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## 3.1 About Expressions and Special Variables

The numbers are manipulated by expressions and statements. Since the language was designed to be interactive, statements and expressions are executed as soon as possible. There is no main program. Instead, code is executed as it is encountered. (Functions, discussed in detail later, are defined when encountered.)

A simple expression is just a constant. `bc`

converts constants
into internal decimal numbers using the current input base, specified by
the variable `ibase`. (There is an exception in functions.) The
legal values of `ibase` are 2 through 16. Assigning a value outside
this range to `ibase` will result in a value of 2 or 16. Input
numbers may contain the characters 0-9 and A-F. (Note: They must be
capitals. Lower case letters are variable names.) Single digit numbers
always have the value of the digit regardless of the value of
`ibase`. (i.e. A = 10.) For multi-digit numbers, `bc`

changes all input digits greater or equal to `ibase` to the value of
`ibase`-1. This makes the number `FFF`

always be the largest
3 digit number of the input base.

Full expressions are similar to many other high level languages.
Since there is only one kind of number, there are no rules for mixing
types. Instead, there are rules on the scale of expressions. Every
expression has a scale. This is derived from the scale of original
numbers, the operation performed and in many cases, the value of the
variable `scale`. Legal values of the variable `scale` are
0 to the maximum number representable by a C integer.