PHP for default email value

Hi,

Me again.

I was thinking about the email field for booking a time slot.
Is it possible to predefine the field with the logged in user his email address?
So checking if a user already has an account on a site and using this email address, else leaving it empty.

<?php global $current_user;
get_currentuserinfo();
$current_user->user_email;
?>

or checking with if state ?

function auto_fill_email() {
	if ( is_user_logged_in() ) {
        	$current_user->user_email;
	}
}
add_action('init', 'auto_fill_email');

Just thinking out loud ??
Thanks again!

Kind regards,
Andreas