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17 lines
674 B
APL
17 lines
674 B
APL
⍝ Sieve of Eratosthenes — the classic APL one-liner
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⍝ primes ← (2=+⌿0=A∘.|A)/A←⍳N
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⍝
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⍝ Read right-to-left:
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⍝ A ← ⍳N : A is 1..N
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⍝ A∘.|A : outer-product residue table — M[i,j] = A[j] mod A[i]
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⍝ 0=... : boolean — true where A[i] divides A[j]
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⍝ +⌿... : column sums — count of divisors per A[j]
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⍝ 2=... : true for numbers with exactly 2 divisors (1 and self) → primes
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⍝ .../A : compress — select A[j] where mask[j] is true
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⍝
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⍝ Examples:
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⍝ primes 10 → 2 3 5 7
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⍝ primes 30 → 2 3 5 7 11 13 17 19 23 29
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primes ← {(2=+⌿0=⍵∘.|⍵)/⍵←⍳⍵}
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