[Haskell-cafe] Flagstone problem
Brent Yorgey
byorgey at seas.upenn.edu
Sat Nov 6 07:03:13 EDT 2010
On Thu, Nov 04, 2010 at 10:50:06AM -0700, michael rice wrote:
> Hi,
>
> I've been looking at a "flagstone problem," where no two adjacent
> n-tuples can be identical. I solved the problem with Icon using
Interesting stuff!
>
> ========= Here's my Haskell code ========
>
> import Data.Bits
> import Data.List
>
> flagstone n = foldl (++) "" (take n (map show (f $ group [if even y
> then 0 else 1 | y <- [bitcount x | x <- [20..]]])))
By the way, I would write this as
flagstone n = concat . take n . map show . f . group
. map (fromEnum . odd . bitcount) $ [20..]
You should never use foldl (++) as it is rather inefficient: you get
things like (((a ++ b) ++ c) ++ d) ... which ends up traversing the
left part of the list repeatedly. And list comprehensions can be nice
at times, but personally using map seems clearer in this case.
> ========= My question ========
>
> A further exercise in the text:
>
> "Exercise 5.-(a) Define a(n) as the sum of the binary
> digits in the binary representation of n. Define b(i) as
> the number of a's between successive zeros as before.
> Then T = b(1) b(2) b(3) b(4) ... gives an infinite
> sequence of *seven* symbols with no repeats. (b) Write
> a routine to generate a sequence for seven colors of
> beads on a string with no repeats."
>
> I may be misreading, but does this make any sense?
Doesn't make much sense to me. The sum of binary digits in the binary
representation of n will not be zero very often...
-Brent
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