TableMeasOffsetDesc.h

This class assists in the definition of the offset component of a TableMeasDesc in the TableMeasures system. Four possibilities exist for specifying the handling of measure offsets in a Measure column. These are:

an offset is not used
all measures in the column have the same offset
a unique (and probably different) offset is stored for each row
a unique offset is stored in each array element per (Array)column row

Note that this last option is only relevant when using ArrayMeasColumns.

Examples of each of these follow.

Example

Specifying a single fixed offset. Note that a Measure offset is itself a measure

    // create an MEpoch to use as the offset in an MEpoch column
    MEpoch offset(MVEpoch(MVTime(1996, 5, 17, (8+18./60.)/24.)), MEpoch::UTC);
    TableMeasOffsetDesc offsetDesc(offset);

Storing an offset per row needs an offset column. Measure offsets are Measures so a Measure column is needed:

    // Need a column for the offsets.  This is to be a Measure column,
    // so the rules for creating a Measure column apply.
    ArrayColumnDesc<Double> cdOffset("OffsetCol", "Variable Offset col");
    ...
    // add the column to the table
    td.addColumn(cdOffset);
    ...
    // Create the Measure column to be used as the measure offset column
    TableMeasValueDesc valDesc(td, "OffsetCol");
    TableMeasDesc<MEpoch> offset(valDesc);
    // Create the offset descriptor
    TableMeasOffsetDesc offsetDesc(offset);

Storing an offset per array element per row requires one change in the constructor used in the previous example:

    ...
    // set up column and TableMeasDesc as before
    ...
    // Setting the asArray parameter to True in the constructor specifies
    // per element offset storage
    TableMeasOffsetDesc offsetDesc(offset, True);

For an example of the use of the TableMeasOffsetDesc class in the context of a full TableMeasDesc declaration see class TableMeasDesc.

Motivation

Creating the required keyword for the definition of a Measure in a Table is somewhat complicated. This class assists in that process.

Thrown Exceptions

AipsError during reconstruction of non-variable offset if a component of the offset measure is missing in the column keywords or is corrupt in some way.
AipsError if getOffset() called on a variable offset object.
AipsError during a reconstruct if the column doesn't have a Unit.

Member Description

TableMeasOffsetDesc (const Measure& offset)

Constructor which defines a constant (non-variable) offset. All measures in the columns will have the same offset.

TableMeasOffsetDesc (const TableMeasDescBase& offsetColumn, Bool asArray=False)

Constructor for defining a variable offset. If asArray is True then the offset is stored per array element. The default is for the offset to be stored (and hence variable) per row.

TableMeasOffsetDesc (const TableMeasOffsetDesc& that)

Copy constructor (copy semantics).

~TableMeasOffsetDesc()

TableMeasOffsetDesc& operator= (const TableMeasOffsetDesc& that)

Assignment operator (copy semantics).

static TableMeasOffsetDesc* reconstruct (const TableRecord& measInfo, const String& prefix, const Table& tab)

Reconstructs the TableMeasOffsetDesc from the measInfo TableRecord.

const Measure& getOffset() const

Get the (non-variable) measure offset for this column. If it doesn't exist (thus if the offset is variable), an exception is thrown.

Bool isVariable() const

Returns True if the offset varies per row.

Bool isArray() const

Returns True if the offset varies per array element.

const String& columnName() const

Gets the name of the column which stores the variable offset. "" is returned if the offset is not variable.

void resetOffset (const Measure& offset)

Reset the offset. It overwrites the value used when defining the TableMeasDesc. It is only possible if it was defined as fixed for the entire column.

void write (TableDesc&, TableRecord& measInfo, const String& prefix)
void write (Table&, TableRecord& measInfo, const String& prefix)

Write the information into the record.

TableMeasOffsetDesc (const TableRecord& measInfo, const String& prefix, const Table&)

Constructor which uses the measInfo TableRecord.

void writeKeys (TableRecord& measInfo, const String& prefix)

Write the actual keywords.