Truncatable primes: Difference between revisions
Content added Content deleted
m (→{{header|REXX}}: simplfied two search loops.) |
m (→{{header|REXX}}: removed some dead code.) |
||
| Line 2,052: | Line 2,052: | ||
=={{header|REXX}}== |
=={{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. |
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 |
<lang REXX>/*REXX program finds largest left─ and right─truncatable primes ≤ 1m (or argument 1).*/ |
||
parse arg high .; if high=='' then high=1000000 /*Not specified? Then use 1m*/ |
parse arg high .; if high=='' then high=1000000 /*Not specified? Then use 1m*/ |
||
!.=0; |
!.=0; w=length(high) /*placeholders for primes; max width. */ |
||
@.1=2; @.2=3; @.3=5; @.4=7; @.5=11; @.6=13; @.7=17 /*define some low primes. */ |
@.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. */ |
!.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. */ |
#=7; s.#=@.#**2 /*number of primes so far; prime². */ |
||
/* [↓] generate more primes ≤ high.*/ |
/* [↓] generate more primes ≤ high.*/ |
||
do j=@.#+2 by 2 to high /*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*/ |
/* [↑] 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*/ |
/* [↓] find largest left truncatable P*/ |
||
do L=# by -1 for #; digs=length(@.L) /*search from top end; get the length.*/ |
do L=# by -1 for #; digs=length(@.L) /*search from top end; get the length.*/ |
||
| Line 2,089: | Line 2,089: | ||
say 'The last prime found is ' @.# " (there are" # 'primes ≤' high")." |
say 'The last prime found is ' @.# " (there are" # 'primes ≤' high")." |
||
say copies('─', 70) /*show a separator line for the output.*/ |
say copies('─', 70) /*show a separator line for the output.*/ |
||
say 'The largest |
say 'The largest left─truncatable prime ≤' high " is " right(@.L, w) |
||
say 'The largest |
say 'The largest right─truncatable prime ≤' high " is " right(@.R, w) |
||
/*stick a fork in it, we're all done. */</lang> |
/*stick a fork in it, we're all done. */</lang> |
||
'''output''' when using the default input: |
'''output''' when using the default input: |
||
| Line 2,096: | Line 2,096: | ||
The last prime found is 999983 (there are 78498 primes ≤ 1000000). |
The last prime found is 999983 (there are 78498 primes ≤ 1000000). |
||
────────────────────────────────────────────────────────────────────── |
────────────────────────────────────────────────────────────────────── |
||
The largest |
The largest left─truncatable prime ≤ 1000000 is 998443 |
||
The largest |
The largest right─truncatable prime ≤ 1000000 is 739399 |
||
</pre> |
</pre> |
||