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Truncatable primes: Difference between revisions

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Truncatable primes (view source)

Revision as of 09:07, 5 March 2016

10 bytes added ,  8 years ago
m
→‎{{header|REXX}}: removed some dead code.
m (→‎{{header|REXX}}: simplfied two search loops.)
m (→‎{{header|REXX}}: removed some dead code.)
Line 2,052:
=={{header|REXX}}==
Extra code was added to the prime number generator as this is the section of the REXX program that consumes the vast majority of the computation time.
<lang REXX>/*REXX program finds largest left-left─ and right-truncatableright─truncatable primes ≤ 1m (or argument 1).*/
parse arg high .; if high=='' then high=1000000 /*Not specified? Then use 1m*/
!.=0; Lpw=0;length(high) Rp=0; w=length(high) /*placeholders for primes; left&right Pmax width. */
@.1=2; @.2=3; @.3=5; @.4=7; @.5=11; @.6=13; @.7=17 /*define some low primes. */
!.2=1; !.3=1; !.5=1; !.7=1; !.11=1; !.13=1; !.17=1 /*set some low prime flags. */
#=7; s.#=@.#**2 /*number of primes so far; prime². */
/* [↓] generate more primes ≤ high.*/
do j=@.#+2 by 2 to high /*only find odd primes from here on out*/
if j// 3==0 if thenj// iterate3==0 then iterate /*is J divisible by three? */
parse var j '' -1 _; if _==5 then iterate /* " " " " five? (right digit)*/
if j// 7==0 if thenj// iterate7==0 then iterate /* " " " " seven? */
if j//11==0 thenif iteratej//11==0 then iterate /* " " " " eleven? */
if j//13==0 thenif iteratej//13==0 then iterate /* " " " " thirteen? */
/* [↑] the above five lines saves time*/
do k=7 while s.k<=j /* [↓] divide by the known odd primes.*/
if j//@.k==0 then iterate j /*Is J divisible by X? Then not prime.*/
end /*k*/
#=#+1 /*bump the number of primes found. */
@.#=j; s.#=j*j; !.j=1 /*assign next prime; prime²; prime #.*/
end /*j*/
/* [↓] find largest left truncatable P*/
do L=# by -1 for #; digs=length(@.L) /*search from top end; get the length.*/
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say 'The last prime found is ' @.# " (there are" # 'primes ≤' high")."
say copies('─', 70) /*show a separator line for the output.*/
say 'The largest left-truncatableleft─truncatable prime ≤' high " is " right(@.L, w)
say 'The largest right-truncatableright─truncatable prime ≤' high " is " right(@.R, w)
/*stick a fork in it, we're all done. */</lang>
'''output''' &nbsp; when using the default input:
Line 2,096:
The last prime found is 999983 (there are 78498 primes ≤ 1000000).
──────────────────────────────────────────────────────────────────────
The largest left-truncatableleft─truncatable prime ≤ 1000000 is 998443
The largest right-truncatableright─truncatable prime ≤ 1000000 is 739399
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