLine [x1,y1,x2,y2] = (a, b, d) where
a = y2 - y1
b = -(x2 - x1)
d = x1*a + y1 * b

within :: (Ord a, Num a, Integral b) => a -> b -> b -> Bool
within a b c | b > c = within a c b
| otherwise = a >= fromIntegral b && a <= fromIntegral c

withinSeg :: (Ord a, Num a) => a -> a -> [Integer] -> Bool
withinSeg x y l@[x1,y1,x2,y2] = within x x1 x2 && within y y1 y2 && not (endpoint x y l)

endpoint :: (Ord a, Num a) => a -> a -> [Integer] -> Bool
endpoint x y [x1,y1,x2,y2] = ((x == fromIntegral x1) && (y == fromIntegral y1)) ||
((x == fromIntegral x2) && (y == fromIntegral y2))

boundingBoxOverlap l1@[l1x1,l1y1,l1x2,l1y2] l2@[l2x1,l2y1,l2x2,l2y2]
| min l1x1 l1x2 > max l2x1 l2x2 = False
| max l1x1 l1x2 < min l2x1 l2x2 = False
| min l1y1 l1y2 > max l2y1 l2y2 = False
| max l1y1 l1y2 < min l2y1 l2y2 = False
| otherwise = True

intersect :: (Fractional a, Ord a) => [Integer] -> [Integer] -> (Bool, a, a)
intersect l1 l2 | boundingBoxOverlap l1 l2 &&
d /= 0 &&
withinSeg x y l1 && withinSeg x y l2 = (True, x, y)
| otherwise = (False, 0, 0)
where
(a1, b1, d1) = implicitLine l1
(a2, b2, d2) = implicitLine l2
d = fromIntegral (a1*b2 - a2*b1)
x = fromIntegral (b2 * d1 - b1 * d2) / d
y = fromIntegral (a1 * d2 - a2 * d1) / d

listIntersects l ls = [(x,y) | l1 <- ls, let (b, x, y) = intersect l l1, b]

allIntersectsList [] = []
allIntersectsList (l:ls) = listIntersects l ls ++ allIntersectsList ls

problem_165 = length . nub . allIntersectsList \$ take 5000 lineSegs```

## 6 Problem 166

Criss Cross

Solution:

```problem_166 =
sum [ product (map count [[0, c, b-d, a-b-d],
[0, b-a, c+d-a, b+d-a],
[0, -b-c, a-b-c-d, -c-d],
[0, a, d, c+d]])|
a <- [-9..9],
b <- [-9+a..9+a],
c <- [-9..9],
d <- [-9+a-c..9+a-c]]
where
count xs
|u<l=0
|otherwise=u-l+1
where
l = -minimum xs
u = 9-maximum xs```

## 7 Problem 167

Investigating Ulam sequences

## 8 Problem 168

Number Rotations

Solution:

```fun e =
sum[n*10+d|
let t=[1..9],
d<-t,
p<-t,
let (n,m)=divMod ((e-p)*d) (10*p-1) ,
m==0,
10*n>=e
]
problem_168=(`mod`(10^5))\$sum[fun e|i<-[1..99],let e=10^i]```

## 9 Problem 169

Exploring the number of different ways a number can be expressed as a sum of powers of 2.

Solution:

```fusc' 0=(1,0)
fusc' n
|even n=(a+b, b)
|odd n=(a,a+b)
where
(a,b)=fusc' \$n`div`2
fusc =fst.fusc'
problem_169=fusc (10^25)```

## 10 Problem 170

Find the largest 0 to 9 pandigital that can be formed by concatenating products.

Solution:

```{-
1) The first integer must be a multiple of 3
(otherwise the digital root of the result is not 9).
2) The first integer contains at most 2 digits
(otherwise the result contains more than 10 digits).
3) The first integer must be less than 49
(otherwise the result contains more than 10 digits).
5) This number must be a multiple of the first factor (f).
In the numbers f and cp/f all digits 1..9 have to occour
once and at least one zeros.
-}
import Data.List
permutationsOf [] = [[]]
permutationsOf xs = [x:xs' | x <- xs, xs' <- permutationsOf (delete x xs)]
digits =reverse.digits'
where
digits' n
|n<10=[n]
|otherwise= y:digits' x
where
(x,y)=divMod n 10
digitsToNum n=foldl dmm 0  n
where
dmm x y=x*10+y
fun k xs c=or [n/=0 && n<100|a<-k,let n=c*xs!!(a+1)]
problem_170 =
maximum[b|
aa<-[7,6..4],
a<-permutationsOf \$delete aa [0..7],
let b=digitsToNum \$[9,8]++(aa:a),
c<-[12,15..48],
let (d,m)=divMod b c ,
m==0,
let xs=digits d,
(digits c++xs) \\t==[0],
let k=elemIndices 0 xs,
last xs/=0,
fun k xs c
]
where
t=[0..9]```