Attributes

Each shape in a vector layer has a record of associated attribute data. There can be zero or more attributes; however, all shapes in one layer must have the same set of attribute fields.

In database applications, the set of fields or columns, their types, their names, and any other descriptive information is often called a schema. Fields can be one of five types:

• Float/Double: The field is a floating point number. Float and Double are similar, except that double-precision fields are more precise than float fields. The actual storage precision depends on the format.
• Integer: The field is an integral number.
• String: The field is a variable length string. Many formats (such as .dbf) have only fixed length string fields, in which case the formatting information indicates the length of the field.
• CountedInt: The field contains a list of numbers. This typically maintains lists of related shapes (for example, ArcIdList in a topological area layer or RingStart for multi-ring polygons).

Many databases have additional field types that are either mapped to GDB field types or are ignored (dates are usually treated as an integer or a string).

Each field has an associated field name. There are no fixed length restrictions on the name, but it is recommended that it be short. It is used as a column header in many applications.

Each field has a format specification, which indicates the number of characters that are used for the field when it is displayed in reports or dialogs. It also indicates whether the field should be left-aligned or right-aligned. In cases of Float and Double variables, it indicates how many decimal places of precision are shown. This value is a clue to applications and is not required. For some formats, the length and precision fields imply actual limitations of the underlying format, while for others it is just a convenience for display purposes.

Each field also has a default value. When not otherwise established, it is zero for numeric values, an empty string for string values, and an empty list for CountedInt lists.