# GDALMDArray C++ API

class `GDALMDArray` : public virtual GDALAbstractMDArray, public GDALIHasAttribute

  Class modeling a multi-dimensional array.

  It has a name, values organized as an array and a list of GDALAttribute.

  This is based on the HDF5 dataset concept

  Since

    GDAL 3.1

  Subclassed by GDALExtractFieldMDArray, GDALMDArrayFromRasterBand, GDALMDArrayMask, GDALMDArrayTransposed, GDALMDArrayUnscaled, GDALSlicedMDArray

  Public Functions

  GUInt64 `GetTotalCopyCost`() const

    Return a total “cost” to copy the array.

    Used as a parameter for CopyFrom()

  virtual bool `CopyFrom`(GDALDataset *poSrcDS, const GDALMDArray *poSrcArray, bool bStrict, GUInt64 &nCurCost, const GUInt64 nTotalCost, GDALProgressFunc pfnProgress, void *pProgressData)

    Copy the content of an array into a new (generally empty) array.

    Return

      true in case of success (or partial success if bStrict == false).

    Parameters

      - `poSrcDS`: Source dataset. Might be nullptr (but for correct behavior of some output drivers this is not recommended)

      - `poSrcArray`: Source array. Should NOT be nullptr.

      - `bStrict`: Whether to enable stict mode. In strict mode, any error will stop the copy. In relaxed mode, the copy will be attempted to be pursued.

      - `nCurCost`: Should be provided as a variable initially set to 0.

      - `nTotalCost`: Total cost from GetTotalCopyCost().

      - `pfnProgress`: Progress callback, or nullptr.

      - `pProgressData`: Progress user data, or nulptr.

  virtual bool `IsWritable`() const = 0

    Return whether an array is writable;.

  virtual CSLConstList `GetStructuralInfo`() const

    Return structural information on the array.

    This may be the compression, etc..

    The return value should not be freed and is valid until GDALMDArray is released or this function called again.

    This is the same as the C function GDALMDArrayGetStruturalInfo().

  virtual const std::string &`GetUnit`() const

    Return the array unit.

    Values should conform as much as possible with those allowed by the NetCDF CF conventions: http://cfconventions.org/Data/cf-conventions/cf-conventions-1.7/cf-conventions.html#units but others might be returned.

    Few examples are “meter”, “degrees”, “second”, … Empty value means unknown.

    This is the same as the C function GDALMDArrayGetUnit()

  virtual bool `SetUnit`(const std::string &osUnit)

    Set the variable unit.

    Values should conform as much as possible with those allowed by the NetCDF CF conventions: http://cfconventions.org/Data/cf-conventions/cf-conventions-1.7/cf-conventions.html#units but others might be returned.

    Few examples are “meter”, “degrees”, “second”, … Empty value means unknown.

    This is the same as the C function GDALMDArraySetUnit()

    Note

      Driver implementation: optionally implemented.

    Return

      true in case of success.

    Parameters

      - `osUnit`: unit name.

  virtual bool `SetSpatialRef`(const OGRSpatialReference *poSRS)

    Assign a spatial reference system object to the the array.

    This is the same as the C function GDALMDArraySetSpatialRef().

  virtual std::shared_ptr<OGRSpatialReference> `GetSpatialRef`() const

    Return the spatial reference system object associated with the array.

    This is the same as the C function GDALMDArrayGetSpatialRef().

  virtual const void *`GetRawNoDataValue`() const

    Return the nodata value as a “raw” value.

    The value returned might be nullptr in case of no nodata value. When a nodata value is registered, a non-nullptr will be returned whose size in bytes is GetDataType().GetSize().

    The returned value should not be modified or freed. It is valid until the array is destroyed, or the next call to GetRawNoDataValue() or SetRawNoDataValue(), or any similar methods.

    This is the same as the C function

    GDALMDArrayGetRawNoDataValue().
    Note

      Driver implementation: this method shall be implemented if nodata is supported.

    Return

      nullptr or a pointer to GetDataType().GetSize() bytes.

  double `GetNoDataValueAsDouble`(bool *pbHasNoData = nullptr) const

    Return the nodata value as a double.

    The value returned might be nullptr in case of no nodata value. When a nodata value is registered, a non-nullptr will be returned whose size in bytes is GetDataType().GetSize().

    This is the same as the C function GDALMDArrayGetNoDataValueAsDouble().

    Return

      the nodata value as a double. A 0.0 value might also indicate the absence of a nodata value or an error in the conversion (*pbHasNoData will be set to false then).

    Parameters

      - `pbHasNoData`: Pointer to a output boolean that will be set to true if a nodata value exists and can be converted to double. Might be nullptr.

  virtual bool `SetRawNoDataValue`(const void *pRawNoData)

    Set the nodata value as a “raw” value.

    The value passed might be nullptr in case of no nodata value. When a nodata value is registered, a non-nullptr whose size in bytes is GetDataType().GetSize() must be passed.

    This is the same as the C function GDALMDArraySetRawNoDataValue().

    Note

      Driver implementation: this method shall be implemented if setting nodata is supported.

    Return

      true in case of success.

  bool `SetNoDataValue`(double dfNoData)

    Set the nodata value as a double.

    If the natural data type of the attribute/array is not double, type conversion will occur to the type returned by GetDataType().

    This is the same as the C function GDALMDArraySetNoDataValueAsDouble().

    Return

      true in case of success.

  virtual double `GetOffset`(bool *pbHasOffset = nullptr, GDALDataType *peStorageType = nullptr) const

    Get the offset value to apply to raw values.

    unscaled_value = raw_value * GetScale() + GetOffset()

    This is the same as the C function GDALMDArrayGetOffset().

    Note

      Driver implementation: this method shall be implemented if gettings offset is supported.

    Return

      the offset value. A 0.0 value might also indicate the absence of a offset value.

    Parameters

      - `pbHasOffset`: Pointer to a output boolean that will be set to true if a offset value exists. Might be nullptr.

      - `peStorageType`: Pointer to a output GDALDataType that will be set to the storage type of the offset value, when known/relevant. Otherwise will be set to GDT_Unknown. Might be nullptr. Since GDAL 3.3

  virtual double `GetScale`(bool *pbHasScale = nullptr, GDALDataType *peStorageType = nullptr) const

    Get the scale value to apply to raw values.

    unscaled_value = raw_value * GetScale() + GetOffset()

    This is the same as the C function GDALMDArrayGetScale().

    Note

      Driver implementation: this method shall be implemented if gettings scale is supported.

    Return

      the scale value. A 1.0 value might also indicate the absence of a scale value.

    Parameters

      - `pbHasScale`: Pointer to a output boolean that will be set to true if a scale value exists. Might be nullptr.

      - `peStorageType`: Pointer to a output GDALDataType that will be set to the storage type of the scale value, when known/relevant. Otherwise will be set to GDT_Unknown. Might be nullptr. Since GDAL 3.3

  virtual bool `SetOffset`(double dfOffset, GDALDataType eStorageType = GDT_Unknown)

    Set the offset value to apply to raw values.

    unscaled_value = raw_value * GetScale() + GetOffset()

    This is the same as the C function GDALMDArraySetOffset() / GDALMDArraySetOffsetEx().

    Note

      Driver implementation: this method shall be implemented if setting offset is supported.

    Return

      true in case of success.

    Parameters

      - `dfOffset`: Offset

      - `eStorageType`: Data type to which create the potential attribute that will store the offset. Added in GDAL 3.3 If let to its GDT_Unknown value, the implementation will decide automatically the data type. Note that changing the data type after initial setting might not be supported.

  virtual bool `SetScale`(double dfScale, GDALDataType eStorageType = GDT_Unknown)

    Set the scale value to apply to raw values.

    unscaled_value = raw_value * GetScale() + GetOffset()

    This is the same as the C function GDALMDArraySetScale() / GDALMDArraySetScaleEx().

    Note

      Driver implementation: this method shall be implemented if setting scale is supported.

    Return

      true in case of success.

    Parameters

      - `dfScale`: scale

      - `eStorageType`: Data type to which create the potential attribute that will store the scale. Added in GDAL 3.3 If let to its GDT_Unknown value, the implementation will decide automatically the data type. Note that changing the data type after initial setting might not be supported.

  std::shared_ptr<GDALMDArray> `GetView`(const std::string &viewExpr) const

    Return a view of the array using slicing or field access.

    The slice expression uses the same syntax as NumPy basic slicing and indexing. See https://www.numpy.org/devdocs/reference/arrays.indexing.html#basic-slicing-and-indexing Or it can use field access by name. See https://www.numpy.org/devdocs/reference/arrays.indexing.html#field-access

    Multiple [] bracket elements can be concatenated, with a slice expression or field name inside each.

    For basic slicing and indexing, inside each [] bracket element, a list of indexes that apply to successive source dimensions, can be specified, using integer indexing (e.g. 1), range indexing (start:stop:step), ellipsis (…) or newaxis, using a comma separator.

    Examples with a 2-dimensional array whose content is [[0,1,2,3],[4,5,6,7]].

    - GetView(“[1][2]”): returns a 0-dimensional/scalar array with the value at index 1 in the first dimension, and index 2 in the second dimension from the source array. That is 5

    - GetView(“[1]”)->GetView(“[2]”): same as above. Above is actually implemented internally doing this intermediate slicing approach.

    - GetView(“[1,2]”): same as above, but a bit more performant.

    - GetView(“[1]”): returns a 1-dimensional array, sliced at index 1 in the first dimension. That is [4,5,6,7].

    - GetView(“[:,2]”): returns a 1-dimensional array, sliced at index 2 in the second dimension. That is [2,6].

    - GetView(“[:,2:3:]”): returns a 2-dimensional array, sliced at index 2 in the second dimension. That is [[2],[6]].

    - GetView(“[::,2]”): Same as above.

    - GetView(“[…,2]”): same as above, in that case, since the ellipsis only expands to one dimension here.

    - GetView(“[:,::2]”): returns a 2-dimensional array, with even-indexed elements of the second dimension. That is [[0,2],[4,6]].

    - GetView(“[:,1::2]”): returns a 2-dimensional array, with odd-indexed elements of the second dimension. That is [[1,3],[5,7]].

    - GetView(“[:,1:3:]”): returns a 2-dimensional array, with elements of the second dimension with index in the range [1,3[. That is [[1,2],[5,6]].

    - GetView(“[::-1,:]”): returns a 2-dimensional array, with the values in first dimension reversed. That is [[4,5,6,7],[0,1,2,3]].

    - GetView(“[newaxis,…]”): returns a 3-dimensional array, with an addditional dimension of size 1 put at the beginning. That is [[[0,1,2,3],[4,5,6,7]]].

    One difference with NumPy behavior is that ranges that would result in zero elements are not allowed (dimensions of size 0 not being allowed in the GDAL multidimensional model).

    For field access, the syntax to use is [“field_name”] or [‘field_name’]. Multiple field specification is not supported currently.

    Both type of access can be combined, e.g. GetView(“[1][‘field_name’]”)

    The returned array holds a reference to the original one, and thus is a view of it (not a copy). If the content of the original array changes, the content of the view array too. When using basic slicing and indexing, the view can be written if the underlying array is writable.

    Note

      When using the GDAL Python bindings, natural Python syntax can be used. That is ar[0,::,1][“foo”] will be internally translated to ar.GetView(“[0,::,1][‘foo’]”)

    Note

      When using the C++ API and integer indexing only, you may use the at(idx0, idx1, …) method.

    This is the same as the C function GDALMDArrayGetView()

    Return

      a new array, that holds a reference to the original one, and thus is a view of it (not a copy), or nullptr in case of error.

    Parameters

      - `viewExpr`: Expression expressing basic slicing and indexing, or field access.

  std::shared_ptr<GDALMDArray> `operator[]`(const std::string &fieldName) const

    Return a view of the array using field access.

    Equivalent of GetView(“[‘fieldName’]”)

    Note

      When operationg on a shared_ptr, use (*array)[“fieldName”] syntax.

  template<typename ...`GUInt64VarArg`>
  inline std::shared_ptr<GDALMDArray> `at`(GUInt64 idx, GUInt64VarArg... tail) const

    Return a view of the array using integer indexing.

    Equivalent of GetView(“[indices_0,indices_1,…..,indices_last]”)

    Example:

        ar->at(0,3,2)

  virtual std::shared_ptr<GDALMDArray> `Transpose`(const std::vector<int> &anMapNewAxisToOldAxis) const

    Return a view of the array whose axis have been reordered.

    The anMapNewAxisToOldAxis parameter should contain all the values between 0 and GetDimensionCount() - 1, and each only once. -1 can be used as a special index value to ask for the insertion of a new axis of size 1. The new array will have anMapNewAxisToOldAxis.size() axis, and if i is the index of one of its dimension, it corresponds to the axis of index anMapNewAxisToOldAxis[i] from the current array.

    This is similar to the numpy.transpose() method

    The returned array holds a reference to the original one, and thus is a view of it (not a copy). If the content of the original array changes, the content of the view array too. The view can be written if the underlying array is writable.

    Note that I/O performance in such a transposed view might be poor.

    This is the same as the C function GDALMDArrayTranspose().

    Return

      a new array, that holds a reference to the original one, and thus is a view of it (not a copy), or nullptr in case of error.

  std::shared_ptr<GDALMDArray> `GetUnscaled`() const

    Return an array that is the unscaled version of the current one.

    That is each value of the unscaled array will be unscaled_value = raw_value * GetScale() + GetOffset()

    Starting with GDAL 3.3, the Write() method is implemented and will convert from unscaled values to raw values.

    This is the same as the C function GDALMDArrayGetUnscaled().

    Return

      a new array, that holds a reference to the original one, and thus is a view of it (not a copy), or nullptr in case of error.

  virtual std::shared_ptr<GDALMDArray> `GetMask`(CSLConstList papszOptions) const

    Return an array that is a mask for the current array.

    This array will be of type Byte, with values set to 0 to indicate invalid pixels of the current array, and values set to 1 to indicate valid pixels.

    The generic implementation honours the NoDataValue, as well as various netCDF CF attributes: missing_value, _FillValue, valid_min, valid_max and valid_range.

    This is the same as the C function GDALMDArrayGetMask().

    Return

      a new array, that holds a reference to the original one, and thus is a view of it (not a copy), or nullptr in case of error.

    Parameters

      - `papszOptions`: NULL-terminated list of options, or NULL.

  virtual GDALDataset *`AsClassicDataset`(size_t iXDim, size_t iYDim) const

    Return a view of this array as a “classic” GDALDataset (ie 2D)

    In the case of > 2D arrays, additional dimensions will be represented as raster bands.

    The “reverse” method is GDALRasterBand::AsMDArray().

    This is the same as the C function GDALMDArrayAsClassicDataset().

    Return

      a new GDALDataset that must be freed with GDALClose(), or nullptr

    Parameters

      - `iXDim`: Index of the dimension that will be used as the X/width axis.

      - `iYDim`: Index of the dimension that will be used as the Y/height axis. Ignored if the dimension count is 1.

  virtual CPLErr `GetStatistics`(GDALDataset *poDS, bool bApproxOK, bool bForce, double *pdfMin, double *pdfMax, double *pdfMean, double *padfStdDev, GUInt64 *pnValidCount, GDALProgressFunc pfnProgress, void *pProgressData)

    Fetch statistics.

    Returns the minimum, maximum, mean and standard deviation of all pixel values in this array.

    If bForce is FALSE results will only be returned if it can be done quickly (i.e. without scanning the data). If bForce is FALSE and results cannot be returned efficiently, the method will return CE_Warning but no warning will have been issued. This is a non-standard use of the CE_Warning return value to indicate “nothing done”.

    When cached statistics are not available, and bForce is TRUE, ComputeStatistics() is called.

    Note that file formats using PAM (Persistent Auxiliary Metadata) services will generally cache statistics in the .aux.xml file allowing fast fetch after the first request.

    Cached statistics can be cleared with GDALDataset::ClearStatistics().

    This method is the same as the C function GDALMDArrayGetStatistics().

    Return

      CE_None on success, CE_Warning if no values returned, CE_Failure if an error occurs.

    Since

      GDAL 3.2

    Parameters

      - `poDS`: Owing dataset. If set to NULL, this method will always call ComputeStatistics(). If set to non-NULL, the method will attempt to retrieve from it (generally its persistent auxiliary metadata) already cached statistics.

      - `bApproxOK`: Currently ignored. In the future, should be set to true if statistics on the whole array are wished, or to false if a subset of it may be used.

      - `bForce`: If false statistics will only be returned if it can be done without rescanning the image.

      - `pdfMin`: Location into which to load image minimum (may be NULL).

      - `pdfMax`: Location into which to load image maximum (may be NULL).-

      - `pdfMean`: Location into which to load image mean (may be NULL).

      - `pdfStdDev`: Location into which to load image standard deviation (may be NULL).

      - `pnValidCount`: Number of samples whose value is different from the nodata value. (may be NULL)

      - `pfnProgress`: a function to call to report progress, or NULL.

      - `pProgressData`: application data to pass to the progress function.

  virtual bool `ComputeStatistics`(GDALDataset *poDS, bool bApproxOK, double *pdfMin, double *pdfMax, double *pdfMean, double *pdfStdDev, GUInt64 *pnValidCount, GDALProgressFunc, void *pProgressData)

    Compute statistics.

    Returns the minimum, maximum, mean and standard deviation of all pixel values in this array.

    Pixels taken into account in statistics are those whose mask value (as determined by GetMask()) is non-zero.

    Once computed, the statistics will generally be “set” back on the owing dataset (if poDS is not NULL).

    Cached statistics can be cleared with GDALDataset::ClearStatistics().

    This method is the same as the C function GDALMDArrayComputeStatistics().

    Return

      true on success

    Since

      GDAL 3.2

    Parameters

      - `poDS`: Owing dataset. If set to non-NULL, the method will attempt to store computed statistics in it (generally its persistent auxiliary metadata) for further retrieval by GetStatistics()

      - `bApproxOK`: Currently ignored. In the future, should be set to true if statistics on the whole array are wished, or to false if a subset of it may be used.

      - `pdfMin`: Location into which to load image minimum (may be NULL).

      - `pdfMax`: Location into which to load image maximum (may be NULL).-

      - `pdfMean`: Location into which to load image mean (may be NULL).

      - `pdfStdDev`: Location into which to load image standard deviation (may be NULL).

      - `pnValidCount`: Number of samples whose value is different from the nodata value. (may be NULL)

      - `pfnProgress`: a function to call to report progress, or NULL.

      - `pProgressData`: application data to pass to the progress function.

  bool `AdviseRead`(const GUInt64 *arrayStartIdx, const size_t *count) const

    Advise driver of upcoming read requests.

    Some GDAL drivers operate more efficiently if they know in advance what set of upcoming read requests will be made. The AdviseRead() method allows an application to notify the driver of the region of interest.

    Many drivers just ignore the AdviseRead() call, but it can dramatically accelerate access via some drivers. One such case is when reading through a DAP dataset with the netCDF driver (a in-memory cache array is then created with the region of interest defined by AdviseRead())

    This is the same as the C function GDALMDArrayAdviseRead().

    Return

      true in case of success (ignoring the advice is a success)

    Since

      GDAL 3.2

    Parameters

      - `arrayStartIdx`: Values representing the starting index to read in each dimension (in [0, aoDims[i].GetSize()-1] range). Array of GetDimensionCount() values. Can be nullptr as a synonymous for [0 for i in range(GetDimensionCount() ]

      - `count`: Values representing the number of values to extract in each dimension. Array of GetDimensionCount() values. Can be nullptr as a synonymous for [ aoDims[i].GetSize() - arrayStartIdx[i] for i in range(GetDimensionCount() ]