Jump to content

Truncatable primes: Difference between revisions

Truncatable primes (view source)

Revision as of 19:51, 7 January 2020

731 bytes removed ,  4 years ago
m
→‎{{header|REXX}}: simplified the code, added/changed whitespace and comments, used a template for the output section.
(Rewrite Pike code to be simpler and faster)
m (→‎{{header|REXX}}: simplified the code, added/changed whitespace and comments, used a template for the output section.)
Line 2,628:
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─ and right─truncatable primes ≤ 1m (or argument 1).*/
parse arg highHI .; if highHI=='' then highHI= 1000000 /*Not specified? Then use default of 1m*/
!.= 0; w=length(high) w= length(HI) /*placeholders for primes; max 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 #=5; s.#= @.# **2 /*number of primes so far; prime². */
/* [↓] generate more primes ≤ high.*/
do j=@.#+2 by 2 for max(0, highHI%2 - @.#%2 - 1) /*only find odd primes from here on out*/
if j// 3==0 then iterate /*is J divisible by three? */
parse var j '' -1 _; if _==5 then iterate /* " " " " five? (right digit)*/
if j// 7==0 then iterate /* " " " " seven? */
if j//11==0 then iterate /* " " " " eleven? */
if j//13==0 then iterate /* " " " " thirteen? */
/* [↑] the above five lines saves time*/
do k=75 while s.k<=j /* [↓] divide by the known odd primes.*/
if j // @.k == 0 then iterate j /*Is J ÷ X? Then not prime. ___ ___ */
end /*k*/ /* [↑] only process upnumers to the≤ √ J */
#= #+1 /*bump the number of primes found. */
@.#= j; s.#=j*j; ! s.j#=1 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.*/
do k=1 for digslength(@.L); _=right(@.L, k) /*validate all left truncatable primes.*/
if \!._ then iterate L /*Truncated number not prime? Skip it.*/
end /*k*/
leave /*egress, found left truncatable prime.*/
end /*L*/
/* [↓] find largest right truncated P.*/
do R=# by -1 for #; digs=length(@.R) /*search from top end; get the length.*/
do k=1 for digslength(@.R); _= left(@.R, k) /*validate all right truncatable primes*/
if \!._ then iterate R /*Truncated number not prime? Skip it.*/
end /*k*/
leave /*egress, found right truncatable prime*/
end /*R*/
/*stick [↓]a fork showin largestit, left/right truncwe're all done. P*/
say 'The lastlargest prime foundleft─truncatable isprime ≤' @.# " (thereHI are" # 'primes ≤'" is high") right(@."L, w)
say copies('─',The 70)largest right─truncatable prime ≤' HI " is " right(@.R, /*show a separator line for the output.*w)</lang>
'''{{out|output''' |text=&nbsp; when using the default inputinputs:}}▼
say 'The largest left─truncatable prime ≤' high " is " right(@.L, w)
say 'The largest right─truncatable prime ≤' high " is " right(@.R, w)
/*stick a fork in it, we're all done. */</lang>
▲'''output''' &nbsp; when using the default input:
<pre>
The last prime found is 999983 (there are 78498 primes ≤ 1000000).
──────────────────────────────────────────────────────────────────────
The largest left─truncatable prime ≤ 1000000 is 998443
The largest right─truncatable prime ≤ 1000000 is 739399
Retrieved from "https://rosettacode.org/wiki/Special:MobileDiff/157004"
Cookies help us deliver our services. By using our services, you agree to our use of cookies.
More information