## 9 Variables

Variables let you give names to values and refer to them later. You have already seen variables in many of the examples. The name of a variable must be a sequence of letters, digits and underscores, but it may not begin with a digit. Octave does not enforce a limit on the length of variable names, but it is seldom useful to have variables with names longer than about 30 characters. The following are all valid variable names

         x
         x15
         __foo_bar_baz__
         fucnrdthsucngtagdjb

However, names like `__foo_bar_baz__` that begin and end with two underscores are understood to be reserved for internal use by Octave. You should not use them in code you write, except to access Octave's documented internal variables and built-in symbolic constants.

Case is significant in variable names. The symbols `a` and `A` are distinct variables.

A variable name is a valid expression by itself. It represents the variable's current value. Variables are given new values with assignment operators and increment operators. See Assignment Expressions.

A number of variables have special built-in meanings. For example, `ans` holds the current working directory, and `pi` names the ratio of the circumference of a circle to its diameter. See Summary of Built-in Variables, for a list of all the predefined variables. Some of these built-in symbols are constants and may not be changed. Others can be used and assigned just like all other variables, but their values are also used or changed automatically by Octave.

Variables in Octave do not have fixed types, so it is possible to first store a numeric value in a variable and then to later use the same name to hold a string value in the same program. Variables may not be used before they have been given a value. Doing so results in an error.