mapCDF

An enhanced map interface for writing NASA CDF files

mapCDF

An enhanced map interface for writing NASA CDF files

Wrapper class around cdflib to make CDF file creation less awkward.

This class presents a CDF files as an enhanced Map. Each member of the map represents a single variable in a CDF file. The data for each variable has more than one row if the Variable is Record Varying (a CDF term) and only a single row if it's a non record-varying variable.

Installation

Copy the +cdf folder somewhere to your MATLAB path. That's it.

Usage

A brief overview of how to use the +CDF module from mapCDF follows.

Value type conversions

MATLAB Type	CDF Type
datetime	CDF_EPOCH
double	CDF_REAL8
float	CDF_REAL4
string	CDF_CHAR
char array	CDF_CHAR
integers	CDF_INT of the same size and sign

The storage type can be set when a Variable is created (see the subsasgn() function), or via the the cdf.Var.type function.

Array storage, Record Varying vs non record varying

Data values are associated with a variable via indexed assignment. The following example creates a variable named status and assigns byte values to it:

   out = cdf.CDF()
   out('Status').data = zeros(1, 5000, 'int8')

The 'Status' variable would be saved as a record varying variable. This is because the size of the last dimension determines the record variability of an array. The zeros function above creates a row-vector of size:

  [1 5000]

Since the last dimension is > 1 then the array will be stored as a record varying variable when the .save() method is called.

For a slightly more complex example, consider the following set of column vectors:

   B_vec_array = [1 0.7071 0; 
                  0 0.7071 0; 
                  0 1      0 ]

The complete set has size [3 3]. Thus the call:

   out('B_gei').data = B_vec_array;

Creates a record varying variable that has 3 values per record. The following column vector:

   Frequency = [10; 
                20; 
                30]

has size [3 1]. Thus it is not a record varying variable, but does have 3 values for it's one, lone constant record. The following time array is record varying and has only one value per record:

   T = [datetime('2019-10-28') datetime('2019-10-28') datetime('2019-10-28')]

Dealing with Rank >= 2 Non-record varying variables

For various reasons matlab drops trailing dimensions of length 1, only if the variable is rank 3 or greater. Thus the language has a broken symmetry around rank 2 variables. Since a trailing rank of 1 is needed to indicate non-record-varying varibles, use the option Rank integer when assigning variables using mkVar() function or manually set the .rank parameter. For example

   Coefficents = [ 0.9946   -0.0393    0.0015   -2.4940; 
                   0.0416    1.0004    0.0450   -1.2780;
                   0.0016   -0.0491    0.9512    9.5738 ]

   out('Coeff').data = Coefficents
   out('Coeff').rank = 3    

Character Data

To store strings as atomic items, use cell-arrays. Otherwise the count of characters along a string is one of the data variable dimensions.
For example:

   cdf.mkVar('char_data', ['Comp A'; 'Comp B'; 'Comp C']);

will generate a two dimensional CDF variable with three records and a size of 6 in the second dimension and an element size of 1. The atomic unit of this data array will be a single character.

On the other hand:

   cdf.mkVar('char_data', {'Comp A'; 'Comp B'; 'Comp C'});

will generate a 1 dimensional CDF variable with three records and with an element size of 6. The atomic uint of this variable is the whole 6-character string.

Properties

Global properties are accessed and set via the '.attrs' map. For ex:

   out = cdf.CDF();
   out.attr('Title') = 'Hyper accurate Irridium Mag vectors (GEI)';

Variable properties are accessed by first indexing to get the variable, and then setting a value for that variable's attrs map:

  out = cdf.CDF();
  out('Epoch').attr('Caution') = 'Leap seconds have been ignored';