Jim Tseng

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Code Challenge 1: List Prime Numbers

In this challenge, you are to list a set of

**prime numbers**from 2 to x (where x can be any number). I have looked at a number of examples out there, most people use two loops (nested) to determine whether a number is divisible by any number within 1 to itself. So if you are to list all prime number up to 100. At 50, it checks to see if it can divide 50 by any number between 1-50. Same goes to 51, 52, 53, etc, until 100. It sounds logical but I really don't like nested loop, and I avoid it at all cost. From my old math class, the first set of prime numbers at:

**2,3,5,7**. So what if I print those numbers first, then I check if the next set of numbers are divisible into any of these numbers. You use these four numbers as your base. For example, 8 can be divided evenly by 2 and 4 so it's not a prime, 9 can be divided evenly by 3 so it's not a prime, 10 can be divided evenly by 2 and 5 so it's not a prime, but 11 cannot be divided by any of these four numbers so it's a prime number. The logic (list) goes on and on. So, to list ALL prime numbers up to 100, you can use the following code snippet:

<?php $out = "2,3,5,7"; echo $out; for ($x=8; $x<=100; $x++) { if ( ( $x % 2 != 0 ) && ( $x % 3 != 0 ) && ( $x % 5 != 0 ) && ( $x % 7 != 0 ) ) echo ",".$x; } ?>

After running this script, you will get the following numbers printed: 2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97