Edit custom field using user_presave hook during account creation

Question

I have made a custom field for user field_custom_xyz from the admin GUI. Now after i saved it i got two tables into the database : one for data and one for revisions.
I already have a huge database into which a lot of users are already there.

I have two questions :

1. How can i make sure that i can assign default value to each user at the time of creation of this field?
2. Let's say if i have to assign a default value of 0 (or null in case i don't follow step 1).
   how can i change that value at the time of new registration using hook_user_presave ?

I want to override the default value of this field using user_presave hook in some custom module lets say 'abc' module

so far i have tried :

$account->field_custom_abc['und'][0]['value'] = 0;

but that doesn't work, any suggestions. thanks in advance.

Answer 1

Try this out:

/**
 * Implements hook_user_presave().
 */
function mymodule_user_presave(&$edit, $account, $category) {
  $edit['field_custom_xyz'][LANGUAGE_NONE][0]['value'] = 0;
}