Topic: access by current position

Summary

Many objects such as files and databases contain components which can not be directly referenced or which are expensive to access randomly. Here a form of pronoun reference is widely used. The current position, or "cursor," refers to location information maintained while the object is accessed. By using current position, the user can ignore the context of a particular component in an object, thereby treating the component as a separate data object. The current position defines a local environment or map which the user can access, relocate, access and relocate, or relocate if the current component is a particular value.

The user interface uses a cursor to indicate the current position. The program counter is a current position indicator. But because its effect is a temporal effect on a sequence of program elements, the program counter is often ignored. In co-routines and parallel processing, multiple program counters are needed. (cbb 5/80)

Subtopic: cursor

Quote: usually design data structures for localized processing, e.g., with an implicit cursor position [»hallJC5_1974]

Quote: in Icon, set the current subject and cursor position by setting variables; programs often set them explicitly [»grisRE4_1980]

Subtopic: next element

QuoteRef: goldA3_1976 ;;22 "the .large:. always fetches in the next literal expression; the .eyeBall. only fetch the expression if there is an exact match

QuoteRef: hallJC5_1974 ;;40 directive needle points into directive while cursor points into data structure. the pair can be pushed on stack with needle pointing to next element in directive

QuoteRef: goldA3_1976 ;;20 "Use of the eyeball, <E, is asking a question: do I see the following token as the next token in the message?

Subtopic: previewing the next element

QuoteRef: demeA3_1979 ;;47 The file operation "Next allows "looking ahead" in the file without "using up" the next value by performing a Read.

Subtopic: cursors not required, cause problems

Quote: cursors are not required within a database; can lead to severe bugs; e.g., CODASYL [»coddEF_1990]

Related Topics

Topic: absolute vs. relative names (12 items)
Topic: access to objects by a path (13 items)
Topic: accessing a sequence (9 items)
Topic: collection class (11 items)
Topic: current position in a user interface; cursor (23 items)
Topic: display console as a user interface (10 items)
Topic: file (22 items)
Topic: file input/output (21 items)
Topic: I/O streams (17 items)
Topic: localized understanding (43 items)
Topic: mouse as the user interface (13 items)
Topic: names defined by context (36 items)
Topic: naming by pointing or recognition (13 items)
Topic: objects without names (7 items)
Topic: pronoun reference (23 items)
Topic: selecting text with a mouse
(14 items)