How to update the value of a single field invoking appropriate validation

I'm making a module to allow users to update single fields on in this case, their user entity.

The code below is an example of the method I have initially been using to get it working and test other elements of the module

global $user;
$account = user_load($user->uid);
$edit = (array) $account;
$edit['field_lastname']['und'][0]['value'] = 'test';
user_save($account, $edit);

However this bypasses any field validation defined elsewhere in Drupal. I don't want to reproduce any validation written elsewhere - it's not the Drupal way!

My question is: Is there a function in Drupal 7 that can be called to update the value of a single field. I imagine such a function would clear the appropriate caches, invoke the fields validation etc.

I am aware the solution will be totally different to my current us开发者_运维问答er object based one. I just can't for the life of me find the appropriate function in the API. I wander whether the fact I am looking for a save function alone is the problem - and that there are some other necessary steps that come before.

Any help gratefully appreciated.

Check out the drupal_form_submit function. It lets you submit forms from code. In this case, you could use it to the user edit form, which would then fire the appropriate validation.