Informats under OpenVMS

RBw.d Informat: OpenVMS

Reads real (floating-point) binary values.

Category:	numeric
Width range:	2 to 8
Default width:	4
Decimal range:	0 to 10
OpenVMS specifics:	native floating-point representation
See:	RBw.d Informat in SAS Language Reference: Dictionary

Syntax
Details
Examples
	Example 1: Single- versus Double-Precision Representation
	Example 2: Writing External Data
See Also

Syntax

RBw.d

w: specifies the width of the input field.
d: specifies the power of 10 by which to divide the input value. This argument is optional. If you specify d, the RBw.d informat divides the input value by the 10^d value. SAS uses the d value even if the input data contains decimal points.

Details

The RBw.d informat reads numeric data that is stored in native real-binary (floating-point) notation. Numeric data for scientific calculations is often stored in floating-point notation. (SAS stores all numeric values in floating-point notation.) A floating-point value consists of two parts: a mantissa that gives the value and an exponent that gives the value's magnitude.

It is usually impossible to key in floating-point binary data directly from a terminal, but many programs write floating-point binary data. Use caution if you are using the RBw.d informat to read floating-point data created by programs other than SAS because the RBw.d informat is designed to read-only double-precision data.

Because the RBw.d informat is designed to read-only double-precision data, it supports widths of less than 8 bytes only for those applications that truncate numeric data for space-saving purposes. RB4., for example, does not expect a single-precision number that is truncated to 4 bytes.

External programs such as those written in C and FORTRAN can produce only single- or double-precision floating-point numbers. No length other than 4 or 8 bytes is allowed. The RBw.d informat allows a length of 3 through 8 bytes, depending on the storage you need to save.

The FLOAT4. informat has been created to read a single-precision, floating-point number. If you read the hexadecimal notation 3F800000 with FLOAT4., the result is a value of 1.

To read data created by a C or FORTRAN program, you need to decide on the proper informat to use. If the floating-point numbers require an 8-byte width, you should use the RB8. informat. If the floating-point numbers require a 4-byte width, you should use FLOAT4.

For more information about OpenVMS floating-point representation, see HP OpenVMS Programming Concepts Manual, Volume II, Part I, OpenVMS Programming Interfaces: Calling a System Routine.

Examples

Example 1: Single- versus Double-Precision Representation

Consider how the value of 1 is represented in single-precision and double-precision notation. For single-precision, the hexadecimal representation of the 4 bytes of data is 3F800000 . For double-precision, the hexadecimal representation is 3FF0000000000000 . The digits at the beginning of the data are different, indicating a different method of storing the data.

Example 2: Writing External Data

Consider this C example:

#include <stdio.h>

main() {

FILE *fp;
float x[3];

fp = fopen("test.dat","wb");
x[0] = 1; x[1] = 2; x[2] = 3;

fwrite((char *)x,sizeof(float),3,fp);
fclose(fp);
}

The file TEST.DAT will contain, in hexadecimal notation, 3f8000004000000040400000 .