The current volume contains a rather fancy parser of command line arguments, together with a detailed help facility. We have found such a system to be indispensable when running large numbers of simulations, using different codes to generate initial data, evolve the N-body systems, and analyze the results. Therefore, we decided to take the time to develop and introduce the whole system here in detail. An additional advantage is that we can show the power and practical usefulness of Ruby, not only for prototyping the scientific modules in a simulation, but also for helping out with the important task of administering N-body runs.

A good piece of software should administer the runs in such a way as to keep the astrophysicists protected from the details, so that they can give their full attention to the scientific problems at hand. If you are not interested in the details of how this administration is carried out, it would be sufficient to just skim this volume, reading only chapters 1, 2, and 11. However, if you plan to use Ruby extensively for your own simulation needs, in new and creative ways, it probably will pay off to spend some time looking at the other chapters as well, in order to get an idea of the rich pallet of possibilities that Ruby has to offer. It may well give you some new ideas for your own applications.

Besides thanking our home institutes, the Institute for Advanced Study in Princeton and the University of Tokyo, we want to convey our special gratitude to the Yukawa Institute of Theoretical Physics in Kyoto, where this volume was written, during a visit in June 2004, made possible by the kind invitations to both of us by Professor Masao Ninomiya.

We thank Bill Guindon for his comments on the manuscript.

Piet Hut and Jun Makino

Kyoto, July 2004

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