65 lines
2.8 KiB
Rexx
65 lines
2.8 KiB
Rexx
/*REXX program counts/displays the number of additive primes less than N. */
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Parse Arg n cols . /*get optional number of primes To find*/
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If n=='' | n==',' Then n= 500 /*Not specified? Then assume default.*/
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If cols=='' | cols==',' Then cols= 10 /* ' ' ' ' ' */
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call genP n /*generate all primes under N. */
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w=5 /*width of a number in any column. */
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title= 'additive primes that are < 'commas(n)
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If cols>0 Then Say ' index ¦'center(title,cols*(w+1)+1)
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If cols>0 Then Say '-------+'center('' ,cols*(w+1)+1,'-')
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found=0
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ol='' /*a list of additive primes (so far). */
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idx=1
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Do j=1 By 1
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p=p.j /*obtain the Jth prime. */
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If p>n Then Leave /* no more needed */
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_=sumDigs(p)
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If !._ Then Do
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found=found+1 /*bump the count of additive primes. */
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c=commas(p) /*maybe add commas To the number. */
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ol=ol right(c,max(w,length(c))) /*add additive prime--?list,allow big# */
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If words(ol)=10 Then Do /* a line is complete */
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Say center(idx,7)'¦' substr(ol,2) /*display what we have so far (cols). */
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ol='' /* prepare for next line */
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idx=idx+10
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End
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End
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End /*j*/
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If ol\=='' Then
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Say center(idx,7)'¦' substr(ol,2) /*possible display residual output. */
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If cols>0 Then
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Say '--------'center('',cols*(w+1)+1,'-')
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Say
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Say 'found ' commas(found) title
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Exit 0 /*stick a fork in it, we're all done. */
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/*--------------------------------------------------------------------------------*/
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commas: Parse Arg ?; Do jc=length(?)-3 To 1 by -3; ?=insert(',',?,jc); End; Return ?
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sumDigs:Parse Arg x 1 s 2; Do k=2 For length(x)-1; s=s+substr(x,k,1); End; Return s
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/*--------------------------------------------------------------------------------*/
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genP:
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Parse Arg n
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pl=2 3 5 7 11 13
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!.=0
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Do np=1 By 1 While pl<>''
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Parse Var pl p pl
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p.np=p
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sq.np=p*p
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!.p=1
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End
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np=np-1
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Do j=p.np+2 by 2 While j<n
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Parse Var j '' -1 _ /*obtain the last digit of the J var.*/
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If _==5 Then Iterate
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If j// 3==0 Then Iterate
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If j// 7==0 Then Iterate
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If j//11==0 Then Iterate
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Do k=6 By 1 While sq.k<=j /*divide J by other primes <=sqrt(j) */
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If j//p.k==0 Then Iterate j /* not prime - try next */
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End /*k*/
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np=np+1 /*bump prime count; assign prime & flag*/
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p.np=j
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sq.np=j*j
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!.j=1
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End /*j*/
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Return
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