Procedure

     POSR ( Position of substring, reverse search)

     INTEGER FUNCTION POSR ( STR, SUBSTR, START )

Abstract

     Find the first occurrence in a string of a substring, starting at
     a specified location, searching in reverse.

Required_Reading

     SCANNING

Keywords

     CHARACTER
     SEARCH
     UTILITY

Declarations

     IMPLICIT NONE

     CHARACTER*(*)         STR
     CHARACTER*(*)         SUBSTR
     INTEGER               START

Brief_I/O

     VARIABLE  I/O  DESCRIPTION
     --------  ---  --------------------------------------------------
     STR        I   A character string
     SUBSTR     I   Substring to locate in the character string.
     START      I   Where to start looking for SUBSTR in STR.

     The function returns the index of SUBSTR in STR preceding START

Detailed_Input

     STR      is any character string.

     SUBSTR   is a substring to look for in STR. Spaces in
              SUBSTR are significant.

     START    is the position in STR to begin looking for SUBSTR.

Detailed_Output

     The function returns the index of the beginning of the last
     substring of STR that begins on or before index START and is
     equal to SUBSTR. If the substring cannot be found starting at or
     before START, the function is returns 0.

Parameters

     None.

Exceptions

     Error free.

     1)  If START is less than 1, POSR returns zero.

     2)  If START is greater than LEN(STRING), the search begins
         at the last character of the string.

Files

     None.

Particulars

     POSR is case sensitive.

     An entire family of related SPICELIB routines (POS, CPOS, NCPOS,
     POSR, CPOSR, NCPOSR) is described in the Required Reading.

     Those familiar with the .TRUE. BASIC language should note that
     these functions are equivalent to the .TRUE. BASIC intrinsic
     functions with the same name.

Examples

     Let STRING = 'AN ANT AND AN ELEPHANT        '
                   123456789012345678901234567890

     Normal (Sequential) Searching:
     ------------------------------

           POSR ( STRING, 'AN',  31 ) = 20
           POSR ( STRING, 'AN',  19 ) = 12
           POSR ( STRING, 'AN',  11 ) =  8
           POSR ( STRING, 'AN',   7 ) =  4
           POSR ( STRING, 'AN',   3 ) =  1
           POSR ( STRING, 'AN',   0 ) =  0

     START out of bounds:
     --------------------

           POSR ( STRING, 'AN', -5 ) =  0
           POSR ( STRING, 'AN',  0 ) =  0
           POSR ( STRING, 'AN', 31 ) = 20
           POSR ( STRING, 'AN', 44 ) = 20

     Significance of Spaces:
     -----------------------

           POSR ( STRING, 'AN',    31 ) =  20
           POSR ( STRING, ' AN',   31 ) =  11
           POSR ( STRING, ' AN ',  31 ) =  11
           POSR ( STRING, ' AN ',  10 ) =   0
           POSR ( STRING, ' AN  ', 31 ) =   0

Restrictions

     None.

Literature_References

     None.

Author_and_Institution

     J. Diaz del Rio    (ODC Space)
     H.A. Neilan        (JPL)
     B.V. Semenov       (JPL)
     W.L. Taber         (JPL)
     K.S. Zukor         (JPL)

Version

    SPICELIB Version 1.1.0, 13-AUG-2021 (JDR)

        Added IMPLICIT NONE statement.

        Edited the header to comply with NAIF standard.

    SPICELIB Version 1.0.4, 31-JAN-2008 (BVS)

        Removed non-standard end-of-declarations marker
        'C%&END_DECLARATIONS' from comments.

    SPICELIB Version 1.0.3, 25-AUG-1994 (HAN) (KSZ)

        $Examples section of the header used POS instead of POSR.
        Also, some examples were incorrect. They have been corrected.

    SPICELIB Version 1.0.2, 10-MAR-1992 (WLT)

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

    SPICELIB Version 1.0.1, 26-MAR-1991 (HAN)

        The Required Reading file POSITION was renamed to SCANNING.
        This header was updated to reflect the change.

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