find all the negative numbers in the array using array_map php

Question

I have this test array

$test = array(-10,20,40,-30,50,-60);

I want the output to be

$out = array (-10, -30, -60);

Here's my solution:

$temp = array();

function neg($x)
{
    if ($x <0) 
    {
        $temp[] = $x; 
        return $x;
    }
}
$negative = array_map("neg", $test);

when I print $negative, I get what I wanted but with some entries that are null. Is there anything I can do in the callback function to not log the null entries?

Array
(
    [0] => -10
    [1] => 
    [2] => 
    [3] => -30
    [4] => 
    [5] => -60
)
1

When I print the $temp array, I thought I would get my answer but it printed an empty array. I don't understand why, I'm clearing adding the $x to the $temp[] array in my callback function. Any thoughts?

print_r($temp);
// outputs
Array
(
)
1

@Orangepill however it's not the concern with array_map right? I'm storing my value $x in a $temp array that is accessible within the callback function. — Wild Widow
– Wild Widow, Commented Aug 20, 2015 at 5:02
why is it so? I declared the $temp outside my callback and I'm appending $x to $temp everytime the callback is called.? — Wild Widow
– Wild Widow, Commented Aug 20, 2015 at 5:09

Sougata Bose · Accepted Answer · 2015-08-20 05:18:11Z

1

array_map will return the value when the condition satisfies and return NULL if conditions fails. In this case you can use array_filter.

$test = array(-10,20,40,-30,50,-60);

$neg = array_filter($test, function($x) {
    return $x < 0;
});

Output

array(3) {
  [0]=>
  int(-10)
  [3]=>
  int(-30)
  [5]=>
  int(-60)
}

And if you continue to use array_map then I would suggest apply array_filter once when it is done -

$negative = array_map("neg", $test);
$negative = array_filter($negative);

The output will be same.

edited Aug 20, 2015 at 5:18

answered Aug 20, 2015 at 5:04

Sougata Bose

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2 Comments

Wild Widow Over a year ago

This is great, answers one of my questions, but why the $temp array is not getting populated in my callback function?

Sougata Bose Over a year ago

$temp is only available inside the function because of the scope. You can't access them like that. Though the $temp is no needed anyway.

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