How to get a field value in form validate

Question

i have a custom form validate function. what is the correct/reliable way to get the field value and why? below two lines get the same value from a Select List.

a) $form['field_a']['und']['#value']

b) $form_state['values']['field_a']['und'][0]['value']

Answer 1

Refering to form API Drupal 7 for #value

$form['field_a']['und']['#value']

Used by: button, hidden, image_button, submit, token, value

Description: Used to set values that cannot be edited by the user.

For $form_state['values']['field_a']['und'][0]['value']

$form_state is one of the arguments passed to a form submission handler, or a form validation handler; its main usage is to retrieve the values entered from the user in the form see the content of $form_state['values'])

So you should use $form_state['values']['field_a']['und'][0]['value']