evaluating values of attributes from computed attribute names in trigger function

Hi,

I need to find all changed fields in a trigger function. I found out how
to get the names of all attributes within plpgsql, but I cannot get the
value of NEW and OLD for the attribute with that name.

That is:

assume there is NEW.foo and OLD.foo

let the name 'foo' be computed and stored in variable attrname:
attrname='foo'

Then, if I write NEW.attrname, it is confused with an attribute called
'attrname'.

So how can I get the changed fields in my row?
Please see my function below.

Thank you very much,

Markus

CREATE FUNCTION trg_001() RETURNS OPAQUE AS
'
DECLARE
rec RECORD;

BEGIN
FOR rec IN SELECT a.attname AS atr FROM pg_attribute
a,pg_class c WHERE a.attrelid = c.oid AND a.attnum > 0 AND
c.relname=TG_RELNAME LOOP
IF NEW.atr <> OLD.atr THEN
INSERT INTO test (txt) VALUES (rec.atr);
END IF;
END LOOP;
RETURN NEW;
END;
'
LANGUAGE 'plpgsql';