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  prime number decomposition (math)  (Read 1735 times)
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Offline Kommi

Junior Devvie

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« Posted 2005-04-05 21:03:09 »

ok I need help coding this up in Java. I really dont understand any explanation of how exaclty a prime number decomposition algorithm works (in math terms). Does anyone know anything about this? Can you help me figure out a Java algo for it?

Offline DanoruX

Senior Newbie

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« Reply #1 - Posted 2005-04-06 11:05:22 »

To find out if a number is a prime number you devide it by every integer starting from 2 up to the square root of the nuber you're analyzing. Of course, the program will run a little faster is you skip as many integers as possible (like storing previously found primes in an arraylist and only deviding by those instead of every integer).

in it's simplest form:

if (number % dividebythisnumber == 0)
    // number is a prime.
Offline CaffeineOverdose

Senior Newbie

Java games rock!

« Reply #2 - Posted 2005-04-08 01:43:58 »

The method will work. It sounds very tedious and will take awhile if you use very large number, but unless you are checking to see if special types of numbers (i.e. (2^n)-1 ) are prime there isn't much else you could do.
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Offline DanoruX

Senior Newbie

Java games rock!

« Reply #3 - Posted 2005-04-08 16:32:16 »

couple of things you should know...

- all primes end with either 1,3,7 or 9
- the last digit is evenly distibuted. I tried it with 100000 primes, and all last digits occured 25000 +- 5 times.
Offline Hansdampf

Senior Devvie

Projects: 3

too offending?

« Reply #4 - Posted 2005-04-08 20:39:00 »

- all primes end with either 1,3,7 or 9


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Offline Malohkan

Senior Devvie

while (true) System.out.println("WOO!!!!");

« Reply #5 - Posted 2005-04-08 23:57:58 »

store them in an array as you create them.  Store 1, 3, 5, 7, 11 in the first indexes and then those rules will suit you fine Smiley

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Offline DanoruX

Senior Newbie

Java games rock!

« Reply #6 - Posted 2005-04-11 12:16:43 »



That's the one exception T_T
Offline Markus_Persson

JGO Wizard

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« Reply #7 - Posted 2005-04-11 12:19:42 »

2  Wink

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