Question

I need to find the highest prime number in a given range.
Here is my code which works for 0-100 but if I give 0-125 it is showing prime number as 125.

<?php
    $flag=0;
    $b=125;
    for($i=$b;$i>=0;$i--)
    {
        if($i%2!=0)
        {
            for($b=3;$b<10;$b++)
            {
                if($flag==0)
                {
                    echo('<br>');
                    if($i%$b!=0)
                    {
                        echo('highest prime number is'.$i);
                        $flag=1;
                        break;
                    }
                    elseif ($i%$b==0)
                    {
                        break;
                    }
                }
            }
        }
    }
?>

In the above code I have taken the range from 0-125

Answer 1

Yup the problem is algorithmic…

1) You’ll need to check up till sqrt($b) i.e. 11 in this case

2) The $flag logic is kinda messed up, no use changing the flag then breaking out right after…

<?php
        $flag=0;
        $b=125;
        $sq=sqrt($b); 

        for($i=$b;$i>=0;$i--)
        {

            if($i%2!=0)
            {
                    for($b=3;$b<=$sq;$b++)
                    {
                            if($i%$b!=0)
                            {
                                $flag=1;
                            }
                            elseif($i%$b==0)
                            {
                                $flag=0;
                                break;
                            }
                    }
            if($flag==1){ 
                echo('highest prime number is '.$i);  
                break;
            }
        }
    }
?>