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The sieve of eratosthenes

J

NB. Sieve of Eratosthenes in J
NB. Written by Rob Hoelz

NB. To run, load this file into the J interpreter
NB. and call main <limit>

NB. NOTE: Although J has a built-in primes function,
NB.       I thought that'd be too cheap to use. =P

    mod =: dyad : 0
z =. x % y
z =. z - (<. z)
z * y
)

    removeMultiples =: dyad : 0
len =. # y
head =. {. y
tail =. }. y
if. len
  do.
  if. (head mod x)
    do. head , (x removeMultiples tail)
    else. x removeMultiples tail
  end.
  else. y
end.
)

    generatePrimes =: verb define
currPrime =. {. y
if. ((currPrime * currPrime) <: {: y)
  do. currPrime , (generatePrimes (currPrime removeMultiples (}. y)))
  else. y
end.
)
    main =: verb define
generatePrimes ((2 }. i. y) , y)
)

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