Truncatable primes: Difference between revisions
Content added Content deleted
(→{{header|REXX}}: added comments & whitespace, support for specifying the upper limit, optimized prime number generator. -- ~~~~) |
m (→{{header|REXX}}: removed two more-or-less useless lines of code. -- ~~~~) |
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n=7; s.7=17**2 /*number of primes so far. */ |
n=7; s.7=17**2 /*number of primes so far. */ |
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/*───────────────────────────────────────generate primes ≤ high. */ |
/*───────────────────────────────────────generate primes ≤ high. */ |
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call time 'Reset' |
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do j=p.n+2 by 2 to high /*only find odd Primes from here.*/ |
do j=p.n+2 by 2 to high /*only find odd Primes from here.*/ |
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if j//3 ==0 then iterate /*divisible by three? */ |
if j//3 ==0 then iterate /*divisible by three? */ |
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!.j=1 /*indicate that J is a prime.*/ |
!.j=1 /*indicate that J is a prime.*/ |
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end /*j*/ |
end /*j*/ |
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say 'generation of primes took' format(time('E'),,2) "seconds." |
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/*───────────────────────────────────────find largest left truncatable P*/ |
/*───────────────────────────────────────find largest left truncatable P*/ |
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do L=n by -1 for n while lp==0; if pos(0,p.L)\==0 then iterate |
do L=n by -1 for n while lp==0; if pos(0,p.L)\==0 then iterate |
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if \!._ then iterate L /*Truncated # ¬prime? Skip it.*/ |
if \!._ then iterate L /*Truncated # ¬prime? Skip it.*/ |
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end /*k*/ |
end /*k*/ |
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leave /*leave the DO loop, we found one*/ |
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lp=p.L |
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end /*L*/ |
end /*L*/ |
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/*───────────────────────────────────────find largest right truncatable P*/ |
/*───────────────────────────────────────find largest right truncatable P*/ |
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if \!._ then iterate R /*Truncated # ¬prime? Skip it.*/ |
if \!._ then iterate R /*Truncated # ¬prime? Skip it.*/ |
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end /*k*/ |
end /*k*/ |
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leave /*leave the DO loop, we found one*/ |
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rp=p.R |
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end /*R*/ |
end /*R*/ |
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/*───────────────────────────────────────show largest left/right trunc P*/ |
/*───────────────────────────────────────show largest left/right trunc P*/ |
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say 'The last prime is' p.n " (there are" n 'primes ≤' high")." |
say 'The last prime is' p.n " (there are" n 'primes ≤' high")." |
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say copies('─',70) /*show a separator line. */ |
say copies('─',70) /*show a separator line. */ |
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say 'The largest left-truncatable prime ≤' high " is " |
say 'The largest left-truncatable prime ≤' high " is " p.L |
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say 'The largest right-truncatable prime ≤' high " is " |
say 'The largest right-truncatable prime ≤' high " is " p.R |
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/*stick a fork in it, we're done.*/</lang> |
/*stick a fork in it, we're done.*/</lang> |
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'''output''' |
'''output''' |
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