Update table using array

Hi,

I want to update a column matching all elements of array. But,I don't
want to iterate over all elements of array.
Basically, I am writing a procedure for

update tx set delete_flag=''true'' where tx_id in (10,20,);

Something like below procedure(which is not currect)

CREATE OR REPLACE FUNCTION t1(bigint[]) RETURNS text AS '
DECLARE
cnt INTEGER;
ret text;BEGIN
update tx set delete_flag=''true'' where tx_id in ($1);
return ret;
end;
'
language 'plpgsql';

Is there anyway to update the table for all elements of array in one go?

- Abhinav