How to get the value of an item from a list field to add it to a computed field
Definitely I found out what to do.
Maybe this will help somebody else.


I wrote this before :


$entity_field[0]["value"] = field_get_items($entity_type, $entity, "field_nb_of_tickets");


This is what's new :


I did put the foreign field's value in a variable before adding it to the current field :

$nb_of_tickets= field_get_items($entity_type, $entity, "field_nb_of_tickets");

$entity_field[0]["value"] = $entity_field[0]["value"] + $nb_of_tickets;

Now it works.