(eric harnden - 920917.0930)
answers to mr powers' stella questions:
1) variables can be plotted with respect to time, plotted with respect
to each other (comparitive mode, they call this), and tabulated.
the newest version allows time plots to be constrained as to their
time window. the version i have plots the entire time sequence, always.
the new version also places time in a column of the table, which my
current one does not. tables can be saved and loaded into spreadsheets
with a minimum of fuss. plots are automatically scaled on the vertical
axis, although this can be overridden.
2) realtime input does not appear to be possible. the newest version does
allow one to halt the simulation, change values, and resume, for gaming
uses. but no joysticks or mouse tracking kind of stuff.
3) data for table functions must be entered by hand, as far as i can tell.
this annoys me, since a large national model might require a fairly large
database. this data can either be typed in, or drawn in on a graph. one of
the tenets of system dynamics is that really precise input data doesn't
affect the dynamics of the system much. certain cusps may take longer
to evolve, threshold may rise or fall a little, but the theory is that it's
the structure that defines the behavior, much more than the exact values
of the parameters. hence, graphic input is deemd appropriate, since getting the
shape of the function is considered the critical part. i will withold
comment.
4) no user functions. there are alot of really useful math and other
functions built in, and again, output can be saved and analyzed with
other tools. but you can't link to exotic homebrew tools. that's one
of the reasons i like dynamo. it links quite easily to C routines.
(one can also call on external data files).
5) it'll run on anything se30 or above. although it is a bit of a
pig. smaller machines should not be running much of anyhting in the
way of inits or such, or stella will run out of memory quickly. but that's
really the only problem. it's fast enough, even on the small machines,
and sims are cheap. you can put in your 4meg and not worry about it.
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Eric Harnden (Ronin)
<HARNDEN@AUVM.BITNET> or <HARNDEN@AMERICAN.EDU>
The American University Physics Dept.
4400 Mass. Ave. NW, Washington, DC, 20016-8058
(202) 885-2748
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