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Status Not under consideration
Workspace PL/I Compilers
Created by Guest
Created on May 12, 2017

RELATED IDEAS

Correct Picture Representation of Floating Point Numbers

When using picture specification to display a floating point number field, (for example P'(15)S9V.(4)9' output for a FLOAT DEC(16) variable), the output is not always accurate. The decimal portion is frequently wrong. For example, a number ending in ".57" might be reported as ".5699" We have to convert the floating point variables to a fixed decimal variable, then display the fixed decimal number to get an accurate report. We'd prefer not to have to do this.

FD: PROCEDURE;

DECLARE
BUFFER CHAR(80) VARYING,
FLTFLD1 FLOAT DEC(16) INIT(15000.57),
FLT2PCKA ENTRY(*,*,FIXED BIN(31), FIXED BIN(31))
OPTIONS(ASM INTER),
PCKNUM194 FIXED DEC(19,4) INIT(0),
FW4 FIXED BIN(31) INIT(4),
FW10 FIXED BIN(31) INIT(10);

PUT STRING(BUFFER) EDIT
('SHARES= ',FLTFLD1)
((1)A,P'(15)S9V.9999');

PUT FILE(SYSPRINT) EDIT
(BUFFER)
(COL(2),A,SKIP);

CALL FLT2PCKA(FLTFLD1,PCKNUM194,FW10,FW4);

PUT STRING(BUFFER) EDIT
('SHARES= ',PCKNUM194)
((1)A,P'(15)S9V.9999');

PUT FILE(SYSPRINT) EDIT
(BUFFER)
(COL(2),A,SKIP);

END FD;

OUTPUT:
SHARES= +15000.5699
SHARES= +15000.5700

Can the accuracy of the picture display be improved for floating point variables?

Idea priority Medium
  • Guest
    Reply
    |     Jun 8, 2017

    this is an inherent problem in using hex (or binary) float to represent decimal values: the representation will be inexact. You can even prove that one cannot represent 1/5 (and hence 1/10) exactly as a finite binary fraction.

    There is, however, already a solution to this problem: use Decimal Floating Point (aka DFP). It is available via the FLOAT(DFP) compiler option

  • Guest
    Reply
    |     Jun 7, 2017

    Or, if IBM does not want to impact existing functionality, can a new picture format be added that will instruct the compiler to do the conversion to packed as part of the display? F2P'(15)S9V.9999'.

    PUT STRING(BUFFER) EDIT
    ('SHARES= ', FLTFLD1)
    ((1)A, F2P'(15)S9V.9999');

  • Guest
    Reply
    |     May 13, 2017

    "... Can the accuracy of the picture display be improved for floating point variables?"

    As you've already found out yourself, the ***only*** way to display floating point variables the way you want is to convert them into fixed point first.

    Except for "a few" floating point values, most floating point variables ***cannot*** exactly be represented, not in IBM HEX format, nor in IEEE (754) floating point.

    Any changes to the compiler, unless a new RULES() option is introduced would break numerous other programs - you may want to see 0.5700, whereas Tom, Dick or Harry absolutely require 0.5699...

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