Primary Key with auto increment field

I would like to increment one field, depending of the value of the
other. here´s an example:

id variable
1 1
2 1
1 2
2 2

can i do that with the following commands?

SELECT setval('sequence',(SELECT max(id) FROM table)) INTO variable;
insert into table values(variable, ..., ...); ?

Will this be transactional? Cause, they say that setval is a command
that its transactional. Using this way i

don´t will need to use a sequence anymore. Is that correct?

Thanks you all.