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Semiprime: Difference between revisions
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→version 2: added/changed comments and whitespace, changed indentations.
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===version 2===
The method used is to examine
If it's composite (the 1<sup>st</sup> factor is prime), then check if the 2<sup>nd</sup> factor is prime. If so, the number is a ''semiprime''.
The '''isPrime''' function could be optimized by utilizing an integer square root function instead of testing if '''j*j>x''' for every divisor.
<lang rexx>/*REXX program determines if any
parse arg bot top . /*obtain optional
if bot=='' | bot=="," then bot=random() /*None given? User wants us to guess.*/
if top=='' | top=="," then top=bot /*maybe define a range of numbers. */
w=max(length(bot), length(top)) /*obtain the maximum width of numbers. */
numeric digits max(9, w) /*ensure there're enough decimal digits*/
do n=bot to top /*show results for a range of numbers. */
if isSemiPrime(n) then say right(n, w) " is semiprime."
else say right(n, w) " isn't semiprime."
end /*n*/
exit /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
isPrime: procedure; parse arg x; if x<2 then return 0
if wordpos(x, '2 3 5 7 11 13 17 19 23')\==0 then return 1
if x//2==0 then return 0; if x//3==0 then return 0
do j=5 by 6 until j*j>x; if x//j==0 then return 0
if x//(j+2)==0 then return 0
end /*j*/
return 1 /*indicate that X is a prime number. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
isSemiPrime: procedure; parse arg x;
do i=2 for 2; if x//i==0 then
else return 0
end /*i*/
/* ___ */
do j=5 by 6; if j*j>x then return 0
do k=j by 2 for 2; if x//k==0 then if isPrime(x%k) then return 1
else return 0
end /*k*/
end /*j*/
'''output''' when using the input
<pre style="height:44ex">
-1 isn't semiprime.
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106 is semiprime.
</pre>
'''output''' when using the input
<pre style="height:44ex">
99888111555 isn't semiprime.
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