Mohammad Masoudian - 1 year ago 103
PHP Question

# A formula to find prime numbers in a loop

I need to find prime numbers with for loop or while loop

I wrote this but this is wrong

``````<?php
\$i = 1;
while(\$i<5)
{
for(\$j=1; \$j<=\$i; \$j++)
{
if (\$j != 1 && \$j != \$i)
{
echo \$i . "/" . \$j . "=" . \$i%\$j . "<br />";
if (\$i%\$j != 0)
{
echo \$i . "<br />";
}
}
}
echo "<br />";
\$i += 1;
}
?>
``````

Is there a way to divide a number with an array to find the remaining?

Here's a little function that I found: (http://icdif.com/computing/2011/09/15/check-number-prime-number/) Seemed to work for me!

``````function isPrime(\$num) {
//1 is not prime. See: http://en.wikipedia.org/wiki/Prime_number#Primality_of_one
if(\$num == 1)
return false;

//2 is prime (the only even number that is prime)
if(\$num == 2)
return true;

/**
* if the number is divisible by two, then it's not prime and it's no longer
* needed to check other even numbers
*/
if(\$num % 2 == 0) {
return false;
}

/**
* Checks the odd numbers. If any of them is a factor, then it returns false.
* The sqrt can be an aproximation, hence just for the sake of
* security, one rounds it to the next highest integer value.
*/
for(\$i = 3; \$i <= ceil(sqrt(\$num)); \$i = \$i + 2) {
if(\$num % \$i == 0)
return false;
}

return true;
}
``````
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