Generate a Kolakoski sequence

Question

Definition¹

A Kolakoski sequence is a self-describing infinite sequence {k_n} of alternating blocks of 1's and 2's, given by the following rules:

• k₀ = 1
• k_n = the length of the (n+1)'th block

The Task

Given a positive integer n, generate the first n elements of the Kolakoski sequence.

Details

Input will be provided as a single command line argument n. Please write a full program that will print the first n elements of the Kolakoski sequence (in order) to STDOUT, with each element separated by your favorite whitespace.

Scoring

Lets count source code bytes this time with all whitespace included. Fewest number of bytes wins. In the event of a tie, the solution with the earliest submission time wins.

The Sequence²

1, 2, 2, 1, 1, 2, 1, 2, 2, 1, 2, 2, 1, 1, 2, 1, 1, 2, 2, 1, 2, 1, 1, 2, 1, 2, 2, 1, 1, 2, 1, 1, 2, 1, 2, 2, 1, 2, 2, 1, 1, 2, 1, 2, 2, 1, 2, 1, 1, 2, 1, 1, 2, 2, 1, 2, 2, 1, 1, 2, 1, 2, 2, 1, 2, 2, 1, 1, 2, 1, 1, 2, 1, 2, 2, 1, 2, 1, 1, 2, 2, 1, 2, 2, 1, 1, 2, 1, 2, 2, 1, 2, 2, 1, 1, 2, 1, 1, 2, 2, 1, 2, 1, 1, 2, 1, 2, 2, ...

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References

1. John Smith, Ariel Scolnicov, yark. "Kolakoski sequence" (version 3). PlanetMath.org. Freely available at http://planetmath.org/KolakoskiSequence.html.
2. The Online Encyclopedia of Integer Sequences http://oeis.org/A000002

Other resources

• Kolakoski Sequence at Wikipedia http://en.wikipedia.org/wiki/Kolakoski_sequence
• Kolakoski Sequence at Wolfram MathWorld http://mathworld.wolfram.com/KolakoskiSequence.html

Answer 1

GolfScript (48 chars)

You can't test this online because the online GS shell doesn't take command-line arguments, but:

;"#{ARGV[-1]}"~[1 2.]2{.2$=[1$2%)]*@\+\)}3$*;<n*

This takes input from stdin instead and works

~[1 2.]2{.2$=[1$2%)]*@\+\)}3$*;<n*

Answer 2

Ruby (73 characters)

s=[1,2,2];n=2;ARGV[0].to_i.times{puts s[n-2];s[n].times{s+=[1+n%2]};n+=1}

Answer 3

Python 93

Adapted from my solution to Calculate the nth term of Golomb's self-describing sequence:

import sys;n=int(sys.argv[1]);a=[1,2,2];
for i in range(3,n):a+=[~i%2+1]*a[i-1]
print a[:n]

Still trying to figure out how to do this with list comprehension which I just learned about today, but I'm running into problems with self-reference.

EDIT: Instead of breaking if list was too long, just printed the appropriate slice.

EDIT: Now pulling arg from command line

Answer 4

Python (118 characters)

Takes n as a command line argument. Could trim four more characters if the printing of first n requirement is relaxed to first n or n + 1. I can't think of any more fat to trim...will be interested to see what a more experienced Python golfer could do

import sys
k,b=[1,2,2],1
n=int(sys.argv[1])
while len(k)<n:k.extend([2,1][b%2]for z in range(k[b+1]));b+=1
print k[:n]

Answer 5

Haskell 157 144

Haskell is kind of verbose so I expect this will be significantly longer than the shortest answer, but here it is.

import System
a=1:2:drop 2(concat.zipWith replicate a.cycle$[1,2])
main=do args<-getArgs;putStr$concatMap((' ':).show)$take(read$args!!0::Int)a

Edit:

Implemented FUZxxl's suggestions and also a few other small fixes.

Answer 6

Perl, 59 chars

say"@{[map{push@a,(2-$_%2)x($b=$a[$_-1]||$_);$b}1..shift]}"

As usual, run with Perl 5.10+ and the -M5.010 switch to enable the say feature.