PHP Program - Find all Prime Numbers in a given Interval
A Prime number is a natural number greater than 1 and divisible by 1 and itself only, for example: 2, 3, 5, 7, etc.
Objective: Write a PHP code to find all prime numbers in a given internal.
Method 1: Using function to find prime number
In the example below, a function called primenumber() is created which takes a number as argument and checks it for prime number by dividing it with all natural numbers starting from 2 to N/2.
<?php function primenumber($MyNum) { $n = 0; for($i = 2; $i < ($MyNum/2+1); $i++) { if($MyNum % $i == 0){ $n++; break; } } if ($n == 0){ echo $MyNum." "; } } $x = 10; $y = 50; echo "Prime numbers between ".$x." and ".$y." are: \n"; for($i = $x; $i < $y + 1; $i++) { primenumber($i); } ?>
The above code will give the following output:
Prime numbers between 10 and 50 are: 11 13 17 19 23 29 31 37 41 43 47
Method 2: Optimized Code
- Instead of checking the divisibility of given number from 2 to N/2, it is checked till square root of N. For a factor larger than square root of N, there must the a smaller factor which is already checked in the range of 2 to square root of N.
- Except from 2 and 3, every prime number can be represented into 6k ± 1.
<?php function primenumber($MyNum) { $n = 0; if ($MyNum == 2 || $MyNum == 3){ echo $MyNum." "; } elseif ($MyNum % 6 == 1 || $MyNum % 6 == 5) { for($i = 2; $i*$i <= $MyNum; $i++) { if($MyNum % $i == 0){ $n++; break; } } if ($n == 0){ echo $MyNum." "; } } } $x = 100; $y = 200; echo "Prime numbers between ".$x." and ".$y." are: \n"; for($i = $x; $i < $y + 1; $i++) { primenumber($i); } ?>
The above code will give the following output:
Prime numbers between 100 and 200 are: 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199
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