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## 18 Programming

There are several ways to “program” the Emacs Calculator, depending on the nature of the problem you need to solve.

1. Keyboard macros allow you to record a sequence of keystrokes and play them back at a later time. This is just the standard Emacs keyboard macro mechanism, dressed up with a few more features such as loops and conditionals.
2. Algebraic definitions allow you to use any formula to define a new function. This function can then be used in algebraic formulas or as an interactive command.
3. Rewrite rules are discussed in the section on algebra commands. See Rewrite Rules. If you put your rewrite rules in the variable `EvalRules`, they will be applied automatically to all Calc results in just the same way as an internal “rule” is applied to evaluate ‘`sqrt(9)`’ to 3 and so on. See Automatic Rewrites.
4. Lisp is the programming language that Calc (and most of Emacs) is written in. If the above techniques aren't powerful enough, you can write Lisp functions to do anything that built-in Calc commands can do. Lisp code is also somewhat faster than keyboard macros or rewrite rules.

Programming features are available through the `z` and `Z` prefix keys. New commands that you define are two-key sequences beginning with `z`. Commands for managing these definitions use the shift-`Z` prefix. (The `Z T` (`calc-timing`) command is described elsewhere; see Troubleshooting Commands. The `Z C` (`calc-user-define-composition`) command is also described elsewhere; see User-Defined Compositions.)