Procedure

     VZERO    ( Is a vector the zero vector? )

     LOGICAL FUNCTION VZERO ( V )

Abstract

     Indicate whether a 3-vector is the zero vector.

Required_Reading

     None.

Keywords

     MATH
     VECTOR

Declarations

     IMPLICIT NONE

     DOUBLE PRECISION      V ( 3 )

Brief_I/O

     VARIABLE  I/O  DESCRIPTION
     --------  ---  --------------------------------------------------
     V          I   Vector to be tested.

     The function returns the value .TRUE. if and only if V is the
     zero vector.

Detailed_Input

     V        is a vector in 3-space.

Detailed_Output

     The function returns the value .TRUE. if and only if V is the
     zero vector.

Parameters

     None.

Exceptions

     Error free.

Files

     None.

Particulars

     This function has the same truth value as the logical expression

        VNORM ( V )  .EQ.  0.D0

     Replacing the above expression by

        VZERO ( V )

     has several advantages: the latter expresses the test more
     clearly, looks better, and doesn't go through the work of scaling,
     squaring, taking a square root, and re-scaling (all of which
     VNORM must do) just to find out that a vector is non-zero.

     A related function is VZEROG, which accepts vectors of arbitrary
     dimension.

Examples

     1)  When testing whether a vector is the zero vector, one
         normally constructs tests like

            IF (  VNORM ( V )  .EQ.  0.D0  ) THEN
                        .
                        .
                        .


         These can be replaced with the code

            IF (  VZERO ( V )  ) THEN
                        .
                        .
                        .


      2)  Check that a normal vector is non-zero before creating
         a plane with PNV2PL:

         IF (  VZERO ( NORMAL )  )  THEN

            [ handle error ]

         ELSE

            CALL PNV2PL ( POINT, NORMAL, PLANE )
                          .
                          .
                          .

Restrictions

     None.

Literature_References

     None.

Author_and_Institution

     N.J. Bachman       (JPL)
     J. Diaz del Rio    (ODC Space)
     W.L. Taber         (JPL)
     I.M. Underwood     (JPL)

Version

    SPICELIB Version 1.1.0, 05-JUL-2021 (JDR)

        Added IMPLICIT NONE statement.

        Edited the header to comply with NAIF standard.

    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, 17-JUL-1990 (NJB) (IMU)