Key Concepts in File Organization and Data Access Methods

Internal vs. External Files in Data Processing

External File: A data structure used to store information in secondary memory (e.g., on a hard drive or SSD).

Internal File: A variable representing a data structure in main memory used to process information within an application.

Understanding File Organization Types

File organization refers to the way data records are stored on a storage medium. The type of organization is determined when the file is created. The primary types are:

• Sequential Organization

  In a sequential file, records are written or recorded consecutively, often in adjacent physical positions, following the same order in which they were introduced. To access the n-th record, you must first read the preceding n-1 records. This results in linear time complexity for access operations.

• Direct Organization

  A file with direct organization allows any record to be accessed directly by its position or address within the file, without reading through other records. This is often achieved using a technique called hashing, where a mathematical function calculates a record's storage address based on the values of its fields.

• Indexed-Sequential Organization

  This method combines features of both sequential and direct organization. Records are stored sequentially, but the file also includes an index. This index allows for the direct lookup of a record's location, enabling access without reading all preceding records.

Handling Logically Deleted Records

Accessing a record that has been marked for logical deletion (flagged as deleted but not yet physically removed) depends on the file's organization and the Abstract Data Type (ADT) used for access:

• In Sequential Files

  Logical deletion is generally not practical. Once a record is marked as deleted, it effectively breaks the sequence. To truly remove it, the file often needs to be rewritten. If you want to re-add a deleted record, it must be appended to the end of the file.

• In Direct Files

  Logical deletion is possible and common. A record can be marked as deleted, and that specific position can be reused later to add a new record. In fact, direct files are often initialized with all record positions marked as available or "logically deleted."

• In Indexed-Sequential Files

  This is not feasible. When a record is marked as deleted, its entry is typically removed from the index. Once it's gone from the index, its position becomes inaccessible through indexed lookup.

Updating Active Location: Direct vs. Indexed Files

The process of updating the "active location" (the current record being pointed to) differs significantly between direct and indexed files.

In a direct file, records are not required to be sorted or follow a specific order. Therefore, after processing one record, the next active position can be any other valid location in the file.

In contrast, with indexed files, records are ordered by a key. When traversing the file, the next active position is determined by the subsequent key in the sequence. Additionally, special handling is required for the last record, as it does not have a successor in the ordered sequence.