Difference between revisions of "Euler problems/1 to 10"
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Solution: |
Solution: |
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− | <haskell> |
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− | problem_1 = |
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− | sum [ x | |
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− | x <- [1..999], |
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− | (x `mod` 3 == 0) || (x `mod` 5 == 0) |
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⚫ | |||
− | </haskell> |
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− | |||
− | <haskell> |
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− | problem_1_v2 = |
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− | sum $ filter (\x -> ( x `mod` 3 == 0 || x `mod` 5 == 0 ) ) [1..999] |
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− | </haskell> |
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− | ---- |
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<haskell> |
<haskell> |
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sumOnetoN n = n * (n+1) `div` 2 |
sumOnetoN n = n * (n+1) `div` 2 |
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− | |||
problem_1 = |
problem_1 = |
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sumStep 3 999 + sumStep 5 999 - sumStep 15 999 |
sumStep 3 999 + sumStep 5 999 - sumStep 15 999 |
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where |
where |
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sumStep s n = s * sumOnetoN (n `div` s) |
sumStep s n = s * sumOnetoN (n `div` s) |
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⚫ | |||
</haskell> |
</haskell> |
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last (primeFactors 317584931803) |
last (primeFactors 317584931803) |
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</haskell> |
</haskell> |
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− | |||
− | This can be improved by using |
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− | <hask>null . tail</hask> |
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− | instead of |
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− | <hask>(== 1) . length</hask>. |
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− | |||
== [http://projecteuler.net/index.php?section=problems&id=4 Problem 4] == |
== [http://projecteuler.net/index.php?section=problems&id=4 Problem 4] == |
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Find the largest palindrome made from the product of two 3-digit numbers. |
Find the largest palindrome made from the product of two 3-digit numbers. |
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<haskell> |
<haskell> |
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problem_4 = |
problem_4 = |
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− | foldr max 0 [ x | |
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− | y <- [100..999], |
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− | z <- [100..999], |
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− | let x = y * z, |
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− | let s = show x, |
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− | s == reverse s |
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− | ] |
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− | </haskell> |
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− | An alternative to avoid evaluating twice the same pair of numbers: |
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− | <haskell> |
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− | problem_4' = |
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foldr1 max [ x | |
foldr1 max [ x | |
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y <- [100..999], |
y <- [100..999], |
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Solution: |
Solution: |
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<haskell> |
<haskell> |
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+ | --http://www.research.att.com/~njas/sequences/A003418 |
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− | problem_5 = |
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⚫ | |||
− | head [ x | |
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− | x <- [2520,5040..], |
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− | all (\y -> x `mod` y == 0) [1..20] |
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− | ] |
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− | </haskell> |
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− | An alternative solution that takes advantage of the Prelude to avoid use of the generate and test idiom: |
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− | <haskell> |
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⚫ | |||
</haskell> |
</haskell> |
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Solution: |
Solution: |
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<haskell> |
<haskell> |
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+ | fun n= |
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⚫ | |||
⚫ | |||
− | sum [ x^2 | x <- [1..100]] - (sum [1..100])^2 |
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⚫ | |||
+ | a=div (n^2 * (n+1)^2) 4 |
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+ | b=div (n * (n+1) * (2*n+1)) 6 |
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⚫ | |||
</haskell> |
</haskell> |
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Line 165: | Line 130: | ||
head $ drop 10000 primes |
head $ drop 10000 primes |
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</haskell> |
</haskell> |
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− | |||
− | As above, this can be improved by using |
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− | <hask>null . tail</hask> |
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− | instead of |
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− | <hask>(== 1) . length</hask>. |
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− | |||
− | Here is an alternative that uses a |
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− | [http://www.haskell.org/pipermail/haskell-cafe/2007-February/022854.html sieve of Eratosthenes]: |
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− | |||
− | <haskell> |
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− | primes' = |
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− | 2 : 3 : sieve (tail primes') [5,7..] |
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− | where |
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− | sieve (p:ps) x = |
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− | h ++ sieve ps (filter (\q -> q `mod` p /= 0) t |
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− | where |
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− | (h, _:t) = span (p*p <) x |
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− | problem_7_v2 = primes' !! 10000 |
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− | </haskell> |
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− | |||
== [http://projecteuler.net/index.php?section=problems&id=8 Problem 8] == |
== [http://projecteuler.net/index.php?section=problems&id=8 Problem 8] == |
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Discover the largest product of five consecutive digits in the 1000-digit number. |
Discover the largest product of five consecutive digits in the 1000-digit number. |
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Solution: |
Solution: |
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− | <haskell> |
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− | problem_9 = |
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− | head [a*b*c | |
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− | a <- [1..500], |
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− | b <- [a..500], |
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− | let c = 1000-a-b, |
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− | a^2 + b^2 == c^2 |
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− | ] |
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− | </haskell> |
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− | |||
− | Another solution using Pythagorean Triplets generation: |
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<haskell> |
<haskell> |
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triplets l = [[a,b,c]| |
triplets l = [[a,b,c]| |
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Solution: |
Solution: |
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<haskell> |
<haskell> |
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+ | --http://www.research.att.com/~njas/sequences/A046731 |
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problem_10 = |
problem_10 = |
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sum (takeWhile (< 1000000) primes) |
sum (takeWhile (< 1000000) primes) |
Revision as of 12:22, 18 February 2008
Problem 1
Add all the natural numbers below 1000 that are multiples of 3 or 5.
Solution:
sumOnetoN n = n * (n+1) `div` 2
problem_1 =
sumStep 3 999 + sumStep 5 999 - sumStep 15 999
where
sumStep s n = s * sumOnetoN (n `div` s)
Problem 2
Find the sum of all the even-valued terms in the Fibonacci sequence which do not exceed one million.
Solution:
problem_2 =
sum [ x |
x <- takeWhile (<= 1000000) fibs,
x `mod` 2 == 0
]
where
fibs = 1 : 1 : zipWith (+) fibs (tail fibs)
The following two solutions use the fact that the even-valued terms in
the Fibonacci sequence themselves form a Fibonacci-like sequence
that satisfies
evenFib 0 = 0, evenFib 1 = 2, evenFib (n+2) = evenFib n + 4 * evenFib (n+1)
.
problem_2_v2 =
sumEvenFibs $ numEvenFibsLessThan 1000000
sumEvenFibs n =
(evenFib n + evenFib (n+1) - 2) `div` 4
evenFib n =
round $ (2 + sqrt 5) ** (fromIntegral n) / sqrt 5
numEvenFibsLessThan n =
floor $ (log (fromIntegral n - 0.5) + 0.5*log 5) / log (2 + sqrt 5)
The first two solutions work because 10^6 is small. The following solution also works for much larger numbers (up to at least 10^1000000 on my computer):
problem_2 = sumEvenFibsLessThan 1000000
sumEvenFibsLessThan n =
(a + b - 1) `div` 2
where
n2 = n `div` 2
(a, b) =
foldr f (0,1) $
takeWhile ((<= n2) . fst) $
iterate times2E (1, 4)
f x y
| fst z <= n2 = z
| otherwise = y
where z = x `addE` y
addE (a, b) (c, d) =
(a*d + b*c - 4*ac, ac + b*d)
where
ac=a*c
times2E (a, b) =
addE (a, b) (a, b)
Problem 3
Find the largest prime factor of 317584931803.
Solution:
primes =
2 : filter ((==1) . length . primeFactors) [3,5..]
primeFactors n =
factor n primes
where
factor n (p:ps)
| p*p > n = [n]
| n `mod` p == 0 = p : factor (n `div` p) (p:ps)
| otherwise = factor n ps
problem_3 =
last (primeFactors 317584931803)
Problem 4
Find the largest palindrome made from the product of two 3-digit numbers.
Solution:
problem_4 =
foldr1 max [ x |
y <- [100..999],
z <- [y..999],
let x = y * z,
let s = show x,
s == reverse s
]
Problem 5
What is the smallest number divisible by each of the numbers 1 to 20?
Solution:
--http://www.research.att.com/~njas/sequences/A003418
problem_5 = foldr1 lcm [1..20]
Problem 6
What is the difference between the sum of the squares and the square of the sums?
Solution:
fun n=
a-b
where
a=div (n^2 * (n+1)^2) 4
b=div (n * (n+1) * (2*n+1)) 6
problem_6=fun 100
Problem 7
Find the 10001st prime.
Solution:
--primes in problem_3
problem_7 =
head $ drop 10000 primes
Problem 8
Discover the largest product of five consecutive digits in the 1000-digit number.
Solution:
import Data.Char
groupsOf _ [] = []
groupsOf n xs =
take n xs : groupsOf n ( tail xs )
problem_8 x=
maximum . map product . groupsOf 5 $ x
main=do
t<-readFile "p8.log"
let digits = map digitToInt $foldl (++) "" $ lines t
print $ problem_8 digits
Problem 9
There is only one Pythagorean triplet, {a, b, c}, for which a + b + c = 1000. Find the product abc.
Solution:
triplets l = [[a,b,c]|
m <- [2..limit],
n <- [1..(m-1)],
let a = m^2 - n^2,
let b = 2*m*n,
let c = m^2 + n^2,
a+b+c==l
]
where limit = floor $ sqrt $ fromIntegral l
problem_9 = product $ head $ triplets 1000
Problem 10
Calculate the sum of all the primes below one million.
Solution:
--http://www.research.att.com/~njas/sequences/A046731
problem_10 =
sum (takeWhile (< 1000000) primes)