LPM: "on break" processing in perl?

[David Hempy]

I keep running into the situation where I am reading rows from a database, 
presenting the rows in groups and subgroups.  For example:

select coursenumber, sectionnumber, ssnum
	from studentview

This might return:

CS101, 1, 000-00-1111
CS101, 1, 000-00-2222
CS101, 1, 000-00-3333
CS101, 2, 000-00-4444
CS101, 2, 000-00-5555
CS117, 1, 000-00-1111
CS117, 1, 000-00-9999


I would like to display something like:

CS101: 7 students
	Section 1: 3 students
		000-00-1111
		000-00-2222
		000-00-3333
	Section 2: 2 students
		000-00-4444
		000-00-5555
CS117: 2 students
	Section 1: 2 students
		000-00-1111
		000-00-9999


(Actually, I am usually plugging these into HTML tables, but the idea is 
the same.)

I would like to do something like:

while fetch {
	on break in $coursenumber
		print count($coursenumber);
	on break in $sectionnumber
		print "\t" . count($sectionnumber);

	print "\t\t $ssnum";
}


It's not so hard to generate the breaks by setting $last_sectionnumber each 
time after comparing the current $sectionnumber to the 
$last_sectionnumber.  That would suffice if I just wanted a <hr> or similar 
between each group.  It becomes more problematic when I want to 
telepathically print the subtotals before I've stepped through the data.

I've gotten around this in the past by building up little chunks of the 
output in different scalars, then outputting all the little pieces in the 
right order when the group changes.  I then have to repeat this break 
processing again after the loop exits, assuming that any were found at 
all.  Ugh.  Double that for two-level grouping.  Double ugh.

I have in some programs saved the break lines with "~SUBTOTAL~" in it, then 
do a s/~SUBTOTAL~/$subtotal/ later.  Effective, but embarassing.


Anyone have any tips or methods to share?  I have a suspicion I should 
separate analysis and presentation into two passes, with some combination 
of arrays/hashes between the two.  I think I'll cipher on that for a while...


-dave

-- 
David Hempy
Internet Database Administrator
Kentucky Educational Television
<hempy at ket.org> -- (859)258-7164 -- (800)333-9764