Optgroup by custom field

Question

I am currently using Wordpress to create and populate a <select> with <option>s.

<?php if( $lh_loop->have_posts() ): ?>
  <select class="mousechoice left">

    <?php while( $lh_loop->have_posts() ) : $lh_loop->the_post();?>
      <optgroup label="<?php echo get_field("company_name") ?>">
        <option data-id="<?php echo $post->post_name; ?>"><?php the_title(); ?></option>
      </optgroup>
    <?php endwhile; ?>

  </select>
<?php endif; ?>

Which is showing this:

<select class="mousechoice left">

  <optgroup label="SteelSeries">
    <option data-id="sensei">Sensei</option>
  </optgroup>
  <optgroup label="Razer">
    <option data-id="deathadder">Deathadder</option>
  </optgroup>
  <optgroup label="SteelSeries">
    <option data-id="kana">Kana</option>
  </optgroup>

</select>

I would like a way of grouping posts with the same company name into an <optgroup>. Which would result in this:

<select class="mousechoice left">

  <optgroup label="SteelSeries">
    <option data-id="sensei">Sensei</option>
    <option data-id="kana">Kana</option>
  </optgroup>
  <optgroup label="Razer">
    <option data-id="deathadder">Deathadder</option>
  </optgroup>

</select>

I have tried comparing the current post's company to the previous post's company but realised that a post with the same company name could be after or before another post with a different company name.

I know that I could also use if statements to specify each company name but I would like a solution that would allow for other company names not yet input into Wordpress.

Answer 1

If you want to group all the elements of an array by certain values, I like iterating over the array to collate it first and then doing the display in a separate loop.

So I would take each element and put them into an array that has the value I want to group on as a key.

So I want to create an array that looks like

 ['company A' =>[ [1 => 'option 1'],
                  [2 => 'option 2'],
                  ...
                ],
  'company B' =>[ [1 => 'option 3'],
                  [2 => 'option 4'],
                  ...
                 ],
   ...
  ]

To create this array you could use something like (untested)

$grouped = array();
while( $lh_loop->have_posts() ) : 
     $lh_loop->the_post();
     $key = get_field("company_name") 
     $subkey = $post->post_name; 
     $display = the_title();   
     if (empty($grouped[$key]){ //if they key isn't set yet
         //add the key to the array as a new element
         $grouped[] = array($key=>array($subkey=>$title));  
     }else{
         //else add to existing key
         $grouped[$key][] = array($subkey=>$title); 
     }         
endwhile;

Then, to display the list, do something like

foreach ($grouped as $comp => $group){
    echo "<optgroup label=\"$comp\">";
     foreach ($group as $key => $val){
          echo "<option data-id=\"$key\">$val</option>";
     }
     echo "</optgroup>";
}