To: J3                                                     J3/18-278r1
From: Robert Corbett
Subject: sign of zero on write
Date: 2018-October-18


1.  Introduction

The Fortran standards up to Fortran 90 treated the sign of a
floating-point zero in formatted output the same as the sign of a
positive value.  If a sign was written for a zero, it was '+'.
Subsequent Fortran standards require processors that distinguish
between +0.0 and -0.0 to write the sign '-' for -0.0.  For the sake of
backward compatibility, some implementations provide an option
that tells the processor to write floating-point zeros using the
old rules.

This paper proposes a language extension to allow a user to choose
how he or she wants the sign of a floating-point zero to be
treated during formatted output.  References are to 18-007r1.


2.  Use Cases

The extension proposed here would allow a user to specify
compatibility with programs that were compiled with older
implementations.  It also allows user to specify consistent
treatment of floating-point zero between implementations that do
and do not distinguish between +0.0 and -0.0, which could be useful
for testing.  An additional way of treating zeros is proposed for
the purpose of writing tables of floating-point values.


3.  Requirements

A new changeable mode (12.5.2) is to be added.  It might be called
"zero sign mode".  The mode could be specified in OPEN statements and
WRITE statements and could be accessed in INQUIRE statements.  The
mode for an output statement with an explicit format could be
overridden by a control edit descriptor.  The mode would affect
list-directed and namelist output as well as output with an
explicit format.


4.  Possible Syntax and Semantics

The form of the specifier for the zero sign mode might be "ZERO_SIGN=".
The possible values might be PLUS, SIGN, and NONE.  The value PLUS
would mean that the sign of a floating-point zero should be treated
the same as the sign of a positive value.  The value SIGN would
mean that a processor that distinguishes between +0 and -0 should
treat the sign of +0 the same as the sign of a positive value and
the sign of -0 the same as a sign of a negative value.  For a
processor that does not distinguish between +0 and -0, the values
PLUS and SIGN have the same effect.

The value NONE specifies that when a floating-point zero is written
using formatted output, no sign is to be written.  This mode could
be used in combination with the sign mode PLUS to distinguish zeros
from small nonzero values when writing using F edit descriptors.
Some tables of mathematical values use such a convention.  For
example, a column in a table might contain the values

      +.00021
      +.00002
      +.00000
       .00000
      -.00000
      -.00001

Here +.00000 represents a small positive value, .00000 represents
zero and -.00000 represents a small magnitude negative value.

The forms ZN, ZP, and ZS might be used as the control edit
descriptors.  The edit descriptors ZN, ZP, and ZS set the zero
sign mode to the values NONE, PLUS, and SIGN, respectively.

The zero sign mode affects formatted output of data whose
internal values are floating-point zeros only.  The zero sign
mode NONE takes precedence over the sign mode (13.8.4) when a
floating-point zero is written.  If the zero sign mode is NONE,
no sign shall be written for a floating-point zero, regardless of
the sign mode.  If the zero sign mode is PLUS, a zero, including
-0.0, shall write the sign '+' if the sign mode would write a
sign for a positive value.  If the zero sign mode is SIGN, the
sign of -0.0 shall be written as if it were negative value, and
the sign of +0.0 shall be written as if it were a positive value.
If the processor does not distinguish between +0.0 and -0.0,
all floating-point zeros are written as if they are +0.0.