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Semiprime: Difference between revisions
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→version 2: added/changed comments, optimized a DO loop. -- ~~~~
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===version 2===
<lang rexx>/*REXX program determines if
parse arg bot top . /*obtain #s from the command line*/
if bot=='' then bot=random() /*so, the user wants us to guess.*/
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w=max(length(bot), length(top)) /*get maximum width of numbers. */
if w>digits() then numeric digits w /*is there enough digits ? */
do n=bot to top /*show results for a range of #s.*/
if isSemiPrime(n) then say right(n,w) ' is semiprime.'
else say right(n,w) " isn't semiprime."
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exit /*stick a fork in it, we're done.*/
/*──────────────────────────────────ISPRIME subroutine──────────────────*/
isPrime: procedure; parse arg x; if x<2 then return 0
if wordpos(x,'2 3 5 7')\==0 then return 1
do i=2 for 2; if x//i==0 then return 0; end /*i*/ /*÷ by 2 & 3*/
do j=5 by 6 until j*j>x;
if x//(j+2)==0 then return 0 /*¬ a prime#*/
end /*j*/
return 1 /*X is a prime number, for sure.*/
/*──────────────────────────────────ISSEMIPRIME subroutine──────────────*/
isSemiPrime: procedure; arg x; if \datatype(x,'W') | x<4 then return 0
x=x/1 /*normalize the X number. */
else return 0
do j=5 by 6; if j*j>x then return 0 /*÷ by #s. */
do k=j by 2 for 2; if x//k==0 then if isPrime(x%k) then return 1
else return 0
end /*k*/ /*see if 2nd factor is prime or ¬*/
end /*j*/ /*[↑] never ÷ by # divisible by 3*/</lang>
'''output''' when the input is: <tt> -1 106 </tt>
<pre style="height:40ex">
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