NCL data types overview

Basic numeric types

In NCL, basic data types are all of the standard types found in nearly all programming languages. Types classified as numeric types support all of the algebraic functions available in NCL (see Expressions). The following is a list of the basic numeric types currently supported; they are listed by their keywords, default size for most systems, and valid numerical ranges:

CAUTION: Currently, arithmetic overflow and underflow are not reported as errors to the user. Assigment of out-of-range values may cause errors. 

	double		64bits		+/- ( 2.22507e-308 ) to (8.98846e+307)
	float		32bits		+/- ( 1.175494e-38 ) to (1.701411e+38)
	long		32bits		+/- ( 2.147483e+09 )
	integer		32bits		+/- ( 2.147483e+09 )
	short		16bits		+/- ( 32767 ) 
	byte		8bits		( 0 ) - ( 255 )

Non-numeric types

Non-numeric types are types for which there is no numeric value and that cannot be coerced into a numeric type. Also, in general, non-numeric types only support the .ne. and .eq. operations, with the exception of the string and logical types. Strings use the '+' operation to concatenate one or more strings. The logical type supports .and., .or., .not., and .xor..

The graphic type is a reference to an instance of an HLU object. The file type is a reference to a disk file. The logical type is generated from relational expressions. Logical values are either True or False. Graphic values are returned by the create statement, and file values are returned by the addfile intrinsic function. 

	string		N/A
	character	8bits
	graphic		N/A
	file		N/A
	logical		N/A

Coercion of types

There are many times that types must be converted from one basic data type to another. In many cases, this coercion occurs automatically. For example, when a float value is multiplied by an integer value, the integer value must be converted to a float value before the multiplication can occur. The following table lists the conversion possibilities for all of the basic types. If a coercion to a common type doesn't exist for both sides of an expression, then a type mismatch error is generated. 

	Type 		Coercible to


	character	string


	short 		character
			string
			integer
			long
			logical
			float
			double


	integer		string
			long
			logical
			float
			double


	long 		logical
			string
			float
			double


	float 		logical
			string
			double


	double		logical
			string

A special set of functions exists for forcing the coercion in the reverse direction. For example, the function doubletoint can be used to convert a double precision number into an integer. This operation will cause the decimal portion to be truncated, and information will be lost. See NCL functions and procedures for other similar functions.

Creating data

All numeric data types, strings, characters, and logical and graphic types can be created using the new statements. There are two types of new calls, one that creates the array of data with a specific missing value assigned to each element, and one that creates the array of data using the default missing value. The new statement takes, as parameters, an array of dimension sizes, the type keyword to create, and optionally a missing value to assign to each element of the new data array. The following creates a three-dimensional float array: 
	a = new((/5,6,7/),float)

The following is an example of how to assign a specific missing value. 

	a = new((/5,6,7/),float,-1e12)

File data types are created differently. Files are only created by the intrinsic function addfile. Arrays of the graphic type can be created with new, but none of the elements of the array will actually reference an existing HLU object. Each element will contain a missing value.

In addition to new, there are ways of entering data manually. Currently, there are four types of literal values. Floating point numbers must be entered with a decimal point. The following show creation of data using literal floating point values: 

	a = 2.
	a = 1.2
	a = (/ 1., 2., 3. /)

Integer values are specified without the decimal point: 

	a = 2
	a = (/ 1, 2, 3 /)
String values are specified as characters enclosed in double quotes ("): 
	a = "NCAR Graphics"

NOTE: There is currently no way to specify characters literally.

All other types must be forcibly coerced. See above.

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$Revision: 1.9 $ $Date: 1995/07/26 23:31:16 $