sort() not affecting original array while inside foreach loop

Question

I have array with structure:

Array
(
    [months] => Array
        (
            [01] => Array
                (
                    [0] => 16
                    [1] => 31
                )

            [02] => Array
                (
                    [0] => 16
                    [1] => 25
                    [2] => 10
                    [3] => 15
                )

        )

)

Now I trying to use uasort in loop:

        foreach ($dates['months'] as $month) {
            uasort($month, function($a, $b) {
                return $a <=> $b;
            });
        }

But it's not sorting by days (sub-array of month) - why?

Hard to say without expected, but perhaps you're just missing making $month a reference in the foreach? I.e. as &$month — Jonnix
– Jonnix, Commented Feb 2, 2018 at 11:01
There's no use for a custom comparison in this particular case, just asort($month) will do the same. — deceze
– deceze ♦, Commented Feb 2, 2018 at 11:14

Robert · Accepted Answer · 2018-02-02 11:06:37Z

2

You need to pass $month with reference in foreach loop

foreach ($dates['months'] as &$month)

uasort() modifies arrays by reference but you modify array that is inside the other array, so months are modified but they are not saved in $dates variable.

It can be easily checked by print_r($dates['months']); after uasort() function in the loop.

edited Feb 2, 2018 at 11:06

answered Feb 2, 2018 at 11:04

Robert

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5 Comments

C3roe Over a year ago

You should add unset($month); after the loop in such a case, to avoid unwanted side effects.

Robert Over a year ago

@CBroe I think it was fixed in PHP 7, In PHP 7, foreach does not use the internal array pointer.

C3roe Over a year ago

This isn’t about any internal array pointer, but that after the loop $month will still contain the reference to the last array element. This has the potential that you later on overwrite that array element without actually intending to do so, if you were to “re-use” that variable name for a different purpose, say $month = 'January'; Therefor the general recommendation is, if you have to use references in a foreach loop, kill the reference immediately after, to avoid falling into that trap. coderwall.com/p/qx3fpa/…

Robert Over a year ago

In that case yes you're right, but it's about naming. If we talk about defensive programming then it's good suggestion to use unset()

C3roe Over a year ago

Sure, a good “programming discipline” can avoid that problem to begin with. But in a more complex script, maybe even with multiple developers working on it, it just has a huge potential to lead to issues, and due to how it manifests itself, this one can be a particular p.i.t.a. to debug ... Therefor I simply consider it a good practice, same as say always using {...} after if/else, even if there currently might be only on single instruction following them.

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