in reply to Stupidest Prime Number detector ever!!

The problem you are coming across isn't Perl, it's your algorithm. Learning algorithms is great -- and there have been recent discussions on algorithms for Primes (use Super Search to look for "Prime") -- but is that what you want to spend your time on right now?

Given your stated goal of learning about Perl and how to make modules, I'd just go down that path with your current algorithm, and just not test or debug it on large prime_candidates to avoid long times.

If you still want to focus on the algorithm, great; here are a few brief comments on your algorithm:

• Despite your assertion, it won't find all non-primes in a flash; if a non-prime has a very large number as its lowest prime factor, then it will take a long time to get that far
• You are testing at least 2x as many divisors as you need to: except for 2, there are no even primes, so don't include those in your loop: foreach my \$each_num ( 2, map { 2*\$_+1 } 1 .. \$prime_candidate/2) -- see perldoc -f map for how map works
• If you also think about odd multiples of 3, that's still too many; other than 3, there are no odd multiples of 3. This means that other than 2 and 3, all primes are +/-1 from a multiple of six (try proving that to yourself based on what I've said already). foreach my \$each_num ( 2, 3, map { (6*\$_-1, 6*\$_+1) } 1 .. 1+\$prime_candidate/6)
• You are still going much higher than you have to for large numbers. The lowest prime factor of a non-prime will always be less than or equal to the number's square root, so divisor checks shouldn't go beyond that. foreach my \$each_num ( 2, 3, map { (6*\$_-1, 6*\$_+1) } 1 .. 1+sqrt(\$prime_candidate)/6)
• Read about the Sieve of Eratosthenes if you want to get deeper into algorithm improvements beyond the 2 and 3 divisors.
• when your prime candidate gets too big, you won't be able to use default math; use bigint to go beyond that limitation

(Before posting, I see ++choroba beat me to most of those algorithm points. I said some things differently, so I am still posting this.)

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Re^2: Stupidest Prime Number detector ever!!
by Anonymous Monk on Jun 23, 2021 at 20:13 UTC

Thank you for such a detailed post, but, the only way I have used map is to gather the output in another array, for example:  my @array (1..1000);  my @odds = map { \$_ % 2 != 0 } @array; .So I am taking a bit of a time getting used to the way you are doing it in the sample code, but that's my bad. Thanks again

.
my @odds = map { \$_ % 2 != 0 } @array;

That's not going to do what you think. Consider the following:-

```johngg@abouriou:~\$ perl -Mstrict -Mwarnings -E '
say for map { \$_ % 2 != 0 } 0 .. 10;'

1

1

1

1

1

The alternating blanks and ones are registering the FALSE and TRUE results of the expression in the map, which essentially has a one to one relation between input on the right and output on the left. What you should be using instead is grep which filters input, only passing to the left those items for which the expression is TRUE.

```johngg@abouriou:~\$ perl -Mstrict -Mwarnings -E '
say for grep { \$_ % 2 != 0 } 0 .. 10;'
1
3
5
7
9

Note also that you could dispense with the != 0 as the expression \$_ % 2 will evaluate to either 0 or 1.

```johngg@abouriou:~\$ perl -Mstrict -Mwarnings -E '
say for grep { \$_ % 2 } 0 .. 10;'
1
3
5
7
9