Editing Computer simulation (section)

== Data preparation ==
The external data requirements of simulations and models vary widely. For some, the input might be just a few numbers (for example, simulation of a waveform of AC electricity on a wire), while others might require terabytes of information (such as weather and climate models).

Input sources also vary widely:
* Sensors and other physical devices connected to the model;
* Control surfaces used to direct the progress of the simulation in some way;
* Current or historical data entered by hand;
* Values extracted as a by-product from other processes;
* Values output for the purpose by other simulations, models, or processes.

Lastly, the time at which data is available varies:
* "invariant" data is often built into the model code, either because the value is truly invariant (e.g., the value of π) or because the designers consider the value to be invariant for all cases of interest;
* data can be entered into the simulation when it starts up, for example by reading one or more files, or by reading data from a [[preprocessor (CAE)|preprocessor]];
* data can be provided during the simulation run, for example by a sensor network.

Because of this variety, and because diverse simulation systems have many common elements, there are a large number of specialized [[simulation language]]s. The best-known may be [[Simula]]. There are now many others.

Systems that accept data from external sources must be very careful in knowing what they are receiving. While it is easy for computers to read in values from text or binary files, what is much harder is knowing what the [[accuracy]] (compared to [[Graphic display resolutions|measurement resolution]] and [[Accuracy and precision|precision]]) of the values are. Often they are expressed as "error bars", a minimum and maximum deviation from the value range within which the true value (is expected to) lie. Because digital computer mathematics is not perfect, rounding and truncation errors multiply this error, so it is useful to perform an "error analysis"<ref name=Taylor>{{cite book |title=An Introduction to Error Analysis: The Study of Uncertainties in Physical Measurements |author=John Robert Taylor |url=https://books.google.com/books?id=giFQcZub80oC&pg=PA128 |pages=128–129 |isbn=978-0-935702-75-0 |year=1999 |publisher=University Science Books |url-status=live |archive-url=https://web.archive.org/web/20150316103343/http://books.google.com/books?id=giFQcZub80oC&pg=PA128 |archive-date=2015-03-16 }}</ref> to confirm that values output by the simulation will still be usefully accurate.