Substring Fields
A substring field represents a discrete portion of an alphanumeric field (specifically, an alpha, format, or text field or a domain-type field based on an alpha, format or text domain). Substrings are defined for those instances where it is beneficial to be able to access both a field in its entirety and a discrete segment of a field, depending uponthe specific requirements of an operation or display.
A substring field occupies a portion of the same physical space as the alphanumeric field it is a part of. For example, if you define a substring to access the three-character area code portion of an alphanumeric telephone number, the substring does not store a separate copy of the data in those referenced three characters. It references the same three characters that are physically stored in the telephone number field. As a result, a change to the data in either the substring or the alphanumeric fields affects both fields.
Because a substring occupies the same physical space as an alphanumeric field, its data is a subset of the data contained in that alphanumeric field. A substring's initial value is also a subset of the alphanumeric field's initial value.
Substrings are discrete segments of the internal formats of physical fields. As such, many of the formatting attributes of a physical field do not flow through to a substring. Those attributes a substring does assume are generally not format-related as, for example, Descriptive.
The set of attributes for a substring is essentially equivalent to the attributes of a basic alphanumeric field. (Remember that a basic alphanumeric field is an alpha field that is not specially formatted.) In other words, all alphanumeric domain and field specifications use their default values except for the Length specification, which you enter.
Refer to the Substring Fields section for additional information. |