explain please

[galaxyAbstractor]

<?php
//1. How many more loops does the program do to find all primes to 200?


$maxArr = 100;
$testRuns = 0;
$noPrimes = 0;
$jrun = 0;

for($i = 0; $i < $maxArr;$i++) {
    $primArr[$i] = $i + 1;
}

$primArr[0] = -1;

for($i = 1; $i < $maxArr;$i++) { 
    $prim = $primArr[$i];
    if($prim != -1) { 
        for($j = ($i + 1); $j < $maxArr; $j++){ //3c. How many numbers does this loop go trought. EXPLAIN WHY
        
            if(($primArr[$j] % $prim) == 0){ //3d. what does this if do?
                $primArr[$j] =-1;
            }
            $testRuns = $testRuns +1;
        }
    }
}


for($i = 0; $i < $maxArr; $i++){
    if($primArr[$i] != -1) {
        echo "$primArr[$i]<br />";
        $noPrimes = $noPrimes+1;
    }
}
echo "Antal loopar: $testRuns<br />";
echo "Antal primtal funna: $noPrimes<br />";

?>

Explain plox. I don't get it...

 

[leafypiggy]

Neither do I. Just looking at the first loop confused me. It subtracts one; then adds one outside of the for(); lol.

I'm confused. xD

 

[garrettroyce]

<?php
//1. How many more loops does the program do to find all primes to 200?


$maxArr = 100;
$testRuns = 0;
$noPrimes = 0;
$jrun = 0;

for($i = 0; $i < $maxArr;$i++) {
    $primArr[$i] = $i + 1;
}

$primArr[0] = -1;

for($i = 1; $i < $maxArr;$i++) { //loop 99 times (1-99)
    $prim = $primArr[$i];
    if($prim != -1) { //true after $i = 0, which is always since loop starts at 1
        for($j = ($i + 1); $j < $maxArr; $j++){ //this will loop 98 times, then 97, then 96... $j is one more than $i and must be less than 100 ($maxArr) and this loop is encountered 99 times ($i from 1 to 99)
        
            if(($primArr[$j] % $prim) == 0){ //if the current number in the array is evenly divisible by ANY other number, it is not prime, change it's value to -1
                $primArr[$j] =-1;
            }
            $testRuns = $testRuns +1;
        }
    }
}


for($i = 0; $i < $maxArr; $i++){ //any value in the array NOT equal to negative one is echoed
    if($primArr[$i] != -1) {
        echo "$primArr[$i]<br />";
        $noPrimes = $noPrimes+1;
    }
}
echo "Antal loopar: $testRuns<br />";
echo "Antal primtal funna: $noPrimes<br />";

?>

Explain plox. I don't get it...

I've seen better ways to do this...

Hope this helps!

 

[VPmase]

3c: It gets the modulus of the $prim number for any number > than $prim and if it equals 0 then $prim not prime.

garrettroyce explained it well in his comments.

 

[galaxyAbstractor]

thanks