Using Drupal 6 for my examples. This is when in CCK you set Number of values: to more then 1.

Computed Code

//Iterate through array. Output x, x, x, x and x

//set temp variables
$temp_field_x_array_length = count($node->field_x);
$temp_field_x_output = "";
$temp_field_x_counter = 0;

//main loop
for ($temp_field_x_counter = 0; $temp_field_x_counter < $temp_field_x_array_length-2; $temp_field_x_counter+= 1)
{
 $temp_field_x_output .= $node->field_x[$temp_field_x_counter]['value'] . ", ";
}

//second to last value
if ($temp_field_x_counter == $temp_field_x_array_length-2)
{
 $temp_field_x_output .= $node->field_x[$temp_field_x_counter]['value'] . " and ";
 $temp_field_x_counter +=1;
}

//last value
if ($temp_field_x_counter == $temp_field_x_array_length-1)
{
 $temp_field_x_output .= $node->field_x[$temp_field_x_counter]['value'] . "";
 $temp_field_x_counter +=1;
}

$node_field[0]['value'] = $temp_field_x_output;

Display Format

$display = $node_field_item['value'];

If your using the Content Taxonomy field change this

 $temp_field_x_output .= $node->field_x[$temp_field_x_counter]['value'] . 

into this for plain text output

 $temp_field_x_term = taxonomy_get_term($node->field_x[$temp_field_x_counter]['value']);
 $temp_field_x_output .= check_plain($temp_field_x_term->name) . 

or this for linked output

 $temp_field_x_term = taxonomy_get_term($node->field_x[$temp_field_x_counter]['value']);
 $temp_field_x_output .= l($temp_field_x_term->name, taxonomy_term_path($temp_field_x_term), array('rel' => 'tag', 'title' => $temp_field_x_term->description));

Grabbed from line 200 of content_taxonomy.module
Add in $temp_field_x_term = ""; at the top in order to declare your variable before using it. Also be aware that the ""; is missing from above so that you can use find and replace for the code.

Computed Code

//display first and last value only: x - x

//set temp variables
$temp_field_x_array_length = count($node->field_x);
$temp_field_x_output = "";

//zero to one value
if ($temp_field_x_array_length == 1)
{
 $temp_field_x_output = $node->field_x[0]['value'];
}
//over one value
else
{
 $temp_field_x_output .= $node->field_x[0]['value'];
 $temp_field_x_output .=  " - ";
 $temp_field_x_output .=  $node->field_x[$temp_field_x_array_length-1]['value'];
}

$node_field[0]['value'] = $temp_field_x_output;

Display Code

$display = $node_field_item['value'];

And is it also possible the display the values as a select box? I don't think thats possible using the "display format"?
It does state however I can supply my own computed_field_field_FIELDNAME_compute() function. But where do I put that function? And are there any examples for this kind of function?

Newbie here... I'm trying to capture the average of a field's multiple fields (its array of fields). Some research into PHP syntax revealed a seemingly easy way to get the sum of an array (array_sum), so I built the below bit of code, to be used in a Computed field type.

I want to use a multi-fielded field, so I don't have to create fields for each score, or write code to look for empty/null's, which if calculated as "0" would render the average inaccurate. The code is probably quite flawed, but I'm at my limit of understanding PHP (but still working at it). Any help is appreciated.

Computed code:
$average = array_sum($entity->field_multiple) / count($entity->field_multiple);

Display Code:
$display_output = $entity_field_item['$average'];

For D6 the solution is posted here: http://drupal.org/node/879404

To count the number of values in a cck multi value field. In my case the number of images in a multi-value image field. When using the Computed Field module, put this code in the Computed Code field:

if (is_null($node->field_images[0]['fid']))
{
$node_field[0]['value'] = 0;
}
else
{
$node_field[0]['value'] = count($node->field_images);
}

Put this in the Display Format field:

$display = $node_field_item['value'];

But how do I achieve this for D7?

i have problem in multiple values using RESFUL API is that
i have field it has multi values in it, but when i did patch Method to add another value, all the values that in the field before will delete and the new value will add
so i want to know how i can add the new value with the old values without change by using RESTFUL API (POST, GET, PATCH, PUT) and which one from these method i have to use ?
anyone he had this problem before or he did it before?
Thanks

First of all, thank you for this example.
My problem is, that it only works if I add the right number of fields to the node while editing it.
I have a php code that generates an array $values with a number of values that is then added with $delta (just like the example).
When I first save the node it will show one value. After the second saving process without changing anything, it shows the first two values and so on until I saved the node n-1 times (n is the number of values in $values).

Is there any way to set the number of values for a multi-value field explicitly?
I can't do this in the "manage fields" page because the number of values is changing from page to page.

Note: Multi-valued computed fields do not work properly in 2.x (even the Dog-Cat-Mouse example above does not work correctly)!
Multi-valued computed fields are only usable starting with 3.x!