Variables

Variables are named repositories where information is stored. A variable can have virtually any size and can contain any of the IDL data types. Variables can be used to store images, spectra, single quantities, names, tables, etc.

Attributes of Variables

Every variable has a number of attributes that can change during the execution of a program or terminal session. Variables have both a structure and a type.

Structure

A variable can contain a single value (a scalar) or a number of values of the same type (an array) or data entities of potentially differing type and size (a structure). Strings are considered as single values, and a string array contains a number of variable-length strings.

In addition, a variable can associate an array structure with a file; these variables are called associated variables. Referencing an associated variable causes data to be read from, or written to, the file. Associated variables are described in "ASSOC" (IDL Reference Guide).

Type

A variable can have one and only one of the following types: undefined, byte, integer, unsigned integer, 32-bit longword, unsigned 32-bit longword, 64-bit integer, unsigned 64-bit integer, floating-point, double-precision floating-point, complex floating-point, double-precision complex floating-point, string, structure, pointer, or object reference.

When a variable appears on the left-hand side of an assignment statement, its attributes are copied from those of the expression on the right-hand side. For example, the statement

ABC = DEF

redefines or initializes the variable ABC with the attributes and value of variable DEF. Attributes previously assigned to the variable are destroyed. Initially, every variable has the single attribute of undefined. Attempts to use the value of an undefined variables result in an error.

Variable Names

IDL variables are named by identifiers. Each identifier must begin with a letter and can contain from 1 to 128 characters. The second and subsequent characters can be letters, digits, the underscore character, or the dollar sign. A variable name cannot contain embedded spaces, because spaces are considered to be delimiters. Characters after the first 128 are ignored. Names are case insensitive. Lowercase letters are converted to uppercase; so the variable name abc is equivalent to the name ABC. The following table illustrates some acceptable and unacceptable variable names.

Table 13-12: Unacceptable and Acceptable IDL Variable Names
Unacceptable	Reason	Acceptable
EOF	Conflicts with function name	A
6A	Does not start with letter	A6
_INIT	Does not start with letter	INIT_STATE
AB@	Illegal character	ABC$DEF
ab cd	Embedded space	My_variable

Tip
Use the IDL_VALIDNAME routine to determine whether a given string is acceptable as an IDL variable name.

Warning
A variable cannot have the same name as a function (either built-in or user-defined) or a reserved word (see "Reserved Words" (IDL Reference Guide)). Giving a variable such a name results in a syntax error or in "hiding" the variable.