Procedure

     OPSGND ( Opposite Sign Double Precision Numbers )

     LOGICAL FUNCTION OPSGND  ( X, Y )

Abstract

     Return .TRUE. if two given double precision numbers have opposite
     signs.

Required_Reading

     None.

Keywords

     NUMBERS

Declarations

     IMPLICIT NONE

     DOUBLE PRECISION   X
     DOUBLE PRECISION   Y

Brief_I/O

     VARIABLE  I/O  DESCRIPTION
     --------  ---  --------------------------------------------------
     X          I   A double precision number
     Y          I   A double precision number

     The function returns .TRUE. when the double precision numbers X
     and Y have opposite signs.

Detailed_Input

     X        is any double precision number.

     Y        is any double precision number.

Detailed_Output

     The function returns .TRUE. if one of the pair X,Y is positive and
     the other is negative. If either of the two values is zero, OPSGND
     will be .FALSE.

Parameters

     None.

Exceptions

     Error free.

Files

     None.

Particulars

     This routine returns the value:

           (      (( X .GT. 0) .AND. (Y .LT. 0))
             .OR. (( X .LT. 0) .AND. (Y .GT. 0)) )

     This is a more stable value than

           ( X*Y .LT. 0 )

     Note that if either of the two values is zero, OPSGND will be
     false.

Examples

     This routine can be used whenever a decision depends upon two
     Double Precision values having opposite signs.

     IF ( OPSGND ( F(X1), F(X2) ) ) THEN
           .
           .
        find a root of F lying between X1 and X2
           .
           .
     ELSE
           .
           .
        do something
           .
           .
     END IF

Restrictions

     None.

Literature_References

     None.

Author_and_Institution

     J. Diaz del Rio    (ODC Space)
     W.L. Taber         (JPL)

Version

    SPICELIB Version 1.1.0, 26-OCT-2021 (JDR)

        Added IMPLICIT NONE statement.

        Edited the header to comply with NAIF standard. Extended
        $Detailed_Output section to indicate the output value for
        the case of either input being zero.

    SPICELIB Version 1.0.1, 10-MAR-1992 (WLT)

        Comment section for permuted index source lines was added
        following the header.

    SPICELIB Version 1.0.0, 31-JAN-1990 (WLT)