PHP function to extract numbers from the list whose squares are exceed some threshold

Question

This is my naive approach:

$sample1 = [3, -20, 1, -19, -4, 6, 77, -1, 0, 6];
$sample3 = [4, 5, -7, 0, 2, 44, -3, -5, 66, -7];
$sample4 = [4, 25, 1, 6, 9, 5, 999, 4, 43, 2];

$sample2 = [];
for ($i = 0; $i < mt_rand(10, 30); $i++) {
  $sample2[] = mt_rand(-1000, 1000);
}

extractNumbersThatSquaresAreGreaterThan($sample1);
extractNumbersThatSquaresAreGreaterThan($sample2);
extractNumbersThatSquaresAreGreaterThan($sample3);
extractNumbersThatSquaresAreGreaterThan($sample4);

var_dump($sample1);
//var_dump($sample2);
var_dump($sample3);
var_dump($sample4);

function extractNumbersThatSquaresAreGreaterThan(&$list, $boundary = 26) {
  foreach ($list as $key => $value) {
    if (pow($value, 2) <= $boundary) {
      unset($list[$key]);
    }
  }
}

Any hints for optimization? Is it possible, I've tried few, but in the end it was more complicated (and slower) solution

@YourCommonSense please provide me more test data, than I'll fix — DevMed
– DevMed, Commented Jun 26, 2018 at 13:13

AJNeufeld · Accepted Answer · 2018-06-26 13:38:13Z

2

Hints? Ok.

Don’t square the values, O(n), instead square-root the boundary. You’ll need an or in your test, though.
Don’t unset values in the list. That requires shifting values in memory. Make a new list of same size, fill in values, without gaps, and then resize result.

answered Jun 26, 2018 at 13:38

AJNeufeld

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Stexxe · Accepted Answer · 2018-06-26 21:02:49Z

1

I don't know about how fast this solution will be, but anyway I suggest to use functional approach to solve your problem. At least you will have concise code:

$sample4 = [4, 25, 1, 6, 9, 5, 999, 4, 43, 2];

var_dump(extractNumbers($sample4, 26));

function extractNumbers($list, $boundary) {
    return array_filter($list, function($x) use ($boundary) {
        return ($x * $x) > $boundary;
    });
}

edited Jun 26, 2018 at 21:02

answered Jun 26, 2018 at 20:22

Stexxe

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Philipp · Accepted Answer · 2018-06-29 10:00:56Z

Instead of remove items with unset, I would use array_filter. And in fact, you could factor out the pow function, if you use basic maths.

$$ num^2 \leq boundary\\ \implies |num| \leq \sqrt{boundary} $$

combining this would result in

function extractNumbersThatSquaresAreGreaterThan(&$list, $boundary = 26) {
    $sqrt_boundary = sqrt($boundary);
    $list = array_filter($list, function ($item) use ($sqrt_boundary) {
        return abs($item) > $sqrt_boundary;
    });
}

In addition, I would avoid the use of referenes for this task and just return your value.

function extractNumbersThatSquaresAreGreaterThan($list, $boundary = 26) {
    $sqrt_boundary = sqrt($boundary);
    return array_filter($list, function ($item) use ($sqrt_boundary) {
        return abs($item) > $sqrt_boundary;
    });
}

Thanks a lot for this. Especially reminding me, that something like ABS exists ;) — DevMed
– DevMed, Commented Jun 30, 2018 at 19:18

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