Dynamic column name troubles

I have a table that gets a column added to it for every insert on another table.
the general form of these columns are g_c_i_avg where i is the unique
identifyer for the insert that caused the column to be added. I need to use this
column to calculate a new value for another column in the same table on an
update to that column. I can find the altered column, and even build a text
string for it: something like column := ''NEW.g_c_'' || 7 || ''_avg'';

but then i can't use the double precision value for that column. Any help would
be appreciated.

matt