Index Page Cells

## Table of Contents

       Cells
          Abstract
          Introduction
          Naming Conventions
          Declaring and Initializing Cells
          Using Cells
          Character Cells
          Cell-based data types
       Summary
       Appendix A --- Revision History
             2017 MAR 13 by N. J. Bachman
             2002 SEP 04 by N. J. Bachman

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# Cells

Last revised on 2017 MAR 13 by N. J. Bachman.

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## Abstract

Cells are SPICE data structures that are vectors of type double precision, integer, or character type carrying with them their own dimension and knowledge of how many components have been used.

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## Introduction

A ``CSPICE cell'' (or simply ``cell'') is a data structure intended to provide safe array access within C applications.

A CSPICE cell consists of two components: a data array, and an associated C structure---called a ``SpiceCell''--- that maintains attribute information regarding the data array.

The SpiceCell structure contains sufficient information to allow CSPICE cell API functions to access the data array ``safely,'' ensuring that out-of-range subscript conditions or mismatched data type errors are avoided. A SpiceCell acts as an array descriptor: rather than passing the address of a cells' data array between functions, a program normally passes the address of the associated SpiceCell.

The SpiceCell structure contains the members:

       dtype:     Data type of cell: character,
                  integer, or double precision.

                  dtype has type
                  SpiceCellDataType.

       length:    For character cells, the
                  declared length of the
                  cell's string array.

       size:      The maximum number of data
                  items that can be stored in
                  the cell's data array.

       card:      The cell's "cardinality": the
                  number of data items currently
                  present in the cell.

       isSet:     Boolean flag indicating whether
                  the cell is a CSPICE set.
                  Sets have no duplicate data
                  items, and their data items are
                  stored in increasing order.

       adjust:    Boolean flag indicating whether
                  the cell's data area has
                  adjustable size.  Adjustable
                  size cell data areas are not
                  currently implemented.

       init:      Boolean flag indicating whether
                  the cell has been initialized.

       base:      is a void pointer to the
                  associated data array.  base
                  points to the start of the
                  control area of this array.

       data:      is a void pointer to the
                  first data slot in the
                  associated data array.  This
                  slot is the element following
                  the control area.

The terms ``size'' and ``cardinality'' refer, respectively, to the maximum number of data elements the cell's data array can hold and the number of data elements currently in the data array.

See the header file SpiceCel.h for details concerning the definition of this structure.

The CSPICE cell's compact representation allows the user to declare, pass, and otherwise manipulate cells without having to keep track of separate pointers and dimensions for each cell array. Thus, a function to merge the contents of two arrays into a third, when coded using cells, looks like this

       merge ( &old, &new, &total );

instead of like this

       merge ( old, n_old, new, n_new, max_total, total, &n_total  );

This is especially convenient for arrays that need to be passed as arguments through several levels of subprograms. This also remedies one of the serious flaws in the implementation of C arrays---the inability of a subprogram to determine the size of an argument array into which it is to place values. Since the size of a cell is always available, functions that manipulate cells can always check for overflow conditions.

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## Naming Conventions

The type of a cell is the same as the type of the cell's data array. The three types are: Character, Double Precision, and Integer. Most CSPICE cell functions are type-independent. The few type-dependent functions have names ending with the suffixes

       c_c, d_c, i_c

where the first character of the suffix indicates the data type. When referring to a class of type-dependent functions, we substitute an ``x'' for the letter denoting type. Thus appndx_c refers to any of appndc_c, appndd_c, or appndi_c.

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## Declaring and Initializing Cells

CSPICE provides declaration macros to simplify the process of declaring and initializing CSPICE cells. There is one macro for each data type. The calling sequences are:

       SPICECHAR_CELL   ( name, size, length );
       SPICEDOUBLE_CELL ( name, size );
       SPICEINT_CELL    ( name, size );

These macros declare both a SpiceCell and a data array having the capacity specified by size, and for character cells, length. The data array is given a name having the input parameter ``name'' as a substring. Normally applications will not refer to the data array by name.

Note that these macros are coded so as to require a semicolon following each call.

The data array created by the declaration macros is dimensioned so as to be compatible with SPICELIB cells. In SPICELIB, a ``cell array"' is an array dimensioned from LBCELL to CMAX, where LBCELL is the standard lower bound of a cell (currently -5) and CMAX is the maximum number of elements that the cell is allowed to contain at any one time---that is, the maximum cardinality of the cell. SPICELIB cell array elements indexed LBCELL through 0 constitute the ``control area'' of the cell---the portion where the cell's bookkeeping data are stored.

Keeping CSPICE cells' data array dimensions compatible with those of SPICELIB cells simplifies passing CSPICE cell data arrays to f2c'd SPICELIB cell routines.

CSPICE cell declaration macros always create cells and their data arrays with static storage duration.

The members of a SpiceCell are initialized by the declaration macros; in particular the base pointer is set to the base address of the data array, and the data pointer is set to the address of the start of the data area (the first address following the control area) in the data array. Unlike SPICELIB cells, CSPICE cells require no initialization action on the user's part at run time.

Every CSPICE cell starts out as a CSPICE ``set.'' The property of being a set (the data elements are sorted in increasing order and contain no duplicates) is maintained if the cell is modified using only CSPICE set functions. Unlike SPICELIB cells, CSPICE cells maintain a flag indicating whether they are in fact sets. See the SETS Required Reading, sets.req, for more information on CSPICE sets.

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## Using Cells

SPICELIB users can access their cells' data elements directly via array subscript notation. In CSPICE, an analogous capability is provided via access macros. These macros allow a user to fetch, assign, or reference CSPICE cells' data array elements. The name of each access macro ends in a letter (C, D, or I) indicating the data type of cells the macro is intended to act upon.

The CSPICE character element reference macro returns a pointer to a specified data array element. The numeric element reference macros return a dereferenced array element. The macros' calling sequences are:

       SPICE_CELL_ELEM_C ( &cell, i );
       SPICE_CELL_ELEM_D ( &cell, i );
       SPICE_CELL_ELEM_I ( &cell, i );

The character macro call

       SPICE_CELL_ELEM_C ( &cell, i );

returns a pointer to the ith string of length

       cell.length

in the cell's data array. However the type of the returned pointer is (SpiceChar *), not (SpiceChar (*)[cell.length]).

The cell assignment macros have the calling sequences

       SPICE_CELL_SET_C ( item, i, &cell );
       SPICE_CELL_SET_D ( item, i, &cell );
       SPICE_CELL_SET_I ( item, i, &cell );

The cell fetch macros have the calling sequences

       SPICE_CELL_GET_C ( &cell, i, lenout, item );
       SPICE_CELL_GET_D ( &cell, i, &item );
       SPICE_CELL_GET_I ( &cell, i, &item );

Here lenout is an input SpiceInt argument indicating the amount of room in the output string item.

CSPICE cells may be populated using the appndx_c functions. These routines ``append'' a datum to a cell: they insert a specified data item into the lowest-indexed free slot in the data area of the cell's data array. The appndx_c functions automatically update the cell's cardinality to reflect the addition of the new datum.

To append three double precision numbers onto an empty double precision cell, we could use the code fragment

       #include "SpiceUsr.h"
         .
         .
         .
       SPICEDOUBLE_CELL ( x, 100 );
         .
         .
         .
       appndd_c ( 0.0, &x );
       appndd_c ( 0.0, &x );
       appndd_c ( 1.0, &x );

Another function scard_c is used to adjust the cardinality of a cell. This is necessary when directly inserting items into or removing items from a cell, as shown below.

       #include "SpiceUsr.h"
         .
         .
         .
       SPICEDOUBLE_CELL ( x, 100 );
         .
         .
         .
       SPICE_CELL_SET_D ( 0.0, 0, &x );
       SPICE_CELL_SET_D ( 0.0, 1, &x );
       SPICE_CELL_SET_D ( 1.0, 2, &x );

       scard_c ( 3, &x );

ssize_c and scard_c should always be used in lieu of altering the size and cardinality members of a SpiceCell directly.

The function copy_c copies the elements of one cell to another cell. This can be useful for modifying temporary or working cells, or for saving copies of cells which are about to be changed. For example,

       #include "SpiceUsr.h"
         .
         .
         .
       SPICEDOUBLE_CELL ( x,    100 );
       SPICEDOUBLE_CELL ( temp, 100 );
         .
         .
         .
       copy_c ( &x, &temp );

copies the contents of `x' into `temp'. In this case, the cells are the same size (each can hold up to 100 elements), so the operation will always succeed. In general, if the output cell is not large enough to hold the contents of the input cell, as many elements as will fit are inserted into the output cell, and the CSPICE error handling mechanism reports the number of excess elements.

An extra check is performed when copy_c is used to copy character cells. In order to avoid truncation problems, copy_c verifies that the operation can be performed without losing any of the non-blank characters in the original cell. The loss of one or more non-blank characters is reported through the CSPICE error handling mechanism.

The integer function card_c returns the cardinality of a cell. This may be used to determine whether a cell is empty or not. (The cardinality of an empty cell is zero.) It may also be used to assist in accessing the elements of a cell individually, as in the following example.

       printf ( "Winners of the Nobel Prize for Physics:\n" );

       for ( i = 0;  i < card_c(&nobel);  i++ )
       {
          printf ( "%s\n",  SPICE_CELL_ELEM_C ( &nobel, i )  );
       }

The integer function size_c returns the size (maximum cardinality) of a cell. This is useful primarily for predicting situations in which overflow can occur, as in the following example.

       if (  card_c( &winners )  <=  size_c( &nobel )  )
       {
          copy_c ( &winners, &nobel );
       }

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## Character Cells

In SPICELIB cells, the size and cardinality of a cell are stored in the control area of the cell (elements LBCELL through 0). For numeric data types, this is accomplished by simple assignment. However, in the case of cell arrays of type character, the values for the size and cardinality must be encoded into character strings.

This is done by storing the numbers in base CHBASE, where CHBASE is the number of distinct characters in the character set supported by the host machine and compiler. (In ASCII environments, CHBASE is always at least 128, and may be as high as 256.) The numbers are encoded and decoded by subroutines ENCHAR and DECHAR respectively. The value of parameter MINLEN (declared in ENCHAR) constrains the minimum length of the elements in a cell array. The nominal value for MINLEN is 5.

Because CSPICE relies on f2c'd Fortran routines to carry out some operations on cells, it is necessary for a character CSPICE cell's data array to be convertible to a SPICELIB character cell. To make this convenient, the string length of a CSPICE character cell should be at least MINLEN.

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## Cell-based data types

CSPICE contains several extended data types based on cells. For example, one family of functions uses cells to implement algebraic sets of all types (character, integer, double precision). Another uses double precision cells to manipulate collections of closed intervals of the real numbers, called windows.

All of these data types are supported by functions designed to manipulate them. However, because they are based on cells, all of these data types can be manipulated by the general cell routines as well. Thus, copy_c can be used to copy sets and windows, just as it can be used to copy vanilla cells.

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# Summary

The following table summarizes the CSPICE cell functions.

ssize_c ( size, cell )
  Initialize (set the size of) a cell.

scard_c ( card, cell )
  Set the cardinality of a cell.

size_c ( cell )
  Return the size of a cell.

card_c ( cell )
  Return the cardinality of a cell.

copy_c ( orig, copy )
  Copy the contents of a cell.

appndc_c ( item, cell )
  Append a SpiceChar item to a cell.

appndd_c ( item, cell )
  Append a SpiceDouble item to a cell.

appndi_c ( item, cell )
  Append a SpiceInt item to a cell.

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# Appendix A --- Revision History

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### 2017 MAR 13 by N. J. Bachman

Moved revision history to this appendix.

Updated description of the element reference macros.

Updated references to CSPICE cell ``append'' functions to name individual functions rather than to refer to them using the notation

       appndx_c

Fixed typos.

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### 2002 SEP 04 by N. J. Bachman

Initial release for CSPICE.