How to merge arrays and add an empty element if key doesn't exist

Question

I have these 3 arrays:

$a1 = array( 'a' => 1, 'b' => 2, 'c' => 3 );
$a2 = array( 'a' => 4, 'b' => 5, 'd' => 6 );
$a3 = array( 'a' => 7, 'c' => 8, 'd' => 9, 'x' => 10 );

I want to merge them so the result looks like this:

Array(
[a] => Array(
        [0] => 1
        [1] => 4
        [2] => 7
    )
[b] => Array(
        [0] => 2
        [1] => 5
        [2] => 
    )
[c] => Array(
        [0] => 3
        [1] => 
        [2] => 8
    )
[d] => Array(
        [0] => 
        [1] => 6
        [2] => 9
    )
[x] => Array(
        [0] => 
        [1] => 
        [2] => 10
    )

... and I will be using the data in a simple table like this:

  col-1 col-2  col-3
a   1     4      7
b   2     5      -
c   3     -      8
d   -     6      9
x   -     -     10

array_merge_recursive is close, but doesn't give me the empty elements, so I believe I need a loop to get the job done. My problem is figuring out which function I need to use in that loop.

I'd be happy just to merge 2 arrays at a time with a custom user function.

I have more than 3 arrays and I need to keep track of which arrays have the "empty" element. — Jeff, Mar 25 '13 at 09:43
Related, but does not tranpose the data structure: [Merge multiple associative arrays and add missing columns with a default value](https://stackoverflow.com/q/25972771/2943403) — mickmackusa, Sep 16 '22 at 08:22

DaveRandom · Answer 1 · 2013-03-25T09:47:46.833

I think the simplest approach to this would be obtain a list of all keys from all input arrays, then iterate the list of keys to construct the result array:

$keys = array_unique(
    array_merge(
        array_keys($a1),
        array_keys($a2),
        array_keys($a3)
    )
);

$result = array();
foreach ($keys as $key) {
    $result[$key] = array(
        isset($a1[$key]) ? $a1[$key] : null,
        isset($a2[$key]) ? $a2[$key] : null,
        isset($a3[$key]) ? $a3[$key] : null
    );
}

print_r($result);

See it working

...or if you want to supply an array of arrays with an arbitrary number of input arrays:

$keys = array();
foreach ($arrs as $arr) {
    $keys = array_merge($keys, array_keys($arr));
}
$keys = array_unique($keys);

$result = array();
foreach ($keys as $key) {
    $result[$key] = array();
    foreach ($arrs as $arr) {
        $result[$key][] = isset($arr[$key]) ? $arr[$key] : null;
    }
}

print_r($result);

See it working

Prasanth Bendra · Answer 2 · 2013-03-25T09:52:04.963

Try this :

<?php
$a1    = array( 'a' => 1, 'b' => 2, 'c' => 3 );
$a2    = array( 'a' => 4, 'b' => 5, 'd' => 6 );
$a3    = array( 'a' => 7, 'c' => 8, 'd' => 9, 'x' => 10 );

$arr   = array($a1,$a2,$a3);
$cnt   = count($arr);

$res   = array();
foreach($arr as $key=>$val){
   foreach($val as $k=>$v){
     $res[$k][$key]  = $v;
   }
}
echo "<table>";
echo "<tr><td>&nbsp;</td><td>col-1</td><td>col-2</td><td>col-3</td></tr>";
foreach($res as $ky=>$vl){
   echo "<tr>";
   echo "<td>".$ky."</td>";
   for($i=0;$i<$cnt;$i++){
      if(array_key_exists($i,$vl)){
         echo "<td>".$vl[$i]."</td>";
      }
      else{
         echo "<td> - </td>";
      }
   }
   echo "</tr>";
}
echo "</table>";
?>

output :

    col-1   col-2   col-3
a   1       4       7
b   2       5       -
c   3       -       8
d   -       6       9
x   -       -       10

score 3 · Accepted Answer · edited May 23 '17 at 11:57

try this

<?php
 $a1 = array( 'a' => 1, 'b' => 2, 'c' => 3 );
 $a2 = array( 'a' => 4, 'b' => 5, 'd' => 6 );
$a3 = array( 'a' => 7, 'c' => 8, 'd' => 9, 'x' => 10 );
  function merge_common_keys(){
$arr = func_get_args();
$num = func_num_args();

$keys = array();
$i = 0;
for($i=0;$i<$num;++$i){
    $keys = array_merge($keys, array_keys($arr[$i]));
}
$keys = array_unique($keys);

$merged = array();

foreach($keys as $key){
    $merged[$key] = array();
    for($i=0;$i<$num;++$i){
        $merged[$key][] = isset($arr[$i][$key])?$arr[$i][$key]:null;
    }
}
  return $merged;
  }

$merged = merge_common_keys($a1,$a2,$a3);

print_r($merged);



$table = "<table>";




$table .= "<tr><td></td><td>col1</td><td>col2</td><td>col3</td></tr>";

foreach($merged as  $key=>$value)
  {
    $row = "<tr><td>$key</td>";
     foreach($value as $subvalue)
 {
   $row .= "<td>$subvalue</td>"; 
 }
 $table .= $row;
}
$table .= "</table>";
echo $table;
?>

ref : PHP, Merging arrays with common keys

see live demo http://codepad.viper-7.com/v8Ol5H

score 1 · Answer 4 · answered Mar 25 '13 at 09:42

1

You need to create something similar like array_merge_recursive().

But there might be a simpler solution, you say:

but doesn't give me the empty elements,

But you do know they don't exist!

So if you know the maximum amount of columns (amount of arrays) you need and you loop then you can always do:

if(isset($row[$column])) {
   echo $row[$column];//your column
} else {
   echo 0;
}

answered Mar 25 '13 at 09:42

Timmetje

7,641
18
36

I need to know which array is producing the empty element. – Jeff Mar 25 '13 at 09:44
Ah then take one of the above ;) And create your own merging function. Good luck! – Timmetje Mar 25 '13 at 09:52

score 1 · Answer 5 · answered Mar 25 '13 at 09:43

function myMergeFunction() {

  $my_new_array = array();

  $arg_list = func_get_args();
  for ($i = 0; $i < count($arg_list); $i++) {

    foreach($arg_list[$i] as $key => $value) {
      if(!is_array($my_new_array[$key]))
        $my_new_array[$key] = array();

      $my_new_array[$key][] = $value

    }

  }

  return $my_new_array;

}

you can call it like:

myMergeFunction($a, $b, $c, $d, [...]);

score 1 · Answer 6 · answered Mar 25 '13 at 11:39

→ Had already started on this before there where any answers, decided to finish it even though there are already acceptable answers. This version is more dynamic though and has some customization for the table's styling:

→ CODE:

/* Arrays from Question */  $a1 = array( 'a' => 1, 'b' => 2, 'c' => 3 ); $a2 = array( 'a' => 4, 'b' => 5, 'd' => 6 ); $a3 = array( 'a' => 7, 'c' => 8, 'd' => 9, 'x' => 10 );
/* Process Arrays       */  $result = handle_arrays( $a1, $a2, $a3 );
/* Create & Echo Table  */  echo ( array_table( $result ) );

function handle_arrays() {
    /* Get Arrays    */  $params = func_get_args(); $result = array();
    /* Merge Arrays  */  foreach( $params as $k=>$v ) { foreach( $v as $key=>$val ) { if( !array_key_exists( $key, $result ) ) $result[ $key ] = array(); $result[ $key ][ $k ] = $val; } }
    /* Handle Blanks */  for( $x = 0; $x < func_num_args(); $x++ ) { foreach( $result as $key=>$val ) { if( !array_key_exists( $x, $val ) ) { $result[ $key ][ $x ] = '-'; } ksort( $result[ $key ] ); } }
    /* Return Array  */  $return = array( func_num_args(), $result );
    return( $return );
}
function array_table( $array ) {
    /* Array # & Result Array */  $n = $array[ 0 ]; $a = $array[ 1 ];
    /* Style & Class Control  */  $padding_left = 10; $padding_right = 10; $col_align = "center"; $key_align = "left"; $table_class = ""; $col_class = ""; $row_class = ""; $field_class = "";
    /* Table & Column Headers */  $html = "<table class='{$table_class}'><tr><td style='padding-right:{$padding_right}px'></td>"; for( $x=0; $x<$n; $x++ ) { $html .= "<td style='padding-left:{$padding_left}px;padding-right:{$padding_right}px' class='{$col_class}'>col-$x</td>"; } $html .= "</tr>";
    /* Row and Fields Values  */  foreach( $a as $key=>$val ) { $html .= "<tr><td align='{$key_align}' class='{$row_class}'>$key</td>"; foreach( $val as $k=>$v ) { $html .= "<td align='{$col_align}' class='{$field_class}'>$v</td>"; } $html .= "</tr>"; } $html .= "</table>";
    return( $html );
}

→ OUTPUT:

    col-0   col-1   col-2
a     1       4       7
b     2       5       -
c     3       -       8
d     -       6       9
x     -       -       10

How to merge arrays and add an empty element if key doesn't exist

6 Answers6