Variable Attributes

All IDL variables have special attributes that return information about the variable. These attributes are equivalent to calling the N_ELEMENTS, SIZE, or TYPENAME functions.

The following variable attributes are available:

Attribute	Definition
DIM	An array giving the dimensions (0 for scalars).
LENGTH	The number of elements in the variable.
NDIM	An integer giving the number of dimensions (0 for scalars).
TNAME	A string giving the raw IDL type name. For structures this returns "STRUCT", while for objects this returns "OBJREF".
TYPECODE	An integer giving the IDL type code.
TYPENAME	A string giving the IDL type name. For structures and objects this returns the actual structure or class name.
TYPESIZE	An integer giving the size in bytes of a single element (always 0 for strings and structures).

Note: For structures and objects, if the structure or object contains a field with the same name as an attribute, then IDL will correctly retrieve the structure field and you will not be able to access the attribute.

Note: For most variables, the TNAME and TYPENAME attributes will return the same result. For structures and objects the TYPENAME will return the actual structure or class name, just like the TYPENAME function.

Examples

Scalars

First print the attributes for a scalar variable:

var = 3.14d

print, var.length

print, var.typename

print, var.typecode

print, var.ndim

print, var.dim

print, var.typesize

IDL prints:

DOUBLE

Arrays

Next, print the attributes for an array:

var = RANDOMU(seed, 100, 50, 3)

print, var.length

print, var.typename

print, var.typecode

print, var.ndim

print, var.dim

print, var.typesize

IDL prints:

FLOAT

100                    50                     3

Structure Fields

The attributes can also be used on fields within structures. Here we use the LENGTH attribute as the loop limit:

var = {mystruct, field1: {substruct, subfield: LONARR(1000)}}

for i=0, var.field1.subfield.LENGTH-1 do var.field1.subfield[i]=i

; Use our static TOTAL method

PRINT, var.field1.subfield.total()

IDL prints:

499500.

Objects

For objects that inherit from IDL_Object, the LENGTH, NDIM, and DIM attributes will be taken from the overloaded ::Size method. For example:

h = HASH('key1', 4, 'key2', 5, 'key3', 6)

PRINT, h.length, h.ndim, h.dim, h.typecode, h.tname, h.typename

IDL prints:

OBJREF

HASH

Notice that the TNAME and TYPENAME attributes are different for objects.

Typesize

The TYPESIZE attribute may be useful when performing unformatted input/output, or converting variables to/from bytes. For example:

IDL> x = [!values.f_infinity, !values.f_nan]

IDL> y = byte(x, 0, x.length * x.typesize)

IDL> print, y

0   0 128 127   0   0 192 127

IDL> print, float(y, 0, y.length / x.typesize)

   Inf            NaN

Note: The TYPESIZE attribute is always 0 for strings and structures because these data types have varying lengths depending upon the stored data.

Version History

8.4	Introduced
9.0	Allow array indexing for the DIM static attribute. Added TYPESIZE variable attribute.