# IDL interface

The Interactive Data Language (IDL) interface consists of a Dynamic Loadable Module (DLM) that provides the harp_import, harp_export, harp_version, and ‘harp_unload’ functions.

Products are represented in IDL as structures which can be manipulated freely from within IDL. A product structure contains a field for each variable contained in the product and contains the global attributes source_product and history (if available). Each variable itself is again a structure containing the variable attributes (unit, description, etc.) and a field data that contains the data of the variable. The structure of a variable also contains a field name that contains the case-sensitive name of the variable and (if the variable is not a scalar) a field dimension that contains a list of dimension names for each dimension of the variable.

Products can be exported as HARP compliant products in any of the file formats supported by the HARP C library (netCDF/HDF4/HDF5). Such exported products can subsequently be processed further using the HARP command line tools.

## Dimension types

Each non-scalar variable will have a dimension field in its structure, which is a list of strings representing the dimension types (e.g. time, vertical, latitude, etc.).

## Data types

The HARP IDL interface takes care of the conversion of product and variables from the C domain to the IDL domain and back. This section describes the relation between types in the C domain and types in the IDL domain.

The table below shows the type map that is used when importing or ingesting a product, i.e. when translating from the C domain to the IDL domain.

Variable data arrays are converted to arrays in the product structure. The data type used for the converted array is determined from the HARP data type of the variable according to the type map shown below. Zero-dimensional arrays of length 1 are converted to IDL scalars. The resulting IDL type is also shown in the type map.

HARP data type | IDL type
--- | ---
harp_type_int8 | byte
harp_type_int16 | int
harp_type_int32 | long
harp_type_float | float
harp_type_double | double
harp_type_string | string

Note that the IDL byte type is an unsigned type (since IDL does not have any 8-bit signed type). The HARP IDL interface will just hard cast signed int8 values to unsigned uint8 values (e.g. -1 will become 255). Make sure that in your operations within IDL on these 8-bit integers you als treat them as mapped signed integers. Note also that this holds for the valid_min and valid_max attributes (e.g. valid_min may end up being higher than valid_max in IDL).

## Unicode

Zero-terminated C strings received from the HARP C library are always converted to instances of type `string` in IDL which are unicode strings.

## Examples

    ; Import a file as a HARP product.
    prod = harp_import("filename.ext")

    ; Print information about the product.
    help, prod, /struct

    ; Print information about the variable 'temperature'.
    help, prod.temperature, /struct

    ; Print the contents of the variable 'temperature'.
    print, prod.temperature.data

    ; Export the updated product as an HDF4 file (the format must be
    ; HDF4, HDF5 or netCDF, if no format is specified netCDF is used).
    result = harp_export(prod, "filename.hdf", "hdf4")

    ; Print the result of the export.
    print, result

    ; Import the HDF4 file and perform an operation to exclude the variable
    ; temperature (variable name must be in uppercase).
    prod2 = harp_import("filename.hdf", "exclude(temperature)");

    ; Print information about the product.
    help, prod2, /struct

## API reference

This section describes the functions defined by the HARP IDL interface.

harp_import(filename, operations='', options='')

  Import a product from a file.

  This will first try to import the file as an HDF4, HDF5, or netCDF file that complies to the HARP Data Format. If the file is not stored using the HARP format then it will try to import it using one of the available ingestion modules.

  If the filename argument is a list of filenames or a globbing (glob.glob()) pattern then the harp.import_product() function will be called on each individual file and the result of harp.concatenate() on the imported products will be returned.

  Parameters:

    - filename (str) – Filename of the product to ingest

    - operations (str) – Actions to apply as part of the import; should be specified as a semi-colon separated string of operations.

    - options (str) – Ingestion module specific options; should be specified as a semi-colon separated string of key=value pairs; only used if the file is not in HARP format.

  Returns:

    Ingested product or error structure.

harp_export(product, filename, file_format='netcdf')

  Export a HARP compliant product.

  Parameters:

    - product (str) – Product to export.

    - filename (str) – Filename of the exported product.

    - file_format (str) – File format to use; one of ‘netcdf’, ‘hdf4’, or ‘hdf5’. If no format is specified, netcdf is used.

  Returns:

    Error structure with result code.

harp_version()

  The harp_version function returns a string containing the current version number of HARP. The version number is always of the format ‘x.y.z’, i.e., major, minor, and revision numbers, separated by dots.

  Returns:

    HARP version number.

harp_unload()

  The harp_unload procedure will clean up any HARP resources. At the first call to a HARP IDL function the HARP C Library will be initialized which will require some memory. A call to harp_unload can then be used to clean up these HARP resources. After a clean up, the first call to a HARP IDL function will initialize the HARP C Library again.

  This function may be (slightly) useful on systems with little memory.