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Week 14 Reading a File

Week 14 Reading a File. Loops. Files. File - collection of records. Logical records - represent entities (persons, places, objects, things, etc.). 1234567890123456 John Smith 12000 500.00. 1234654321098765 Richard Roe 12000 500.00. Fields - describe properties of the entities.

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Week 14 Reading a File

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  1. Week 14Reading a File Loops

  2. Files File - collection of records Logical records - represent entities (persons, places, objects, things, etc.) 1234567890123456 John Smith 12000 500.00 1234654321098765 Richard Roe 12000 500.00 Fields - describe properties of the entities

  3. Basic Concepts DASD (direct access storage device) • Performance: • Seek time • Rotational delay • Transfer time Track Magnetic disks Cylinder (vertical alignment of tracks)

  4. Access Data Tracks  Rotational delay Seek time Locating the beginning sector Locating the right track  Transfer time: Transferring data from the disk to memory Read/write Head

  5. Basic Concepts DASD (direct access storage device) Tracks Physical records (blocks, pages) Block overhead Logical records

  6. File Organization and Access Methods Progression Magnetic tape • Heap (sequential) files • Hashed • Hashed files • Sorted* • Indexed sequential • B+ trees (e.g., VSAM files) Sequential access method Hash methods DSAD Secondary storage devices Indexed access methods *Applies to accessing records, not just storing them

  7. File Organization and Access Methods • File OrganizationThe physical arrangement of data in a file into records and pages on secondary storage • Types: • Heap (unordered)-sequential • Ordered (ordered by value) • Hash • Access methodThe steps involved in storing and retrieving records from a file

  8. Heap or Sequential Files Queue Records are accessed in the same sequence they were placed into the file (first in, first out).

  9.  Insert at the very top  Define variables  Define file stream  Open file For-nextLoop

  10. Copy-and-pastefrom For-next loop While loop End of file

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