Dynamic Lists, or...?

[Lorn]

I'm trying to figure out a way to create dynamic lists or possibly
antother solution for the following problem. I have multiple lines in a
text file (every line is the same format) that are iterated over and
which need to be compared to previous lines in the file in order to
perform some simple math. Each line contains 3 fileds: a descriptor and
two integers. Here is an example:

rose, 1, 500
lilac, 1, 300
lilly, 1, 400
rose, 0, 100

The idea is that the 0/1 values are there to let the program know
wether to add or subtract the second integer value for a specific
descriptor (flower in this case). So the program comes upon rose, adds
the 500 to an empty list, waits for the next appearance of the rose
descriptor and then (in this case) subtracts 100 from 500 and prints
the value. If the next rose was a 1 then it would have added 100.

I'm uncertain on how to approach doing this though. My idea was to
somehow be able to create lists dynamically upon each new occurence of
a descriptor that currently has no list and then perform the
calculations from there. Unfortunately, the list of descriptors is
potentially infinte, so I'm unable to previously create lists with the
descriptor names. Could anyonw give any suggestions on how to best
approach this problem, hopefully I've been clear enough? Any help would
be very gratly appreciated.

Best regards,
Lorn