Row Locking

I have a few basic question and either need an affirmation or correction.
Thank you very much for help in advance.

When using Row locks, when does the lock start, which rows does it lock and
when does it end? I was unable to find this info in documentation (maybe
because it is supposed to be implicit). I have the following idea:

BEGIN;
LOCK TABLE A IN ROW EXCLUSIVE MODE;
SELECT c FROM A WHERE id=1; # The row with d=1 is locked
if c then
DELETE FROM A WHEE id=1;
COMMIT; # The row (if remaining) is unlocked

Also, could I do something like this:

BEGIN;
LOCK TABLE A IN ROW EXCLUSIVE MODE;
cursor = SELECT c, id FROM A WHERE d=1;
while cursor.fetchone() # Next row is locked, previous row
if c then # (if remaining) is unlocked
DELETE FROM A WHERE id=id;

COMMIT;

Thanks.