IDL Analyst Reference Guide: Math and Statistics Utilities

IMSL_MACHINE

The IMSL_MACHINE function returns information describing the computer's arithmetic.

Syntax

Result = IMSL_MACHINE( [, /DOUBLE] [, /FLOAT] )

Return Value

The information describing the computer's arithmetic is returned in a structure.

Keywords

DOUBLE

If present and nonzero, a structure containing the information describing the single-precision, floating-point arithmetic is returned.

FLOAT

If present and nonzero, a structure containing the information describing the single-precision, floating-point arithmetic is returned.

Discussion

The IMSL_MACHINE function returns information describing the computer's arithmetic. This can be used to make programs machine independent. The information returned by IMSL_MACHINE is in the form of a structure. A different structure is used for each type: integer, float, and double. Depending on how IMSL_MACHINE is called, a different structure is returned.

The default action of IMSL_MACHINE is to return the structure IMACHINE which contains integer information on the computer's arithmetic. By using either the keywords Float or Double, information about the floating- or double-precision arithmetic is returned in structures FMACHINE or DMACHINE.

The contents of the these structures are described below.

Integer Information: IMACHINE

Assume that integers are represented in M-digit, base A form as:

where s is the sign and 0 £ x_k < A for k = 0, ..., M. Then, Table 25-4 describes the tags:

Table 25-4: Integer Tags

Table 25-4: Integer Tags

Tag	Definition
BITS_PER_CHAR	C, bits per character
INTEGER_BASE	A, the base
INTEGER_DIGITS	Ms, the number of base-A digits in a short int
MAX_INTEGER	, the largest short int
LONG_DIGITS	Ml, the number of base-A digits in a long int
MAX_LONG	, the largest long int

Assume that floating-point numbers are in N-digit, base B form as:

where s is the sign and 0 £ x_k < B for k = 1, ..., N for and E_min £ E £ E_max. Then, Table 25-5 describes the tags:

Table 25-5: Floating Point Tags 

Table 25-5: Floating Point Tags 

Tag	Definition
FLOAT_BASE	B, the base
FLOAT_DIGITS	Nf, the number of base-B digits in float
FLOAT_MIN_EXP	, the smallest float exponent
FLOAT_MAX_EXP	, the largest float exponent
DOUBLE_DIGETS	Nd, the number of base-B digits in double
DOUBLE_MIN_EXP	, the largest long int
DOUBLE_MAX_EXP	, the number of base-B digits in double

Floating- and Double-precision Information: FMACHINE and DMACHINE

Information concerning the floating- or double-precision arithmetic of the computer is contained in the structures FMACHINE and DMACHINE. These structures are returned into named variables by calling IMSL_MACHINE with the keywords Float for FMACHINE and Double for DMACHINE.

Assume that float numbers are represented in N_f- digit, base B form as:

where s is the sign, 0 £ x_k < B for k = 1, 2, ..., N_f  and

Note that if we make the assignment imach = IMSL_MACHINE( ), then B = imach.FLOAT_BASE, N_f = imach.FLOAT_DIGITS,

and:

The ANSI/IEEE 754-1985 standard for binary arithmetic uses NaN (Not a Number) as the result of various otherwise illegal operations, such as computing 0/0. If the assignment amach = IMSL_MACHINE(/Float) is made, then on computers that do not support NaN, a value larger than amach. MAX_POS is returned in amach.NAN. On computers that do not have a special representation for infinity, amach.POS_INF contains the same value as amach.MAX_POS.

The structure IMACHINE is defined by Table 25-6:

The structure DMACHINE contains machine constants that define the computer's double arithmetic. Note that for double, if the assignment imach = IMSL_MACHINE( ) is made, then:

B = imach.FLOAT_BASE, N_f = imach.DOUBLE_DIGITS

and:

Missing values in IDL Analyst procedures and functions are often indicated by NaN. There is no missing-value indicator for integers. Users usually have to convert from their missing value indicators to NaN.

Example

In this example, all values returned by IMSL_MACHINE are printed on a machine with IEEE (Institute for Electrical and Electronics Engineering) arithmetic.

i = IMSL_MACHINE()  
f = IMSL_MACHINE(/FLOAT)  
d = IMSL_MACHINE(/DOUBLE)  
; Call HELP with the keyword STRUCTURE set to view the contents   
; of the structures.  
HELP, i, f, d, /STRUCTURE  
  
** Structure IMACHINE, 13 tags, length=52:  
    BITS_PER_CHAR 	LONG 	            8  
    INTEGER_BASE 	LONG	             2  
    INTEGER_DIGITS 	LONG 	            15  
    MAX_INTEGER 	LONG	         32767  
    LONG_DIGITS 	LONG	            31  
    MAX_LONG	LONG	    2147483647  
    FLOAT_BASE 	LONG	             2  
    FLOAT_DIGITS 	LONG	            24  
    FLOAT_MIN_EXP 	LONG 	          -125  
    FLOAT_MAX_EXP 	LONG	           128  
    DOUBLE_DIGITS 	LONG	            53  
    DOUBLE_MIN_EXP 	LONG	         -1021  
    DOUBLE_MAX_EXP 	LONG 	          1024  
** Structure FMACHINE, 8 tags, length=32:  
    MIN_POS 	FLOAT	   1.17549e-38  
    MAX_POS 	FLOAT 	   3.40282e+38  
    MIN_REL_SPACE 	FLOAT	   5.96046e-08  
    MAX_REL_SPACE 	FLOAT 	   1.19209e-07  
    LOG_10 	FLOAT 	      0.301030  
    NAN 	FLOAT 	           NaN  
    POS_INF 	FLOAT	           Inf  
    NEG_INF 	FLOAT	          -Inf  
** Structure DMACHINE, 8 tags, length=64:  
    MIN_POS 	DOUBLE 	2.2250739e-308  
    MAX_POS 	DOUBLE 	1.7976931e+308  
    MIN_REL_SPACE 	DOUBLE	 1.1102230e-16  
    MAX_REL_SPACE 	DOUBLE	 2.2204460e-16  
    LOG_10 	DOUBLE 	    0.30102998  
    NAN 	DOUBLE 	           NaN  
    POS_INF 	DOUBLE	      Infinity  
    NEG_INF 	DOUBLE 	     -Infinity