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, Jan 28 '10 at 14:12

score 22 · Answer 1 · edited Jan 03 '16 at 15:36

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 03 '16 at 15:36

Peter Mortensen

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answered Nov 02 '09 at 20:24

Franz

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score 9 · Answer 2 · answered Nov 02 '09 at 20:28

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 02 '09 at 20:28

Ewan Todd

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score 4 · Answer 3 · answered Nov 02 '09 at 20:25

4

They're not opposites...

Check out de Morgan's laws.

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

should be

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

answered Nov 02 '09 at 20:25

Austin Salonen

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score 2 · Answer 4 · edited Jan 03 '16 at 15:38

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 03 '16 at 15:38

Peter Mortensen

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answered Nov 02 '09 at 20:25

LiraNuna

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score 2 · Answer 5 · edited Jan 03 '16 at 15:39

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 03 '16 at 15:39

Peter Mortensen

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answered Nov 02 '09 at 20:26

meder omuraliev

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score 1 · Answer 6 · edited Jan 03 '16 at 15:37

1

This:

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

should be:

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

edited Jan 03 '16 at 15:37

Peter Mortensen

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answered Nov 02 '09 at 20:25

schneck

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score 1 · Answer 7 · edited Jan 03 '16 at 15:40

1

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

edited Jan 03 '16 at 15:40

Peter Mortensen

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answered Nov 02 '09 at 20:32

Peter Bailey

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PHP != and == operators

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