Use subquery variable inside to_date - postgresql

Question

I need to update my date with a subquery like this, i aint got a day number .
UPDATE grh
SET date = to_date('subquery.year-subquery.week_number','YYYYWW')
FROM (SELECT year,week_number
FROM grh) AS subquery
how could i use the subquery.year variable inside the to_date fonction ?
TRY 1 : So I've tried this :
UPDATE grh SET date = to_date(week_number, 'WW');
And i have this kind of error :
function to_date(integer,unknown) doesnt exists.
but if you look at this doc : https://www.techonthenet.com/postgresql/functions/to_date.php
They say that WW exists, for specifiing a week number. My 'date' column is a date format .
TRY 2 : this is working :
UPDATE grh SET date = to_date('42018', 'WWYYYY');
As soon as I try to use a variable like that, it doesnt work :
UPDATE grh SET date = to_date(string_agg(week_number,2018), 'WWYYYY');

Answer 1

In Postgres( and most other SQL flavors ), the concatenation operator is ||
UPDATE grh SET date = to_date(week_number||'2018', 'WWYYYY');

Answer 2

The linked documentation on to_date function :
to_date( string1, format_mask )
and the error is
function to_date(integer,unknown) doesnt exists.
so i think that you have a mismatch on the type of your first parameter
Edit1:
for the statement
UPDATE grh SET date = to_date(string_agg(week_number,2018), 'WWYYYY');
if you chekc the documentation of the function string_agg you will see the expected type of input params are string and at least the second one is a integer