Compare array items against other items in array one time

Question

I have a PHP array that has from 2-5 items in it.

$z=["itema", "itemb", "itemc", "itemd","iteme"];

I need to have a way to go through the array and match each item against each other one time. So the output would look like

itema vs itemb
itema vs itemc
itema vs itemd
itema vs iteme

itemb vs itemc
itemb vs itemd
itemb vs iteme

itemc vs itemd...ect

I can not seem to get the loop to hit every item. I think it has to be a loop in a loop since my just one loop doesn't work.

And it has to go to each only once. I am so lost on how to get this to work. Any help would be much appreciated.

Please provide the code that you have so far - it will help users to pinpoint possible problems with your solution. Also describe how exactly the code fails to achieve your goal. — Olga
– Olga, Commented Nov 16, 2015 at 18:43

Philipp · Accepted Answer · 2018-07-25 13:48:59Z

6

You could use two for loops

$count = count($z);
for ($i = 0; $i < $count - 1; $i++) {
    for ($j = $i + 1; $j < $count; $j++) {
        // compare $z[$i] with $z[$j]...
    }
}

The other loop goes from 0 to n - 2 and the inner loop takes the value from the outer value as initial and goes to the end.

If you also need to handle associative arrays, like one commenter mentioned, you could use array_values (and array_keys) to handle this case.

$values = array_values($z);
$keys = array_keys($z); // optional - only used, if you need the keys..
$count = count($values);
for ($i = 0; $i < $count - 1; $i++) {
    for ($j = $i + 1; $j < $count; $j++) {
        // compare $values[$i] with $values[$j]...
        // or $keys[$i] with $keys[$j];
    }
}

Note: A foreach loop won't work in either case, because you can't told the inner loop to skip all values, already seen in the outher loop. And teaching the foreach loop this behavior would result in a foreach, that effectly looks like a simple for loop and is slower. There's simply no benefit in using a foreach.

edited Jul 25, 2018 at 13:48

answered Nov 16, 2015 at 18:21

Philipp

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10 Comments

workingxx Over a year ago

Thats it ! than you very much. I figured I needed an inner loop but I could not get it right.

ProgrammingWithRandy Over a year ago

@Philipp congrats on 10k

Julian Over a year ago

I down voted it because this approach doesn't work with string key array's. It's better to use foreach instead of for, please see this approach: stackoverflow.com/questions/33741890/…

Philipp Over a year ago

@Julian - 1. assoc arrays wasn't the authors problem. 2. The "better" approach is not what the author wants, as this compares [1,2] and also [2,1]. If you can't use numeric indices, you could first apply array_values to $z and if you also need the key, there is more to done. But just using a foreach doesn't work!

Julian Over a year ago

@Philipp You are right about array_values() but it's wrong to say that a foreach won't work. Please see here: stackoverflow.com/questions/33741890/…

|

David Poirier · Accepted Answer · 2015-11-16 18:19:26Z

0

Not tested but something like:

foreach($z as $item){
  foreach($z as $item2){
    if($item!=$item2){
      echo $item . ' vs ' . $item2;
    }
  }
}

answered Nov 16, 2015 at 18:19

David Poirier

1711 silver badge7 bronze badges

Comments

Nana Partykar · Accepted Answer · 2015-11-16 18:46:33Z

0

<?php 
$z=["itema", "itemb", "itemc", "itemd","iteme"];

$sizeZ=count($z);
for ($i = 0; $i < $sizeZ-1; $i++) 
{
    for ($j = $i+1; $j < $sizeZ; $j++) 
    {
        echo $z[$i]." vs ".$z[$j]."<br>";
    }
}
?>

Output :

itema vs itemb
itema vs itemc
itema vs itemd
itema vs iteme
itemb vs itemc
itemb vs itemd
itemb vs iteme
itemc vs itemd
itemc vs iteme
itemd vs iteme

answered Nov 16, 2015 at 18:46

Nana Partykar

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Comments

Julian · Accepted Answer · 2018-07-25 09:01:23Z

In my opinion it's better to use a foreach() then a for().

The approach with the foreach() looks cleaner to me. The approach with the for() is not wrong, but it's strange.

See the example with a foreach:

$z = array('xxApple' => "itema", 'B' => "itemb", 'C' => "itemc", 'D' => "itemd", 'AAAA' => "iteme");
//$z = array("itema", "itemb", "itemc", "itemd", "iteme"); // works also with this array

$x = array_values($z);
$y = array_keys($z); // Optional
foreach($x as $key1 => $item){
  foreach($x as $key2 => $item2){

    if($key2 <= $key1){
        continue;
    }

    if($item == $item2){
        continue;
    }

    echo $item . ' vs ' . $item2 . '<br>';
    // echo $y[$key1] . ' vs ' . $y[$key2] . '<br>'; // Optional
  }
}


//OUTPUT
itema vs itemb
itema vs itemc
itema vs itemd
itema vs iteme
itemb vs itemc
itemb vs itemd
itemb vs iteme
itemc vs itemd
itemc vs iteme
itemd vs iteme

Here are the benchmark results:

Benchmark #1

$z = range(0,20000);

$start = time();
$values = array_values($z);
for ($i = 0; $i < count($values) - 1; $i++) {
    for ($j = $i + 1; $j < count($values); $j++) {
    }
}
$end = time();                  
$elapsedTimeA = $end - $start;

// Case A, elapsed time in seconds: 275
echo 'Case A, elapsed time in seconds: ' . $elapsedTimeA; 


$start = time();
$x = array_values($z);
foreach($x as $key1 => $item){
  foreach($x as $key2 => $item2){

    if($key2 <= $key1){
        continue;
    }

    if($item == $item2){
        continue;
    }
  }
}
$end = time();                   
$elapsedTimeB = $end - $start;

// Case B, elapsed time in seconds: 103
echo 'Case B, elapsed time in seconds: ' . $elapsedTimeB;

Benchmark #2

$z = range(0,20000);

$start = time();
$values = array_values($z);
$count = count($values);
for ($i = 0; $i < $count - 1; $i++) {
    for ($j = $i + 1; $j < $count; $j++) {
    }
}

$end = time();                  
$elapsedTimeA = $end - $start;

// Case A, elapsed time in seconds: 17
echo 'Case A, elapsed time in seconds: ' . $elapsedTimeA;


$start = time();
$x = array_values($z);
foreach($x as $key1 => $item){
  foreach($x as $key2 => $item2){

    if($key2 <= $key1){
        continue;
    }

    if($item == $item2){
        continue;
    }
  }
}
$end = time();                   
$elapsedTimeB = $end - $start;

// Case B, elapsed time in seconds: 103
echo 'Case B, elapsed time in seconds: ' . $elapsedTimeB;

Why the big difference in case A? Also, what hardware is the benchmark running on?

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