Using Variables
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## Using Variables

Syntax:

    VariableName

Example:

    genius> e
    = 2.71828182846

To evaluate a variable by itself, just enter the name of the variable. This will return the value of the variable. You can use a variable anywhere you would normally use a number or string. In addition, variables are necessary when defining functions that take arguments (see the section called “Defining Functions”).

### Using Tab completion

You can use Tab completion to get Genius to complete variable names for you. Try typing the first few letters of the name and pressing `Tab`.

### Variable names are case sensitive

The names of variables are case sensitive. That means that variables named `hello`, `HELLO` and `Hello` are all different variables.

### Setting Variables

Syntax:

    <identifier> = <value>
    <identifier> := <value>

Example:

    x = 3
    x := 3

To assign a value to a variable, use the `=` or `:=` operators. These operators set the value of the variable and return the value you set, so you can do things like

    a = b = 5

This will set `b` to 5 and then also set `a` to 5.

The `=` and `:=` operators can both be used to set variables. The difference between them is that the `:=` operator always acts as an assignment operator, whereas the `=` operator may be interpreted as testing for equality when used in a context where a Boolean expression is expected.

For more information about the scope of variables, that is when are what variables visible, see the section called “Global Variables and Scope of Variables”.

### Built-in Variables

GEL has a number of built-in ‘variables’, such as `e`, `pi` or `GoldenRatio`. These are widely used constants with a preset value, and they cannot be assigned new values. There are a number of other built-in variables. See the section called “Constants” for a full list. Note that `i` is not by default the square root of negative one (the imaginary number), and is undefined to allow its use as a counter. If you wish to write the imaginary number you need to use `1i`.

### Previous Result Variable

The `Ans` and `ans` variables can be used to get the result of the last expression. For example, if you had performed some calculation, to add 389 to the result you could do:

    Ans+389

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