PHP != and == operators

Question

This has boggled me for a while. I am running through a directory and echo'ing out its contents, and I want to exclude the ".." and "." files.

Now, this code works:

if ($files = scandir("temp/"))
{
    foreach ($files as $file)
    {
        if ($file == ".." OR $file == ".")
        {
        }
        else {
            echo $file;
            echo "<br>";
        }
    }
}

But this doesn't...

if ($files = scandir("temp/"))
{
    foreach ($files as $file)
    {
        if ($file != ".." OR $file != ".")
        {
            echo $file;
            echo "<br>";
        }
    }
}

For obvious reasons the second lump of code is more what I want, because I really hate having the true statement do nothing.

You really need to choose an accepted answer on some your questions. — Josh Mein
– Josh Mein, Commented Jan 28, 2010 at 14:12

Peter Mortensen · Accepted Answer · 2016-01-03 15:36:12Z

22

If you negate a condition consisting of two single conditions and a conjunction ("and" or "or"), you need to negate each condition separately and use the other conjunction.

So try this instead:

if ($file != ".." AND $file != ".")

edited Jan 3, 2016 at 15:36

Peter Mortensen

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answered Nov 2, 2009 at 20:24

Franz

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Comments

Ewan Todd · Accepted Answer · 2009-11-02 20:28:08Z

9

This is one of deMorgan's Laws.

not (A OR B) = (not A) AND (not B)

The change you are making is a refactoring called Reverse Conditional

answered Nov 2, 2009 at 20:28

Ewan Todd

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Comments

Austin Salonen · Accepted Answer · 2009-11-02 20:25:35Z

4

They're not opposites...

Check out de Morgan's laws.

if($file != ".." OR $file != ".")

should be

if($file != ".." AND $file != ".")

answered Nov 2, 2009 at 20:25

Austin Salonen

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Comments

Peter Mortensen · Accepted Answer · 2016-01-03 15:38:22Z

2

You have to negate the entire expression, just like -(-x + 2) in math negates everything:

if ($file == ".." OR $file == ".")

Is not the negation of

if ($file != ".." OR $file != ".")

Because you didn't negate the OR. The opposite of OR is AND, resulting a:

if ($file != ".." AND $file != ".")

edited Jan 3, 2016 at 15:38

Peter Mortensen

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answered Nov 2, 2009 at 20:25

LiraNuna

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Comments

Peter Mortensen · Accepted Answer · 2016-01-03 15:39:26Z

2

$file != ".." evaluates to true. Instead, simply use an AND operator:

if ( $file != '..' && $file != '.' ) { }

However, I would use DirectoryIterator instead:

foreach (new DirectoryIterator('temp') as $fileInfo) {
    if ($fileInfo->isDot())
        continue;
    echo $fileInfo->getFilename() . "<br>\n";
}

edited Jan 3, 2016 at 15:39

Peter Mortensen

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answered Nov 2, 2009 at 20:26

meder omuraliev

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Comments

Peter Mortensen · Accepted Answer · 2016-01-03 15:37:21Z

1

This:

    if ($file != ".." OR $file != ".")

should be:

    if ($file != ".." && $file != ".")

edited Jan 3, 2016 at 15:37

Peter Mortensen

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answered Nov 2, 2009 at 20:25

schneck

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Comments

Community · Accepted Answer · 2023-11-21 15:10:29Z

1

Alternatively, you could always use DirectoryIterator and specifically its isDot method.

edited Nov 21, 2023 at 15:10

CommunityBot

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answered Nov 2, 2009 at 20:32

Peter Bailey

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