Copy xprofile field data to usermeta -Success ! but one little php tweak needed

Found this code that copies (on registration) profile field data to usermeta fields at https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/xprofile-vs-usermeta-syncing-problems/

function synchro_wp_usermeta($user_id, $password, $meta) {
	global $bp, $wpdb;

	$uid = get_userdata($user_id);
	$email = $uid->user_email;

	$fullname = $meta[field_1]; 

... whatever you need here ....

	update_usermeta( $user_id, 'nickname', $fullname );
	update_usermeta( $user_id, 'first_name', $firstname );
	update_usermeta( $user_id, 'last_name', $lastname );

$wpdb->query( $wpdb->prepare( "UPDATE {$wpdb->users} SET display_name = %s WHERE ID = %d", $fullname, $user_id ) );
$wpdb->query( $wpdb->prepare( "UPDATE {$wpdb->users} SET user_url = %s WHERE ID = %d", bp_core_get_user_domain( $user_id ), $user_id ) );
}
add_action( 'wpmu_activate_user', 'synchro_wp_usermeta', 10, 3);

I have tested it and it works.

In the line above
$fullname = $meta[field_1];

field_1 refers to the id number of the field in wp_XX_bp_xprofile_fields (where XX is the site id number).

I want to use the field name here, not the number.

what do I change $fullname = $meta[field_1]; to use the field name ? Please.