Difference between revisions of "Filling Out the Table Delimited Data Structure"
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Revision as of 20:52, 28 April 2015
The <Table_Delimited> class contains the information needed to parse a string of character bytes in a delimited table format into a table structure in programmatic memory. It is very similar to the <Table_Character>, the principal difference being on the I/O end of things - Table_Character, as a fixed-width table, could potentially allow users to directly access individual rows and fields; Table_Delimited allows only serial access to the rows and fields.
|N.B.: PDS has defined a standard comprising parsing rules for extracting tabular data from a text file with delimited fields and records. That standard defines the I/O processing for the <Table_Delimited> data.|
For additional explanation, see the PDS4 Standards Reference, or contact your PDS node consultants.
Following are the attributes and subclasses you'll find in <Table_Delimited>, in label order.
Note that in the PDS4 master schema, all classes have capitalized names; attributes never do.
- 1 <name>
- 2 <local_identifier>
- 3 <offset>
- 4 <object_length>
- 5 <parsing_standard_id>
- 6 <description>
- 7 <records>
- 8 <record_delimiter>
- 9 <field_delimiter>
- 10 <Uniformly_Sampled>
- 11 <Record_Delimited>
- 11.1 <fields>
- 11.2 <groups>
- 11.3 <maximum_record_length>
- 11.4 A Note about Fields and Group Fields
- 11.5 <Field_Delimited>
- 11.6 <Group_Field_Delimited>
If you'd like to give your table a name, do it here.
If you need to reference this <Table_Delimited> from elsewhere in the same label, give it an identifier here. If the identifier uses the same syntax as an average variable name in a typical programming language, you should be OK syntactically.
This is the offset, in bytes, from the beginning of the file to the beginning of the Table_Delimited data. Offsets begin at zero. You must indicate a unit of bytes for this attribute:
If you know the total length of the Table_Delimited data, including all delimiter, line break characters and filler space, you can list it here. You must include a unit of bytes for this value. For example:
This attribute must have the standard value PDS DSV V1.0.
This is a free-format text field for any additional comments you might care to include at this point.
This attribute must contain the total number of records in the Table_Delimited data.
This attribute must contain the standard value carriage-return line-feed. Note that the data must have carriage-return/linefeed delimited records.
This must have one of the standard values:
- horizontal tab
- vertical bar'
Note that the parsing rules standard will contain additional information about leading and trailing blanks on the record, possible additional delimiters around text fields, the significance of leading and trailing space in a field, and so on.
If this Table_Delimited contains records which are uniformly spaced in some dimension (time, wavelength, distance, etc.), you can use this class to define that dimension and interval rather than including an additional field in each row to hold the value explicitly. The details are on the Filling Out the Uniformly Sampled Class page.
Here's a quick summary of what's in that class. All but sampling_parameter_scale are required:
This class defines the repeating series of fields contained in one complete record of the Table_Delimited data.
The number of Field_Delimited classes directly under (that is, in the first nesting level of) the Record_Delimited class. Do not count Field_Delimited classes nested under Group_Field_Delimited classes.
If your Record_Delimited contains only one or more Group_Field_Delimited classes, this will have a value of zero.
The number of Group_Field_Delimited classes directly under (that is, in the first nesting level of) the Record_Delimited class. Do not count Group_Field_Delimited classes nested under other Group_Field_Delimited classes.
If your Record_Delimited contains only one or more Field_Delimited classes, this will have a value of zero.
The length of the longest single record in the Table_Delimited data, including all fields, all repetitions of group fields, any space between fields, and the record delimiters. You must specify a unit of bytes for this value:
A Note about Fields and Group Fields
Records are composed of Fields and Group Fields. A Record must have at least one of those (either will do), and can have an arbitrary number of them, in any order (that is, you can have Fields and Group Fields interspersed). Note, however, that Group Fields are never necessary - they are a notational convenience to save writing out large numbers of essentially identical Field definitions.
The class defines a single scalar field.
The name of the field. SBN recommends that this be something fairly human-readable that can be easily turned into a variable name for use in applications, or displayed as a meaningful column heading.
This is the sequential number of the Field_Delimited definition. For SBN data products, the field_number is intended to be a help to human readers trying to map field definitions to columns in a print-out of the Table.
The type of the values in the field. This must be one of the values listed in the Standard Values Quick Reference.
The greatest number of bytes in the longest instance of this field in the delimited table. You must specify the unit:
Note: This value is slightly ambiguous within the context of a scalar field within a <Group_Field_Delimited> class. It should be the maximum length of a single scalar value, not the maximum length of all the repetitions of the value within a group.
The value of this attribute is a string representing the read/print format for the data in the field, using a subset of the POSIX print conventions defined in the Standards Reference, and also described on the PDS4 field format Conventions page.
SBN will require that this attribute be present in all Field definitions. It is used for validation of the Table contents.
If the value in this field has an associated unit, this is where it goes. This value is case sensitive, and you may use characters from the UTF-8 character set (like the Angstrom symbol) where appropriate.
Note: If a field contains a unitless value, then there should be no <unit> attribute. NEVER include a null unit value, or even worse, this:
If the data in this field are scaled, this attribute should contain the value by which the data must be multiplied to get back to the original value. Scaling factors are applied prior to adding any offset.
If the values in the field have been shifted by an offset, this attribute should contain the value that must be added to each field value to get back to the original value. Offsets and added after the scaling factor, if any.
Free-format text describing the content of the field.
Note: While not required, SBN expects to see a useful definition for every Field, as do both reviewers and users. Omit this attribute at your peril.
This class defines flag values used to indicate that a particular field value is unknown for one reason or another. It is identical to the <Special_Constants> class used in the Array classes. For details, check the Filling Out the Array 2D Data Structure - <Special_Constants> page. Here is a quick list of the special constants available in this class:
If you want to include things like extrema, mean value, and such for all the values that occur in this field through all the records in the table, this is the place to do it. This class is identical for all Field types. For details, see Filling Out the Field Statistics Class. Here is a quick list of the field statistics available in this class:
This class defines a set of Field_Delimited and nested Group_Field_Delimited classes that repeats a given number of times in each record.
|Note: Unless you have three good reasons, don't use Group_Field_Delimited in SBN data.|
If you'd like to give your group a name, this is the place to do it. Names are often useful for helping users quickly understand what relationship the repeating fields have with each other.
Analogous to field_number for scalar fields, this is a sequential number useful for referencing Group_Field_Delimited classes at a single nesting level of a complex Record_Delimited definition.
The number of times the complete set of Field_Delimiteds and Group_Field_Delimiteds comprising this <Group_Field_Character> repeats.
|Note: The minimum value for this field listed in the data dictionary is one, but as a rule no product will pass SBN review unless this value is at least two.|
The count of Field_Delimited classes directly under (i.e., at the first nesting level below) the Group_Field_Delimited definition. This will be zero if the group contains no Field_Delimited classes.
The count of Group_Field_Delimited classes directly under (i.e., at the first nesting level below) the present Group_Field_Delimited definition. This will be zero if the group contains no nested Group_Field_Delimited classes.
This free-format text field is available to provide additional text about why this group exists or what it represents.
Fields and Nested Groups
As in the Record_Delimited, the Group_Field_Delimited may contain either Field_Delimited classes, or Group_Field_Delimited classes, or both intermixed. Group_Field_Delimited classes may be nested arbitrarily deeply. The requirements for these data structure classes inside a <Group_Field_Delimited> are identical to those above.