Variables
=========

A "variable" is somewhere between a storage register on a conventional
calculator, and a variable in a programming language.  (In fact, a
Calc variable is really just an Emacs Lisp variable that contains a
Calc number or formula.)  A variable's name is normally composed of
letters and digits.  Calc also allows apostrophes and `#' signs in
variable names.  The Calc variable `foo' corresponds to the Emacs Lisp
variable `var-foo'.  Commands like `s s' (`calc-store') that operate
on variables can be made to use any arbitrary Lisp variable simply by
backspacing over the `var-' prefix in the minibuffer.

In a command that takes a variable name, you can either type the full
name of a variable, or type a single digit to use one of the special
convenience variables `var-q0' through `var-q9'.  For example, `3 s s
2' stores the number 3 in variable `var-q2', and `3 s s foo RET'
stores that number in variable `var-foo'.

To push a variable itself (as opposed to the variable's value) on the
stack, enter its name as an algebraic expression using the apostrophe
(') key.  Variable names in algebraic formulas implicitly have `var-'
prefixed to their names.  The `#' character in variable names used in
algebraic formulas corresponds to a dash `-' in the Lisp variable
name.  If the name contains any dashes, the prefix `var-' is *not*
automatically added.  Thus the two formulas `foo + 1' and `var#foo +
1' both refer to the same variable.

The `=' (`calc-evaluate') key "evaluates" a formula by replacing all
variables in the formula which have been given values by a
`calc-store' or `calc-let' command by their stored values.  Other
variables are left alone.  Thus a variable that has not been stored
acts like an abstract variable in algebra; a variable that has been
stored acts more like a register in a traditional calculator.  With a
positive numeric prefix argument, `=' evaluates the top N stack
entries; with a negative argument, `=' evaluates the Nth stack entry.

A few variables are called "special constants".  Their names are `e',
`pi', `i', `phi', and `gamma'.  (See Scientific Functions.)  When
they are evaluated with `=', their values are calculated if necessary
according to the current precision or complex polar mode.  If you wish
to use these symbols for other purposes, simply undefine or redefine
them using `calc-store'.

The variables `inf', `uinf', and `nan' stand for infinite or
indeterminate values.  It's best not to use them as regular variables,
since Calc uses special algebraic rules when it manipulates them.
Calc displays a warning message if you store a value into any of these
special variables.

See Store and Recall, for a discussion of commands dealing with variables.