## Question

An irrational decimal fraction is created by concatenating the positive integers:

$\displaystyle 0.123456789101112131415161718192021...$

It can be seen that the 12th digit of the fractional part is 1.

If $d_n$ represents the nth digit of the fractional part, find the value of the following expression.

$\displaystyle d_1 \times d_{10} \times d_{100} \times d_{1000} \times d_{10000} \times d_{100000} \times d_{1000000}$

champernowne :: String
champernowne = foldr (\x acc -> (show x) ++ acc) "" [1..]

main :: IO ()
main = print $product [read [champernowne !! (n - 1)] | n <- [10^x | x <- [0..6]]] $ ghc -O2 -o champernowne champernowne.hs
$time ./champernowne real 0m0.079s user 0m0.064s sys 0m0.012s ## Python #!/usr/bin/env python d = [int(digit) for digit in ''.join((str(digit) for digit in range(1, 10000001)))] print(d[0] * d[9] * d[99] * d[999] * d[9999] * d[99999] * d[999999]) $ time python3 irrational-part.py
real   0m15.868s
user   0m15.324s
sys    0m0.428s

## Ruby

#!/usr/bin/env ruby
s = ('1'..'1000000').to_a.join ''
puts (0..6).map { |i|
s[(10**i)-1].to_i
}.reduce(1, :*)
\$ time ruby irrational-part.rb
real   0m0.494s
user   0m0.460s
sys    0m0.016s